Description of the operation of the audio power amplifier on MOSFET transistors. The simplest low-frequency amplifiers on ULF transistors on 2 transistors of different conductivity circuit

10.02.2022

Now on the Internet you can find a huge number of circuits for various amplifiers on microcircuits, mainly the TDA series. They have fairly good characteristics, good efficiency and are not so expensive, in connection with this they are so popular. However, against their background, transistor amplifiers remain undeservedly forgotten, which, although difficult to set up, are no less interesting.

Amplifier circuit

In this article, we will consider the assembly process of a very unusual amplifier operating in class "A" and containing only 4 transistors. This scheme was developed back in 1969 by the English engineer John Linsley Hood, despite his old age, it remains relevant to this day.

Unlike IC amplifiers, transistor amplifiers require careful tuning and selection of transistors. This scheme is no exception, although it looks extremely simple. Transistor VT1 - input, PNP structures. You can experiment with various low-power PNP transistors, including germanium ones, for example, MP42. Transistors such as 2N3906, BC212, BC546, KT361 have proven themselves well in this circuit as VT1. Transistor VT2 - NPN structures, medium or low power, KT801, KT630, KT602, 2N697, BD139, 2SC5707, 2SD2165 are suitable here. Particular attention should be paid to the output transistors VT3 and VT4, or rather, their gain. KT805, 2SC5200, 2N3055, 2SC5198 are well suited here. It is necessary to select two identical transistors with the closest possible gain, while it should be more than 120. If the gain of the output transistors is less than 120, then a transistor with a high gain (300 or more) must be placed in the driver stage (VT2).

Selection of amplifier ratings

Some ratings on the diagram are selected based on the supply voltage of the circuit and the load resistance, some possible options are shown in the table:

It is not recommended to raise the supply voltage more than 40 volts, the output transistors may fail. A feature of class A amplifiers is a large quiescent current, and, consequently, a strong heating of transistors. With a supply voltage of, for example, 20 volts and a quiescent current of 1.5 amperes, the amplifier consumes 30 watts, regardless of whether a signal is applied to its input or not. At the same time, 15 watts of heat will be dissipated on each of the output transistors, and this is the power of a small soldering iron! Therefore, transistors VT3 and VT4 must be installed on a large radiator using thermal paste.
This amplifier is prone to self-excitation, therefore, a Zobel circuit is placed at its output: a 10 Ohm resistor and a 100 nF capacitor connected in series between ground and the common point of the output transistors (this circuit is shown in the diagram by a dotted line).
When you first turn on the amplifier in the gap of its supply wire, you need to turn on the ammeter to control the quiescent current. Until the output transistors have warmed up to operating temperature, it may float a little, this is quite normal. Also, when you turn it on for the first time, you need to measure the voltage between the common point of the output transistors (collector VT4 and emitter VT3) and ground, there should be half the supply voltage. If the voltage differs up or down, you need to turn the tuning resistor R2.

Amplifier board:

(downloads: 523)

The board is made by the LUT method.

Amplifier built by me

A few words about capacitors, input and output. The capacitance of the input capacitor in the diagram is indicated as 0.1 uF, but this capacitance is not enough. A film capacitor with a capacitance of 0.68 - 1 μF should be installed as an input, otherwise an undesirable low-frequency cutoff is possible. The output capacitor C5 should be taken for a voltage no less than the supply voltage, you should not be greedy with a capacitance either.
The advantage of this amplifier circuit is that it does not pose a danger to the speakers of the acoustic system, because the speaker is connected through a separating capacitor (C5), which means that when a constant voltage appears at the output, for example, when the amplifier fails, the speaker will remain intact, because the capacitor will not pass a constant voltage.

- The neighbor got tired of knocking on the battery. He turned the music up louder so that he could not be heard.
(From audiophile folklore).

The epigraph is ironic, but the audiophile is not necessarily “sick in the head” with the physiognomy of Josh Earnest at a briefing on relations with the Russian Federation, who is “rushing” because the neighbors are “happy”. Someone wants to listen to serious music at home as in the hall. The quality of the equipment for this is necessary, which for fans of the decibel of loudness as such simply does not fit where sane people have a mind, but for the latter, this mind comes from the prices of suitable amplifiers (UMZCH, audio frequency power amplifier). And someone along the way has a desire to join useful and exciting areas of activity - the technique of sound reproduction and electronics in general. Which in the digital age are inextricably linked and can become a highly profitable and prestigious profession. The first step in this matter, optimal in all respects, is to make an amplifier with your own hands: it is UMZCH that allows, with initial training based on school physics, on the same table, to go from the simplest structures for half an evening (which, nevertheless, “sing” well) to the most complex units, through which a good rock band will play with pleasure. The purpose of this publication is to cover the first stages of this path for beginners and, perhaps, to tell something new to experienced ones.

Protozoa

So, for starters, let's try to make a sound amplifier that just works. In order to thoroughly delve into sound engineering, you will have to gradually master quite a lot of theoretical material and do not forget to enrich your knowledge base as you progress. But any “smartness” is easier to digest when you see and feel how it works “in hardware”. In this article, further, too, it will not do without theory - in what you need to know at first and what can be explained without formulas and graphs. In the meantime, it will be enough to be able to use the multitester.

Note: if you have not soldered electronics yet, please note that its components must not be overheated! Soldering iron - up to 40 W (better than 25 W), the maximum allowable soldering time without interruption is 10 s. The soldered lead for the heat sink is held 0.5-3 cm from the place of soldering from the side of the device case with medical tweezers. Acid and other active fluxes must not be used! Solder - POS-61.

On the left in fig.- the simplest UMZCH, "which just works." It can be assembled on both germanium and silicon transistors.

On this crumb, it is convenient to master the basics of setting up the UMZCH with direct connections between the cascades, which give the clearest sound:

Before the first power-up, the load (speaker) is turned off;
Instead of R1, we solder a chain of a constant resistor of 33 kOhm and a variable (potentiometer) of 270 kOhm, i.e. first note. four times smaller, and the second approx. twice the face value against the original according to the scheme;
We supply power and, by rotating the potentiometer slider, at the point marked with a cross, set the specified collector current VT1;
We remove the power, solder the temporary resistors and measure their total resistance;
As R1, we set the nominal resistor from the standard row closest to the measured one;
We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
The same as according to paragraphs. 3-5, incl. a set the voltage equal to half the supply voltage.

Point a, from where the signal is taken to the load, is the so-called. middle point of the amplifier. In UMZCH with unipolar power, half of its value is set in it, and in UMZCH with bipolar power - zero relative to the common wire. This is called adjusting the balance of the amplifier. In unipolar UMZCH with capacitive load decoupling, it is not necessary to turn it off during setup, but it’s better to get used to doing it reflexively: an unbalanced 2-polar amplifier with a connected load can burn its own powerful and expensive output transistors, or even “new, good” and very expensive powerful speaker.

Note: components that require selection when setting up a device in a layout are indicated on the diagrams either with an asterisk (*) or an apostrophe dash (‘).

In the center in the same Fig.- a simple UMZCH on transistors, which already develops power up to 4-6 W at a load of 4 ohms. Although it works, like the previous one, in the so-called. class AB1, not intended for Hi-Fi sound, but if you replace a pair of such class D amplifier (see below) in cheap Chinese computer speakers, their sound improves markedly. Here we learn another trick: powerful output transistors must be placed on radiators. Components that require additional cooling are circled in the diagrams with a dotted line; however, not always; sometimes - with an indication of the required dissipating area of the heat sink. Adjustment of this UMZCH - balancing with R2.

On the right in fig.- not yet a 350 W monster (as was shown at the beginning of the article), but already quite a solid beast: a simple 100 W transistor amplifier. You can listen to music through it, but not Hi-Fi, the work class is AB2. However, for scoring a picnic area or an outdoor meeting, a school assembly or a small trading floor, it is quite suitable. An amateur rock band, having such an UMZCH for an instrument, can perform successfully.

In this UMZCH, 2 more tricks appear: firstly, in very powerful amplifiers, the buildup cascade of a powerful output also needs to be cooled, so VT3 is placed on a radiator from 100 sq. see. For output VT4 and VT5, radiators from 400 square meters are needed. see Secondly, UMZCH with bipolar power supply are not balanced at all without load. Either one or the other output transistor goes into cutoff, and the conjugated one goes into saturation. Then, at full supply voltage, current surges during balancing can destroy the output transistors. Therefore, for balancing (R6, did you guess?), the amplifier is powered from +/-24 V, and instead of the load, a 100 ... 200 Ohm wire resistor is included. By the way, the squiggles in some of the resistors in the diagram are Roman numerals, denoting their required heat dissipation power.

Note: a power source for this UMZCH needs a power of 600 watts or more. Smoothing filter capacitors - from 6800 uF to 160 V. In parallel with the electrolytic capacitors of the IP, ceramic ones of 0.01 uF are turned on to prevent self-excitation at ultrasonic frequencies, which can instantly burn out the output transistors.

On the field workers

On the trail. rice. - another option for a fairly powerful UMZCH (30 W, and with a supply voltage of 35 V - 60 W) on powerful field-effect transistors:

The sound from it already draws on the requirements for entry-level Hi-Fi (if, of course, the UMZCH works on the corresponding acoustic systems, speakers). Powerful field workers do not require much power for buildup, so there is no pre-power cascade. Even powerful field-effect transistors do not burn the speakers under any malfunctions - they themselves burn out faster. Also unpleasant, but still cheaper than changing an expensive bass speaker head (GG). Balancing and generally adjustment to this UMZCH are not required. It has only one drawback, like a design for beginners: powerful field-effect transistors are much more expensive than bipolar ones for an amplifier with the same parameters. IP requirements are the same as before. occasion, but its power is needed from 450 watts. Radiators - from 200 sq. cm.

Note: no need to build powerful UMZCH on field-effect transistors for switching power supplies, for example. computer. When trying to “drive” them into the active mode necessary for the UMZCH, they either simply burn out, or they give a weak sound, but “none” in quality. The same applies to powerful high-voltage bipolar transistors, for example. from the horizontal scanning of old TVs.

Right up

If you have already taken the first steps, then it will be quite natural to want to build UMZCH class Hi-Fi, without going too deep into the theoretical jungle. To do this, you will have to expand the instrument park - you need an oscilloscope, an audio frequency generator (GZCH) and an alternating current millivoltmeter with the ability to measure the constant component. It is better to take the UMZCH E. Gumeli, described in detail in Radio No. 1 for 1989, as a prototype for repetition. To build it, you will need a few inexpensive affordable components, but the quality meets very high requirements: power up to 60 W, bandwidth 20-20,000 Hz, frequency response unevenness 2 dB, non-linear distortion factor (THD) 0.01%, self-noise level -86 dB. However, setting up the Gumeli amplifier is quite difficult; if you can handle it, you can take on any other. However, some of the circumstances now known greatly simplify the establishment of this UMZCH, see below. Bearing this in mind and the fact that not everyone succeeds in getting into the Radio archives, it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

UMZCH Gumeli schemes and specifications for them are given in the illustration. Radiators of output transistors - from 250 sq. see for UMZCH according to fig. 1 and from 150 sq. see for variant according to fig. 3 (numbering is original). The transistors of the pre-output stage (KT814/KT815) are mounted on radiators bent from aluminum plates 75x35 mm 3 mm thick. It is not worth replacing KT814 / KT815 with KT626 / KT961, the sound does not noticeably improve, but it is seriously difficult to establish.

This UMZCH is very critical to the power supply, installation topology and general, therefore, it must be adjusted in a structurally finished form and only with a standard power source. When trying to power from a stabilized IP, the output transistors burn out immediately. Therefore, in fig. drawings of original printed circuit boards and instructions for setting up are given. It can be added to them that, firstly, if “excitation” is noticeable at the first start, they fight with it by changing the inductance L1. Secondly, the leads of the parts installed on the boards must be no longer than 10 mm. Thirdly, it is highly undesirable to change the installation topology, but if it is very necessary, there must be a frame screen on the side of the conductors (ground loop, highlighted in the figure), and the power supply paths must pass outside it.

Note: breaks in the tracks to which the bases of powerful transistors are connected - technological ones, for establishing, after which they are sealed with drops of solder.

The establishment of this UMZCH is greatly simplified, and the risk of encountering "excitation" in the process of use is reduced to zero if:

Minimize interconnect wiring by placing boards on high-power transistor heatsinks.
Completely abandon the connectors inside, performing the entire installation only by soldering. Then you will not need R12, R13 in a powerful version or R10 R11 in a less powerful one (they are dotted on the diagrams).
Use the minimum length of oxygen-free copper audio wires for indoor wiring.

When these conditions are met, there are no problems with excitation, and the establishment of UMZCH is reduced to a routine procedure, described in Fig.

Wires for sound

Audio wires are not idle fiction. The need for their use at the present time is undeniable. In copper with an admixture of oxygen, the thinnest oxide film is formed on the faces of metal crystallites. Metal oxides are semiconductors and if the current in the wire is weak without a constant component, its shape is distorted. In theory, distortions on myriads of crystallites should compensate each other, but very little (it seems, due to quantum uncertainties) remains. Enough to be noticed by discerning listeners against the background of the purest sound of modern UMZCH.

Manufacturers and traders without a twinge of conscience slip ordinary electrical copper instead of oxygen-free copper - it is impossible to distinguish one from the other by eye. However, there is a scope where a fake does not go unambiguously: a twisted-pair cable for computer networks. Put a grid with long segments on the left, it will either not start at all, or it will constantly fail. Dispersion of impulses, you know.

The author, when there was still talk about audio wires, realized that, in principle, this was not empty chatter, especially since oxygen-free wires by that time had long been used in special-purpose equipment, with which he was well acquainted by the nature of his activity. Then I took it and replaced the regular cord of my TDS-7 headphones with a home-made one from a “vitukha” with flexible stranded wires. The sound, by ear, has steadily improved for analog tracks through, i.e. on the way from the studio microphone to the disc, never digitized. Recordings on vinyl made using DMM technology (Direct Meta lMastering, direct metal deposition) sounded especially bright. After that, the interblock editing of all home audio was converted to "vitushny". Then completely random people began to notice the improvement in sound, they were indifferent to music and not forewarned in advance.

How to make interconnect wires from twisted pair, see next. video.

Video: do-it-yourself twisted-pair interconnect wires

Unfortunately, the flexible "vituha" soon disappeared from sale - it did not hold well in crimped connectors. However, for the information of readers, flexible “military” wire MGTF and MGTFE (shielded) is made only from oxygen-free copper. Forgery is impossible, because. on ordinary copper, fluoroplastic tape insulation spreads rather quickly. MGTF is now widely available and is much cheaper than branded, guaranteed audio wires. It has one drawback: it cannot be done colored, but this can be corrected with tags. There are also oxygen-free winding wires, see below.

Theoretical interlude

As you can see, already at the very beginning of mastering sound engineering, we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity of sound reproduction. Hi-Fi comes in different levels, which are ranked next. main parameters:

Band of reproducible frequencies.
Dynamic range - the ratio in decibels (dB) of the maximum (peak) output power to the level of self-noise.
Self-noise level in dB.
Nonlinear distortion factor (THD) at rated (long-term) output power. SOI at peak power is assumed to be 1% or 2% depending on the measurement technique.
Irregularities in the amplitude-frequency characteristic (AFC) in the reproducible frequency band. For speakers - separately at low (LF, 20-300 Hz), medium (MF, 300-5000 Hz) and high (HF, 5000-20,000 Hz) audio frequencies.

Note: the ratio of the absolute levels of any values of I in (dB) is defined as P(dB) = 20lg(I1/I2). If I1

You need to know all the subtleties and nuances of Hi-Fi when designing and building speakers, and as for a home-made Hi-Fi UMZCH for the home, before moving on to these, you need to clearly understand the requirements for their power required for scoring a given room, dynamic range (dynamics), self-noise level and SOI. To achieve a frequency band of 20-20,000 Hz from the UMZCH with a blockage at the edges of 3 dB and a frequency response unevenness at the midrange of 2 dB on a modern element base is not very difficult.

Volume

The power of the UMZCH is not an end in itself, it should provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. Natural noise in residential premises is quieter than 20 dB; 20 dB is the wilderness in complete calm. The volume level of 20 dB relative to the threshold of hearing is the threshold of intelligibility - you can still make out the whisper, but the music is perceived only as a fact of its presence. An experienced musician can tell which instrument is playing, but not exactly what.

40 dB - the normal noise of a well-insulated city apartment in a quiet area or a country house - represents the threshold of intelligibility. Music from the threshold of intelligibility to the threshold of intelligibility can be listened to with a deep frequency response correction, primarily in bass. To do this, the MUTE function is introduced into modern UMZCH (mute, mutation, not mutation!), Which includes resp. corrective circuits in UMZCH.

90 dB is the volume level of a symphony orchestra in a very good concert hall. 110 dB can give out an expanded orchestra in a hall with unique acoustics, of which there are no more than 10 in the world, this is the threshold of perception: louder sounds are perceived even as distinguishable in meaning with an effort of will, but already annoying noise. The loudness zone in residential premises of 20-110 dB is the zone of full audibility, and 40-90 dB is the zone of the best audibility, in which unprepared and inexperienced listeners fully perceive the meaning of the sound. If, of course, he is in it.

Power

Calculating the power of the equipment for a given volume in the listening area is perhaps the main and most difficult task of electroacoustics. For yourself, in conditions it is better to go from acoustic systems (AS): calculate their power using a simplified method, and take the nominal (long-term) power of the UMZCH equal to the peak (musical) speakers. In this case, the UMZCH will not noticeably add its distortions to those speakers, they are already the main source of non-linearity in the audio path. But the UMZCH should not be made too powerful: in this case, the level of its own noise may be above the threshold of audibility, because. it is considered from the voltage level of the output signal at maximum power. If it is quite simple to consider, then for a room of an ordinary apartment or house and speakers with normal characteristic sensitivity (sound output), you can take a trace. UMZCH optimal power values:

Up to 8 sq. m - 15-20 W.
8-12 sq. m - 20-30 W.
12-26 sq. m - 30-50 W.
26-50 sq. m - 50-60 W.
50-70 sq. m - 60-100 watts.
70-100 sq. m - 100-150 watts.
100-120 sq. m - 150-200 watts.
Over 120 sq. m - is determined by calculation according to acoustic measurements on site.

Dynamics

The dynamic range of UMZCH is determined by equal loudness curves and threshold values for different degrees of perception:

Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. Sound with dynamics of 80-85 dB in a city apartment will not be distinguished from ideal by any expert.
Other serious musical genres - 75 dB is excellent, 80 dB is over the roof.
Pops of any kind and movie soundtracks - 66 dB for the eyes is enough, because. these opuses are already compressed in levels up to 66 dB and even up to 40 dB during recording, so that you can listen to anything.

The dynamic range of the UMZCH, correctly selected for a given room, is considered equal to its own noise level, taken with a + sign, this is the so-called. signal-to-noise ratio.

SOI

Nonlinear distortions (NI) UMZCH are components of the spectrum of the output signal, which were not in the input. Theoretically, it is best to “push” the NI under the level of its own noise, but technically this is very difficult to implement. In practice, they take into account the so-called. masking effect: at volume levels below approx. 30 dB the range of frequencies perceived by the human ear narrows, as does the ability to distinguish sounds by frequency. Musicians hear notes, but it is difficult to assess the timbre of the sound. In people without a musical ear, the masking effect is already observed at 45-40 dB of volume. Therefore, UMZCH with a THD of 0.1% (-60 dB from a volume level of 110 dB) will be assessed as a Hi-Fi by an ordinary listener, and with a THD of 0.01% (-80 dB) can be considered not distorting the sound.

Lamps

The last statement, perhaps, will cause rejection, up to furious, among adherents of tube circuitry: they say that only tubes give real sound, and not just any, but certain types of octal ones. Calm down, gentlemen - a special tube sound is not fiction. The reason is fundamentally different distortion spectra for electronic tubes and transistors. Which, in turn, are due to the fact that the electron flow in the lamp moves in vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minor charge carriers (electrons and holes) move in a crystal, which is generally impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly traced in it, and there are very few combinational components (sums and differences of the frequencies of the input signal and their harmonics). Therefore, in the days of vacuum circuitry, SOI was called the harmonic coefficient (KH). In transistors, the distortion spectrum (if they are measurable, the reservation is random, see below) can be traced up to the 15th and higher components, and there are more than enough combination frequencies in it.

At the beginning of solid-state electronics, the designers of transistorized UMZCH took for them the usual "tube" SOI of 1-2%; a sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as clean. By the way, the very concept of Hi-Fi did not exist then. It turned out - they sound dull and deaf. In the process of the development of transistor technology, an understanding was developed of what Hi-Fi is and what is needed for it.

At present, the growing pains of transistor technology have been successfully overcome and side frequencies at the output of a good UMZCH are hardly captured by special measurement methods. And lamp circuitry can be considered to have passed into the category of art. Its basis can be any, why can't electronics go there? An analogy with photography would be appropriate here. No one can deny that a modern digital SLR gives an image immeasurably clearer, more detailed, deeper in terms of brightness and color range than a plywood box with an accordion. But someone with the coolest Nikon "clicks pictures" like "this is my fat cat got drunk like a bastard and sleeps with his paws spread", and someone with Smena-8M on a Svemov b / w film takes a picture in front of which people are crowding at a prestigious exhibition.

Note: and once again calm down - not everything is so bad. To date, low-power lamp UMZCHs have at least one application left, and not of the least importance, for which they are technically necessary.

Experimental stand

Many audio lovers, having barely learned how to solder, immediately "go into the lamps." This is by no means deserving of condemnation, on the contrary. Interest in the origins is always justified and useful, and electronics has become such on lamps. The first computers were tube-based, and the on-board electronic equipment of the first spacecraft was also tube-based: there were already transistors at that time, but they could not withstand extraterrestrial radiation. By the way, then, under the strictest secrecy, tube ... microcircuits were also created! Cold cathode microlamps. The only known mention of them in open sources is in the rare book by Mitrofanov and Pickersgil "Modern receiving-amplifying lamps".

But enough of the lyrics, let's get down to business. For those who like to tinker with the lamps in fig. - a diagram of a bench lamp UMZCH, designed specifically for experiments: SA1 switches the operating mode of the output lamp, and SA2 - the supply voltage. The circuit is well known in the Russian Federation, a slight refinement touched only the output transformer: now you can not only “drive” your native 6P7S in different modes, but also select the screen grid switching ratio for other lamps in ultra-linear mode; for the vast majority of output pentodes and beam tetrodes, it is either 0.22-0.25, or 0.42-0.45. See below for output transformer manufacturing.

Guitarists and rockers

This is the case when you can not do without lamps. As you know, the electric guitar became a full-fledged solo instrument after the pre-amplified signal from the pickup was passed through a special prefix - a fuser - deliberately distorting its spectrum. Without this, the sound of the string was too sharp and short, because. an electromagnetic pickup reacts only to the modes of its mechanical oscillations in the plane of the soundboard of the instrument.

Soon an unpleasant circumstance came to light: the sound of an electric guitar with a fuser gains full strength and brightness only at high volumes. This is especially evident for guitars with a humbucker pickup, which gives the most "evil" sound. But what about a beginner, forced to rehearse at home? Do not go to the hall to perform, not knowing exactly how the instrument will sound there. And just rock lovers want to listen to their favorite things in full juice, and rockers are generally decent and non-conflict people. At least those who are interested in rock music, and not outrageous surroundings.

So, it turned out that the fatal sound appears at volume levels acceptable for residential premises, if the UMZCH is tube. The reason is the specific interaction of the signal spectrum from the fuser with a clean and short spectrum of tube harmonics. Here again, an analogy is appropriate: a b / w photo can be much more expressive than a color one, because. leaves only the contour and the light for viewing.

Those who need a tube amplifier not for experiments, but because of technical necessity, have no time to master the intricacies of tube electronics for a long time, they are passionate about others. UMZCH in this case, it is better to do transformerless. More precisely, with a single-ended matching output transformer that operates without constant bias. This approach greatly simplifies and speeds up the manufacture of the most complex and critical assembly of the lamp UMZCH.

“Transformerless” UMZCH tube output stage and preamplifiers for it

On the right in fig. a diagram of a transformerless output stage of a tube UMZCH is given, and on the left are options for a preamplifier for it. Above - with a tone control according to the classic Baksandal scheme, which provides a fairly deep adjustment, but introduces small phase distortions into the signal, which can be significant when operating the UMZCH on a 2-way speaker. Below is a simpler preamplifier with tone control that does not distort the signal.

But let's get back to the end. In a number of foreign sources, this circuit is considered a revelation, however, identical to it, with the exception of the capacity of electrolytic capacitors, is found in the Soviet Radio Amateur's Handbook of 1966. A thick book of 1060 pages. There was no Internet then and databases on disks.

In the same place, on the right in the figure, the shortcomings of this scheme are briefly but clearly described. Improved, from the same source, given on the trail. rice. on right. In it, the screen grid L2 is powered from the midpoint of the anode rectifier (the anode winding of the power transformer is symmetrical), and the screen grid L1 through the load. If, instead of high-impedance speakers, you turn on a matching transformer with a conventional speaker, as in the previous. circuit, the output power is approx. 12 W, because the active resistance of the primary winding of the transformer is much less than 800 ohms. SOI of this final stage with a transformer output - approx. 0.5%

How to make a transformer?

The main enemies of the quality of a powerful signal low-frequency (sound) transformer are the magnetic stray field, the lines of force of which are closed, bypassing the magnetic circuit (core), eddy currents in the magnetic circuit (Foucault currents) and, to a lesser extent, magnetostriction in the core. Because of this phenomenon, a carelessly assembled transformer "sings", buzzes or squeaks. Foucault currents are fought by reducing the thickness of the plates of the magnetic circuit and additionally isolating them with varnish during assembly. For output transformers, the optimal thickness of the plates is 0.15 mm, the maximum allowable is 0.25 mm. Thinner plates should not be taken for the output transformer: the filling factor of the core (the central core of the magnetic circuit) with steel will fall, the cross section of the magnetic circuit will have to be increased to obtain a given power, which will only increase distortion and losses in it.

In the core of an audio transformer operating with a constant bias (eg, anode current of a single-ended output stage), there must be a small (determined by calculation) non-magnetic gap. The presence of a non-magnetic gap, on the one hand, reduces signal distortion from constant bias; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a larger core. Therefore, the non-magnetic gap must be calculated at the optimum and performed as accurately as possible.

For transformers operating with magnetization, the optimal type of core is made of Shp plates (punched), pos. 1 in fig. In them, a non-magnetic gap is formed during the penetration of the core and therefore is stable; its value is indicated in the passport for the plates or measured with a set of probes. The stray field is minimal, because the side branches through which the magnetic flux closes are solid. Shp plates are often used to assemble transformer cores without magnetization, because Shp plates are made of high quality transformer steel. In this case, the core is assembled in an overlap (the plates are placed with a notch in one direction or the other), and its cross section is increased by 10% against the calculated one.

It is better to wind transformers without magnetization on USh cores (reduced height with widened windows), pos. 2. In them, the reduction of the stray field is achieved by reducing the length of the magnetic path. Since USh plates are more accessible than Shp, transformer cores with magnetization are often also made from them. Then the assembly of the core is carried out in a cut: a package of W-plates is assembled, a strip of non-conductive non-magnetic material is laid with a thickness equal to the value of the non-magnetic gap, covered with a yoke from a package of jumpers and pulled together by a clip.

Note:"Audio" signal magnetic circuits of the ShLM type for output transformers of high-quality tube amplifiers are of little use, they have a large stray field.

At pos. 3 is a diagram of the dimensions of the core for calculating the transformer, at pos. 4 winding frame design, and on pos. 5 - patterns of its details. As for the transformer for the "transformerless" output stage, it is better to do it on the SLMme with an overlap, because. the bias is negligible (the bias current is equal to the current of the screen grid). The main task here is to make the windings as compact as possible in order to reduce the stray field; their active resistance will still turn out to be much less than 800 ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings wind turn to turn (if there is no winding machine, this is a terrible machine) from the thinnest possible wire, the anode winding laying coefficient for the mechanical calculation of the transformer is taken as 0.6. The winding wire is of the PETV or PEMM brands, they have an oxygen-free core. It is not necessary to take PETV-2 or PEMM-2, they have an increased outer diameter due to double varnishing and the scattering field will be larger. The primary winding is wound first, because. it is its stray field that most affects the sound.

Iron for this transformer must be looked for with holes in the corners of the plates and clamps (see the figure on the right), because. "For complete happiness" the assembly of the magnetic circuit is carried out in the following. order (of course, the windings with leads and outer insulation should already be on the frame):

Prepare half-diluted acrylic varnish or, in the old fashioned way, shellac;
Plates with jumpers are quickly varnished on one side and put into the frame as quickly as possible, without pressing hard. The first plate is placed with the lacquered side inward, the next - with the unvarnished side to the lacquered first, etc.;
When the frame window is full, staples are applied and tightened tightly with bolts;
After 1-3 minutes, when the extrusion of varnish from the gaps apparently stops, the plates are added again until the window is filled;
Repeat paragraphs. 2-4 until the window is tightly packed with steel;
The core is pulled tightly again and dried on a battery or the like. 3-5 days.

The core assembled using this technology has very good plate insulation and steel filling. Losses due to magnetostriction are not detected at all. But keep in mind - for the cores of their permalloy, this technique is not applicable, because. from strong mechanical influences, the magnetic properties of permalloy irreversibly deteriorate!

On microchips

UMZCH on integrated circuits (ICs) are most often made by those who are satisfied with sound quality up to average Hi-Fi, but are more attracted by cheapness, speed, ease of assembly and the complete absence of any adjustment procedures that require special knowledge. Simply, an amplifier on microcircuits is the best option for dummies. The classic of the genre here is UMZCH on the TDA2004 IC, standing on the series, God forbid, for 20 years, on the left in fig. Power - up to 12 W per channel, supply voltage - 3-18 V unipolar. Radiator area - from 200 sq. see for maximum power. The advantage is the ability to work on a very low-resistance, up to 1.6 Ohm, load, which allows you to remove full power when powered from the 12 V on-board network, and 7-8 W - with a 6-volt power supply, for example, on a motorcycle. However, the TDA2004 output in class B is non-complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 gives no better sound, but more powerful, up to 25 W, because. the upper limit of the supply voltage has been increased to 25V. TDA7261 can be run from almost all on-board networks, except for aircraft 27 V. With the help of hinged components (strapping, on the right in the figure), TDA7261 can operate in mutation mode and with the St-By (Stand By, wait) function, which switches the UMZCH to the minimum power consumption mode when there is no input signal for a certain time. Amenities cost money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: if you are attracted to amplifiers with the St-By function, keep in mind that you should not expect speakers wider than 66 dB from them.

"Super-economical" in terms of power TDA7482, on the left in the figure, working in the so-called. class D. Such UMZCH are sometimes called digital amplifiers, which is not true. For true digitization, level samples are taken from an analog signal at a quantization frequency of at least twice the highest of the reproducible frequencies, the value of each sample is recorded in an error-correcting code and stored for future use. UMZCH class D - pulsed. In them, the analogue is directly converted into a sequence of high-frequency pulse-width modulated (PWM) pulses, which is fed to the speaker through a low-pass filter (LPF).

Class D sound has nothing to do with Hi-Fi: THD of 2% and dynamics of 55 dB for UMZCH class D are considered very good indicators. And TDA7482 here, I must say, the choice is not optimal: other companies specializing in class D produce UMZCH ICs cheaper and require less strapping, for example, the Paxx D-UMZCH series, on the right in Fig.

Of the TDAs, it should be noted the 4-channel TDA7385, see the figure, on which you can assemble a good amplifier for speakers up to medium Hi-Fi inclusive, with frequency separation into 2 bands or for a system with a subwoofer. The filtering of low-frequency and mid-high frequencies in both cases is done at the input on a weak signal, which simplifies the design of the filters and allows for a deeper separation of the bands. And if the acoustics are subwoofer, then 2 channels of the TDA7385 can be allocated for the sub-ULF of the bridge circuit (see below), and the remaining 2 can be used for midrange-high frequencies.

UMZCH for subwoofer

A subwoofer, which can be translated as a "subwoofer" or, literally, "a subwoofer" reproduces frequencies up to 150-200 Hz, in this range, human ears are practically unable to determine the direction to the sound source. In speakers with a subwoofer, the “subwoofer” speaker is placed in a separate acoustic design, this is the subwoofer as such. The subwoofer is placed, in principle, as it is more convenient, and the stereo effect is provided by separate MF-HF channels with their own small-sized speakers, for the acoustic design of which there are no particularly serious requirements. Connoisseurs agree that it is still better to listen to stereo with full channel separation, but subwoofer systems significantly save money or labor on the bass path and make it easier to place acoustics in small rooms, which is why they are popular with consumers with normal hearing and not particularly demanding.

“Leakage” of midrange-high frequencies into the subwoofer, and from it into the air, greatly spoils the stereo, but if you sharply “cut off” the subbass, which, by the way, is very difficult and expensive, then a sound jump effect that is very unpleasant to the ear will occur. Therefore, channel filtering in subwoofer systems is done twice. At the input, MF-HF with bass "tails" are distinguished by electric filters, which do not overload the MF-HF path, but provide a smooth transition to sub-bass. Bass with midrange "tails" are combined and fed to a separate UMZCH for the subwoofer. The midrange is filtered out so that the stereo does not deteriorate, it is already acoustic in the subwoofer: the subwoofer is placed, for example, in the partition between the resonator chambers of the subwoofer, which do not let the midrange out, see on the right in Fig.

A number of specific requirements are imposed on the UMZCH for a subwoofer, of which the "dummies" consider the greatest possible power as the main one. This is completely wrong, if, say, the calculation of acoustics for a room gave peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if speakers S-30 are suitable for the room, then a subwoofer is needed 1.6x30 \u003d 48 watts.

It is much more important to ensure the absence of phase and transient distortions: if they go, there will definitely be a sound jump. As for THD, it is acceptable up to 1%. Bass distortions of this level are not audible (see curves of equal loudness), and the “tails” of their spectrum in the best audible midrange region will not get out of the subwoofer.

In order to avoid phase and transient distortions, the amplifier for the subwoofer is built according to the so-called. bridge circuit: the outputs of 2 identical UMZCH are turned on in the opposite direction through the speaker; the signals to the inputs are in antiphase. The absence of phase and transient distortion in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers that form the shoulders of the bridge is ensured by the use of paired UMZCH on ICs, made on the same chip; this is perhaps the only case when an amplifier on microcircuits is better than a discrete one.

Note: the power of the bridge UMZCH does not double, as some people think, it is determined by the supply voltage.

An example of a bridge UMZCH circuit for a subwoofer in a room up to 20 sq. m (without input filters) on the TDA2030 IC is given in fig. left. Additional midrange filtering is carried out by the R5C3 and R'5C'3 circuits. Radiator area TDA2030 - from 400 sq. see Bridge UMZCHs with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current that can disable the speaker, and protection circuits on the subbass often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive “dubovo” woofer with non-polar batteries of electrolytic capacitors (highlighted in color, and the diagram of one battery is given in the sidebar.

A little about acoustics

The acoustic design of a subwoofer is a special topic, but since a drawing is given here, explanations are also needed. Case material - MDF 24 mm. The resonator tubes are made of sufficiently durable non-ringing plastic, for example, polyethylene. The internal diameter of the pipes is 60 mm, the protrusions inward are 113 mm in the large chamber and 61 in the small one. For a specific speaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, for the least effect on the stereo effect. To tune the pipes, they take obviously longer lengths and, pushing in and out, achieve the desired sound. The outward protrusions of the pipes do not affect the sound, they are then cut off. The pipe settings are interdependent, so you have to tinker.

Headphone Amplifier

A headphone amplifier is made by hand most often for 2 reasons. The first is for listening "on the go", i.e. outside the home, when the power of the audio output of the player or smartphone is not enough to build up "buttons" or "burdocks". The second is for high-end home headphones. Hi-Fi UMZCH for an ordinary living room is needed with dynamics up to 70-75 dB, but the dynamic range of the best modern stereo headphones exceeds 100 dB. An amplifier with such dynamics is more expensive than some cars, and its power will be from 200 watts per channel, which is too much for an ordinary apartment: listening at a very low power level spoils the sound, see above. Therefore, it makes sense to make a low-power, but with good dynamics, a separate amplifier specifically for headphones: the prices for household UMZCH with such a makeweight are obviously too high.

The diagram of the simplest headphone amplifier on transistors is given in pos. 1 fig. Sound - except for Chinese "buttons", works in class B. It also does not differ in economy - 13-mm lithium batteries last for 3-4 hours at full volume. At pos. 2 - TDA classic for on-the-go headphones. The sound, however, gives quite decent, up to average Hi-Fi, depending on the parameters of the track digitization. Amateur improvements to the TDA7050 strapping are innumerable, but no one has yet achieved the transition of sound to the next level of class: the “mikruha” itself does not allow. TDA7057 (pos. 3) is simply more functional, you can connect the volume control on a regular, not dual, potentiometer.

UMZCH for headphones on the TDA7350 (pos. 4) is already designed to build up good individual acoustics. It is on this IC that headphone amplifiers are assembled in most household UMZCHs of the middle and high class. The UMZCH for headphones on the KA2206B (pos. 5) is already considered professional: its maximum power of 2.3 W is enough to build up such serious isodynamic "burdocks" as TDS-7 and TDS-15.

Low-frequency amplifiers (ULF) are used to convert weak signals of a predominantly audio range into more powerful signals that are acceptable for direct perception through electrodynamic or other sound emitters.

Note that high-frequency amplifiers up to frequencies of 10 ... 100 MHz are built according to similar schemes, the whole difference most often comes down to the fact that the values of the capacitances of the capacitors of such amplifiers decrease as many times as the frequency of the high-frequency signal exceeds the frequency of the low-frequency signal.

A simple single transistor amplifier

The simplest ULF, made according to the scheme with a common emitter, is shown in Fig. 1. A telephone capsule was used as a load. The allowable supply voltage for this amplifier is 3 ... 12 V.

It is desirable to determine the value of the bias resistor R1 (tens of kΩ) experimentally, since its optimal value depends on the supply voltage of the amplifier, the resistance of the telephone capsule, and the transmission coefficient of a particular instance of the transistor.

Rice. 1. Scheme of a simple ULF on one transistor + capacitor and resistor.

To select the initial value of the resistor R1, it should be taken into account that its value should be about a hundred or more times greater than the resistance included in the load circuit. To select a bias resistor, it is recommended to connect in series a constant resistor with a resistance of 20 ... 30 kOhm and a variable resistor with a resistance of 100 ... 1000 kOhm, after which, by applying a small amplitude sound signal to the amplifier input, for example, from a tape recorder or player, by rotating the variable resistor knob, achieve the best signal quality at the highest volume.

The capacitance value of the transition capacitor C1 (Fig. 1) can be in the range from 1 to 100 microfarads: the greater the value of this capacitance, the lower frequencies the ULF can amplify. To master the technique of amplifying low frequencies, it is recommended to experiment with the selection of the values of the elements and the operating modes of the amplifiers (Fig. 1 - 4).

Improved Single Transistor Amplifier Options

Complicated and improved in comparison with the scheme in fig. 1 amplifier circuits are shown in fig. 2 and 3. In the diagram in fig. 2, the amplification stage additionally contains a frequency-dependent negative feedback circuit (resistor R2 and capacitor C2), which improves the signal quality.

Rice. 2. Scheme of a single-transistor ULF with a chain of frequency-dependent negative feedback.

Rice. 3. A single-transistor amplifier with a divider to supply a bias voltage to the base of the transistor.

Rice. 4. Single transistor amplifier with automatic bias setting for the base of the transistor.

In the diagram in fig. 3, the bias to the base of the transistor is set more “rigidly” using a divider, which improves the quality of the amplifier when its operating conditions change. An “automatic” bias setting based on an amplifying transistor is used in the circuit in fig. 4.

Two-stage transistor amplifier

By connecting in series two simple amplification stages (Fig. 1), you can get a two-stage ULF (Fig. 5). The gain of such an amplifier is equal to the product of the gains of the individual stages. However, it is not easy to obtain a large stable gain with a subsequent increase in the number of stages: the amplifier will most likely self-excite.

Rice. 5. Scheme of a simple two-stage bass amplifier.

New developments of low-frequency amplifiers, the circuits of which are often cited on the pages of magazines of recent years, are aimed at achieving a minimum coefficient of non-linear distortion, increasing output power, expanding the bandwidth of amplified frequencies, etc.

At the same time, when setting up various devices and conducting experiments, a simple ULF is often needed, which can be assembled in a few minutes. Such an amplifier must contain a minimum number of deficient elements and operate in a wide range of supply voltage and load resistance.

ULF circuit on field-effect and silicon transistors

A diagram of a simple low-frequency power amplifier with a direct connection between the cascades is shown in fig. 6 [Rl 3/00-14]. The input impedance of the amplifier is determined by the value of the potentiometer R1 and can vary from hundreds of ohms to tens of megohms. The output of the amplifier can be connected to a load with a resistance of 2 ... 4 to 64 ohms and higher.

With a high-resistance load, the KT315 transistor can be used as VT2. The amplifier is operable in the supply voltage range from 3 to 15 V, although its acceptable performance is maintained even when the supply voltage is reduced to 0.6 V.

Capacitor C1 can be selected from 1 to 100 microfarads. In the latter case (C1 \u003d 100 μF), the ULF can operate in the frequency band from 50 Hz to 200 kHz and above.

Rice. 6. Scheme of a simple low-frequency amplifier on two transistors.

The amplitude of the ULF input signal should not exceed 0.5 ... 0.7 V. The output power of the amplifier can vary from tens of mW to units of W, depending on the load resistance and the magnitude of the supply voltage.

Setting up the amplifier consists in selecting resistors R2 and R3. With their help, the voltage at the drain of the transistor VT1 is set, equal to 50 ... 60% of the voltage of the power source. Transistor VT2 must be installed on a heat sink plate (radiator).

Track-cascade ULF with direct connection

On fig. 7 shows a diagram of another outwardly simple ULF with direct connections between the cascades. This kind of connection improves the frequency response of the amplifier in the low-frequency region, the circuit as a whole is simplified.

Rice. 7. Schematic diagram of a three-stage ULF with a direct connection between the stages.

At the same time, the tuning of the amplifier is complicated by the fact that each amplifier resistance has to be selected individually. Roughly, the ratio of resistors R2 and R3, R3 and R4, R4 and R BF should be within (30 ... 50) to 1. Resistor R1 should be 0.1 ... 2 kOhm. The calculation of the amplifier shown in fig. 7 can be found in the literature, eg [P 9/70-60].

Schemes of cascade ULF on bipolar transistors

On fig. 8 and 9 show cascode ULF circuits on bipolar transistors. Such amplifiers have a rather high gain Ku. The amplifier in fig. 8 has Ku=5 in the frequency band from 30 Hz to 120 kHz [MK 2/86-15]. ULF according to the scheme in Fig. 9 with a harmonic coefficient of less than 1% has a gain of 100 [RL 3/99-10].

Rice. 8. Cascade ULF on two transistors with gain = 5.

Rice. 9. Cascade ULF on two transistors with gain = 100.

Economical ULF on three transistors

For portable electronic equipment, an important parameter is the efficiency of VLF. The scheme of such a ULF is shown in fig. 10 [RL 3/00-14]. Here, a cascade connection of a field-effect transistor VT1 and a bipolar transistor VT3 is used, and the transistor VT2 is turned on in such a way that it stabilizes the operating point of VT1 and VT3.

With an increase in the input voltage, this transistor shunts the emitter-base VT3 junction and reduces the value of the current flowing through the transistors VT1 and VT3.

Rice. 10. Scheme of a simple economical low-frequency amplifier on three transistors.

As in the above circuit (see Fig. 6), the input impedance of this ULF can be set in the range from tens of ohms to tens of megohms. A telephone primer, for example, TK-67 or TM-2V, was used as a load. A telephone capsule connected with a plug can simultaneously serve as a power switch for the circuit.

The ULF supply voltage ranges from 1.5 to 15 V, although the device remains operational even when the supply voltage drops to 0.6 V. In the supply voltage range of 2 ... 15 V, the current consumed by the amplifier is described by the expression:

1(µA) = 52 + 13*(Upit)*(Upit),

where Upit is the supply voltage in Volts (V).

If you turn off the transistor VT2, the current consumed by the device increases by an order of magnitude.

Two-cascade ULF with direct connection between the cascades

Examples of ULF with direct connections and a minimum selection of the operating mode are the circuits shown in Fig. 11 - 14. They have high gain and good stability.

Rice. 11. A simple two-stage ULF for a microphone (low noise level, high gain).

Rice. 12. Two-stage low-frequency amplifier based on KT315 transistors.

Rice. 13. Two-stage low-frequency amplifier based on KT315 transistors - option 2.

The microphone amplifier (Fig. 11) is characterized by a low level of intrinsic noise and a high gain [MK 5/83-XIV]. An electrodynamic type microphone was used as the BM1 microphone.

A telephone capsule can also act as a microphone. Stabilization of the operating point (initial bias based on the input transistor) of the amplifiers in fig. 11 - 13 is carried out due to the voltage drop across the emitter resistance of the second amplification stage.

Rice. 14. Two-stage ULF with a field-effect transistor.

The amplifier (Fig. 14), which has a high input resistance (about 1 MΩ), is made on a field-effect transistor VT1 (source follower) and bipolar - VT2 (with a common one).

A cascade low-frequency field-effect transistor amplifier, which also has a high input impedance, is shown in fig. 15.

Rice. 15. diagram of a simple two-stage ULF on two field-effect transistors.

ULF circuits for working with low-ohm load

Typical ULF, designed to operate on a low-resistance load and having an output power of tens of mW or more, are shown in Fig. 16, 17.

Rice. 16. A simple ULF for working with a low-resistance load.

Electrodynamic head BA1 can be connected to the output of the amplifier, as shown in fig. 16, or in the diagonal of the bridge (Fig. 17). If the power source is made of two batteries (accumulators) connected in series, the output of the BA1 head, right according to the diagram, can be connected to their midpoint directly, without capacitors C3, C4.

Rice. 17. Low-frequency amplifier circuit with the inclusion of a low-resistance load in the diagonal of the bridge.

If you need a circuit for a simple tube ULF, then such an amplifier can be assembled even on a single lamp, see our electronics website in the appropriate section.

Literature: Shustov M.A. Practical Circuitry (Book 1), 2003.

Corrections in the post: in fig. 16 and 17 instead of the diode D9, a chain of diodes is installed.

The simplest transistor amplifier can be a good tool for studying the properties of devices. The schemes and designs are quite simple, you can independently manufacture the device and check its operation, measure all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature microphone amplifier literally from three elements. And connect it to a personal computer to improve the sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.

Frequency characteristics

Low-frequency (sound) frequency amplifiers are available in almost all household appliances - music centers, televisions, radios, radios, and even personal computers. But there are also high-frequency amplifiers on transistors, lamps and microcircuits. Their difference is that ULF allows you to amplify the signal of only the audio frequency, which is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.

Therefore, even the simplest device is able to amplify the signal in this range. And it does it as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of the dependence of these quantities is almost a straight line. If, on the other hand, a signal with a frequency outside the range is applied to the input of the amplifier, the quality of work and the efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, on transistors operating in the low and medium frequency ranges.

Classes of operation of audio amplifiers

All amplifying devices are divided into several classes, depending on what degree of current flow through the cascade during the period of operation:

Class "A" - the current flows non-stop during the entire period of operation of the amplifying stage.
In the class of work "B" current flows for half the period.
Class "AB" indicates that the current flows through the amplifying stage for a time equal to 50-100% of the period.
In "C" mode, the electric current flows for less than half of the operating time.
Mode "D" ULF has been used in amateur radio practice quite recently - a little over 50 years. In most cases, these devices are implemented on the basis of digital elements and have a very high efficiency - over 90%.

The presence of distortion in various classes of low-frequency amplifiers

The working area of a class "A" transistor amplifier is characterized by rather small non-linear distortions. If the incoming signal throws out higher voltage pulses, this causes the transistors to saturate. In the output signal, higher harmonics (up to 10 or 11) begin to appear near each harmonic. Because of this, a metallic sound, characteristic only for transistor amplifiers, appears.

With an unstable power supply, the output signal will be modeled in amplitude near the mains frequency. The sound will become harsher on the left side of the frequency response. But the better the power stabilization of the amplifier, the more complex the design of the entire device becomes. ULF operating in class "A" have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly on and current flows through it constantly.

To increase (albeit insignificant) efficiency, you can use push-pull circuits. One disadvantage is that the half-waves of the output signal become asymmetrical. If you transfer from class "A" to "AB", the non-linear distortion will increase by 3-4 times. But the efficiency of the entire circuit of the device will still increase. ULF classes "AB" and "B" characterizes the increase in distortion with a decrease in the signal level at the input. But even if you turn up the volume, it will not help to completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers "A +". In it, the transistors at the input (low-voltage) operate in the "A" mode. But high-voltage, installed in the output stages, work either in "B" or in "AB". Such amplifiers are much more economical than those operating in class "A". A noticeably smaller number of non-linear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The principle of operation of amplifiers on these elements will be discussed below.

But still there are a large number of higher harmonics in the output signal, which makes the sound characteristic metallic. There are also amplifier circuits that work in the "AA" class. In them, non-linear distortion is even less - up to 0.0005%. But the main drawback of transistor amplifiers is still there - a characteristic metallic sound.

"Alternative" designs

It cannot be said that they are alternative, just some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction are increasingly preferring tube designs. Tube amplifiers have the following advantages:

Very low level of non-linear distortion in the output signal.
There are fewer higher harmonics than in transistor designs.

But there is one huge minus that outweighs all the advantages - you must definitely install a device for coordination. The fact is that the tube cascade has a very high resistance - several thousand ohms. But the speaker winding resistance is 8 or 4 ohms. To match them, you need to install a transformer.

Of course, this is not a very big drawback - there are also transistor devices that use transformers to match the output stage and the speaker system. Some experts argue that the most effective circuit is hybrid - which uses single-ended amplifiers that are not covered by negative feedback. Moreover, all these cascades operate in the ULF class "A" mode. In other words, a transistorized power amplifier is used as a repeater.

Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not speak of the high quality of sound reproduction by the amplifier. Much more important are the linearity of the characteristics and their quality. Therefore, you need to pay attention first of all to them, and not to power.

Scheme of a single-ended ULF on a transistor

The simplest amplifier, built according to the common emitter circuit, operates in class "A". The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, which limits the flowing current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative. In the case of using semiconductor transistors with a p-n-p structure, the circuit will be exactly the same, only the polarity will need to be reversed.

With the help of a coupling capacitor C1, it is possible to separate the AC input signal from the DC source. In this case, the capacitor is not an obstacle to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction, together with resistors R1 and R2, is the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - the most typical values \u200b\u200bfor such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 Volts, you can see that the value of the current gain h21 will be 150. It should be noted that HF amplifiers on transistors are made according to similar circuits, only they work a little differently.

In this case, the emitter voltage is 9 V and the drop in the “E-B” circuit section is 0.7 V (which is typical for transistors based on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “E-B” section will be equal to 0.3 V. The current in the collector circuit will be equal to that which flows in the emitter. You can calculate by dividing the emitter voltage by the resistance R2 - 9V / 1 kOhm = 9 mA. To calculate the value of the base current, it is necessary to divide 9 mA by the gain h21 - 9mA / 150 \u003d 60 μA. ULF designs usually use bipolar transistors. The principle of its work is different from the field.

On the resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found by the formula - the sum of the characteristics of the emitter and the "E-B" transition. When powered by a 20 Volt source: 20 - 9.7 \u003d 10.3. From here, you can calculate the resistance value R1 = 10.3V / 60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary for the implementation of the circuit through which the alternating component of the emitter current can pass.

If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only when a bias voltage is applied to the output of the base of the transistor.

But it must be borne in mind that absolutely always, regardless of the presence of bias, the collector leakage current necessarily flows through the base circuit. In circuits with a common emitter, the leakage current is increased by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the "K-B" circuit is very small, this value is simply neglected.

MIS transistor amplifiers

The field-effect transistor amplifier shown in the diagram has many analogues. Including using bipolar transistors. Therefore, we can consider as a similar example the design of a sound amplifier assembled according to a common emitter circuit. The photo shows a circuit made according to a circuit with a common source. R-C connections are assembled on the input and output circuits so that the device operates in the class “A” amplifier mode.

Alternating current from the signal source is separated from the DC supply voltage by capacitor C1. Be sure the field-effect transistor amplifier must have a gate potential that will be lower than that of the source. In the presented diagram, the gate is connected to a common wire through a resistor R1. Its resistance is very large - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the signal at the input is not shunted.

This resistance almost does not pass electric current, as a result of which the potential of the gate (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential is higher than that of the ground, only due to the voltage drop across the resistance R2. From this it is clear that the potential of the gate is lower than that of the source. Namely, this is required for the normal functioning of the transistor. It should be noted that C2 and R3 in this amplifier circuit have the same purpose as in the design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.

ULF with output transformer

You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class "A". The design is the same as discussed above - with a common emitter. One feature - it is necessary to use a transformer for matching. This is a disadvantage of such a transistor audio amplifier.

The collector circuit of the transistor is loaded with a primary winding, which develops an output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. With the help of this circuit, a bias voltage is supplied to the base. All other components have the same purpose as the circuits discussed above.

push-pull audio amplifier

This is not to say that this is a simple transistor amplifier, since its operation is a little more complicated than that of those discussed earlier. In push-pull ULF, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex transformations can cause signal distortion, since the dynamic and frequency properties of two, even of the same type, transistors will be different.

As a result, the sound quality at the output of the amplifier is significantly reduced. When a push-pull amplifier in class "A" is working, it is not possible to reproduce a complex signal with high quality. The reason is that the increased current flows constantly through the arms of the amplifier, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.

Transformerless ULF

The low-frequency amplifier on a transistor, made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's best to get rid of them. A much more efficient circuit is made on complementary semiconductor elements with different types of conductivity. Most of the modern ULFs are performed exactly according to such schemes and work in class "B".

Two powerful transistors used in the design work according to the emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss and amplification. If there is no signal at the input, then the transistors are on the verge of turning on, but still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in cutoff mode at this time.

Therefore, only positive half-waves can pass through the load. But negative ones open the second transistor and completely block the first one. In this case, only negative half-waves are in the load. As a result, the signal amplified in power is at the output of the device. Such a transistor amplifier circuit is quite effective and is able to provide stable operation, high-quality sound reproduction.

ULF circuit on one transistor

Having studied all the above features, you can assemble an amplifier with your own hands on a simple element base. The transistor can be used domestically KT315 or any of its foreign analogues - for example BC107. As a load, you need to use headphones, the resistance of which is 2000-3000 ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 µF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, current - 0.3-0.5 A.

If the resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here, you can make a simple calculation of the transistor amplifier and find out the collector current - it turns out to be 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect the load - headphones to the output.

Touch the input of the amplifier with your finger - a characteristic noise should appear. If it is not there, then most likely the design is assembled incorrectly. Recheck all connections and element ratings. To make the demonstration clearer, connect a sound source to the ULF input - the output from the player or phone. Listen to music and appreciate the sound quality.

The transistor amplifier, despite its already long history, remains a favorite subject of study for both beginners and venerable radio amateurs. And this is understandable. It is an indispensable component of the most massive and low (sound) frequency amplifiers. We will look at how the simplest transistor amplifiers are built.

Amplifier frequency response

In any television or radio receiver, in every music center or sound amplifier, you can find transistor sound amplifiers (low frequency - LF). The difference between audio transistor amplifiers and other types lies in their frequency response.

The transistor audio amplifier has a uniform frequency response in the frequency band from 15 Hz to 20 kHz. This means that all input signals with a frequency within this range are converted (amplified) by the amplifier in approximately the same way. The figure below shows the ideal frequency response curve for an audio amplifier in terms of "amplifier gain Ku - input signal frequency".

This curve is almost flat from 15 Hz to 20 kHz. This means that such an amplifier should be used specifically for input signals with frequencies between 15 Hz and 20 kHz. For input signals with frequencies above 20 kHz or below 15 Hz, its efficiency and performance will rapidly decrease.

The type of frequency response of the amplifier is determined by the electrical radio elements (ERE) of its circuit, and above all by the transistors themselves. An audio amplifier based on transistors is usually assembled on the so-called low- and mid-frequency transistors with a total bandwidth of input signals from tens and hundreds of Hz to 30 kHz.

Amplifier class

As you know, depending on the degree of continuity of current flow throughout its period through the transistor amplifying stage (amplifier), the following classes of its operation are distinguished: "A", "B", "AB", "C", "D".

In the class of operation, current "A" flows through the stage for 100% of the period of the input signal. The operation of the cascade in this class is illustrated in the following figure.

In the class of operation of the amplifier stage "AB", the current flows through it for more than 50%, but less than 100% of the period of the input signal (see figure below).

In the class of operation of the "B" stage, the current flows through it exactly 50% of the period of the input signal, as illustrated in the figure.

And finally, in the class of operation of the "C" stage, the current through it flows less than 50% of the period of the input signal.

Low-frequency amplifier on transistors: distortion in the main classes of work

In the working area, a class "A" transistor amplifier has a low level of non-linear distortion. But if the signal has impulse surges in voltage, leading to saturation of the transistors, then higher harmonics (up to the 11th) appear around each “standard” harmonic of the output signal. This causes the phenomenon of the so-called transistorized or metallic sound.

If low-frequency power amplifiers on transistors have an unstabilized power supply, then their output signals are modulated in amplitude near the mains frequency. This leads to harshness of the sound at the left edge of the frequency response. Various methods of voltage stabilization make the design of the amplifier more complex.

The typical efficiency of a single-ended class A amplifier does not exceed 20% due to the always-on transistor and the continuous flow of the DC component. You can make a class A amplifier push-pull, the efficiency will increase slightly, but the half-waves of the signal will become more asymmetric. The transfer of the cascade from the work class "A" to the work class "AB" quadruples the nonlinear distortion, although the efficiency of its circuit increases.

In amplifiers of classes "AB" and "B", distortion increases as the signal level decreases. You involuntarily want to turn up such an amplifier louder to complete the sensations of the power and dynamics of the music, but often this does not help much.

Intermediate classes of work

The class of work "A" has a variety - the class "A +". In this case, the low-voltage input transistors of the amplifier of this class operate in class "A", and the high-voltage output transistors of the amplifier, when their input signals exceed a certain level, go into classes "B" or "AB". The efficiency of such cascades is better than in the pure class "A", and the non-linear distortion is less (up to 0.003%). However, their sound is also "metallic" due to the presence of higher harmonics in the output signal.

For amplifiers of another class - "AA" the degree of nonlinear distortion is even lower - about 0.0005%, but higher harmonics are also present.

A return to a class "A" transistor amplifier?

Today, many experts in the field of high-quality sound reproduction advocate a return to tube amplifiers, since the level of non-linear distortion and higher harmonics introduced by them into the output signal is obviously lower than that of transistors. However, these advantages are largely offset by the need for a matching transformer between the high-impedance tube output stage and the low-impedance speakers. However, a simple transistorized amplifier can also be made with a transformer output, as will be shown below.

There is also a point of view that only a hybrid tube-transistor amplifier can provide the ultimate sound quality, all stages of which are single-ended, not covered and work in class "A". That is, such a power follower is an amplifier on a single transistor. Its scheme can have the maximum achievable efficiency (in class "A") no more than 50%. But neither the power nor the efficiency of the amplifier are indicators of the quality of sound reproduction. In this case, the quality and linearity of the characteristics of all EREs in the circuit are of particular importance.

Since single-ended circuits are gaining this perspective, we will look at their options below.

single-ended amplifier with one transistor

Its circuit, made with a common emitter and R-C connections for input and output signals for operation in class "A", is shown in the figure below.

It shows the npn transistor Q1. Its collector is connected to the +Vcc positive terminal via a current-limiting resistor R3, and its emitter is connected to -Vcc. The p-n-p transistor amplifier will have the same circuit, but the power supply leads will be reversed.

C1 is a decoupling capacitor by which the AC input source is separated from the DC voltage source Vcc. At the same time, C1 does not prevent the passage of an alternating input current through the base-emitter junction of transistor Q1. Resistors R1 and R2, together with the resistance of the "E - B" junction, form Vcc to select the operating point of the transistor Q1 in static mode. Typical for this circuit is the value of R2 = 1 kOhm, and the position of the operating point is Vcc / 2. R3 is the load resistor of the collector circuit and serves to create a variable voltage output signal on the collector.

Assume that Vcc = 20 V, R2 = 1 kΩ, and the current gain h = 150. We select the emitter voltage Ve = 9 V, and the voltage drop at the E-B junction is Vbe = 0.7 V. This value corresponds to the so-called silicon transistor. If we were considering an amplifier based on germanium transistors, then the voltage drop across the open junction "E - B" would be equal to Vbe \u003d 0.3 V.

Emitter current, approximately equal to collector current

Ie = 9 V/1 kΩ = 9 mA ≈ Ic.

Base current Ib = Ic/h = 9 mA/150 = 60 µA.

Voltage drop across resistor R1

V(R1) = Vcc - Vb = Vcc - (Vbe + Ve) = 20V - 9.7V = 10.3V,

R1 \u003d V (R1) / Ib \u003d 10.3 V / 60 μA \u003d 172 kOhm.

C2 is needed to create a circuit for the passage of the variable component of the emitter current (actually the collector current). If it were not there, then the resistor R2 would severely limit the variable component, so that the bipolar transistor amplifier in question would have a low current gain.

In our calculations, we assumed that Ic = Ib h, where Ib is the base current flowing into it from the emitter and arising when a bias voltage is applied to the base. However, through the base always (both with and without bias) the leakage current from the collector Icb0 also flows. Therefore, the real collector current is Ic = Ib h + Icb0 h, i.e. the leakage current in the circuit with OE is amplified by 150 times. If we were considering an amplifier based on germanium transistors, then this circumstance would have to be taken into account in the calculations. The fact is that they have a significant Icb0 of the order of several μA. In silicon, it is three orders of magnitude smaller (about a few nA), so it is usually neglected in calculations.

Single ended amplifier with MIS transistor

Like any field-effect transistor amplifier, the circuit under consideration has its own analogue among amplifiers. Therefore, we will consider an analogue of the previous circuit with a common emitter. It is made with a common source and R-C connections for input and output signals for operation in class "A" and is shown in the figure below.

Here C1 is the same decoupling capacitor, by means of which the source of the alternating input signal is separated from the source of the constant voltage Vdd. As you know, any field-effect transistor amplifier must have the gate potential of its MIS transistors below the potentials of their sources. In this circuit, the gate is grounded by R1, which is typically high resistance (100 kΩ to 1 MΩ) so that it does not shunt the input signal. There is practically no current through R1, so the gate potential in the absence of an input signal is equal to the ground potential. The source potential is higher than the ground potential due to the voltage drop across the resistor R2. Thus, the gate potential is lower than the source potential, which is necessary for normal operation of Q1. Capacitor C2 and resistor R3 have the same purpose as in the previous circuit. Since this is a common-source circuit, the input and output signals are out of phase by 180°.

Amplifier with transformer output

The third single-stage simple transistor amplifier, shown in the figure below, is also made according to the common emitter circuit for operation in class "A", but it is connected to a low-impedance speaker through a matching transformer.

The primary winding of the transformer T1 is the load of the collector circuit of the transistor Q1 and develops the output signal. T1 sends the output signal to the speaker and ensures that the output impedance of the transistor matches the low (on the order of a few ohms) speaker impedance.

The voltage divider of the collector power supply Vcc, assembled on resistors R1 and R3, provides the choice of the operating point of the transistor Q1 (supplying a bias voltage to its base). The purpose of the remaining elements of the amplifier is the same as in the previous circuits.

Push-Pull Audio Amplifier

A two-transistor push-pull low-frequency amplifier splits the input frequency into two anti-phase half-waves, each of which is amplified by its own transistor stage. After such amplification, the half-waves are combined into a complete harmonic signal, which is transmitted to the speaker system. Such a conversion of the low-frequency signal (splitting and re-merging), of course, causes irreversible distortion in it, due to the difference in the frequency and dynamic properties of the two transistors of the circuit. These distortions reduce the sound quality at the output of the amplifier.

Push-pull amplifiers operating in class "A" do not reproduce complex audio signals well enough, since a constant current of increased magnitude continuously flows in their arms. This leads to asymmetry of the half-waves of the signal, phase distortions and, ultimately, to the loss of sound intelligibility. When heated, two powerful transistors double the signal distortion in the low and infra-low frequencies. But still, the main advantage of the push-pull circuit is its acceptable efficiency and increased output power.

A push-pull transistor power amplifier circuit is shown in the figure.

This is an amplifier for class "A", but class "AB" and even "B" can also be used.

Transformerless Transistor Power Amplifier

Transformers, despite the success in their miniaturization, are still the most bulky, heavy and expensive ERE. Therefore, a way was found to eliminate the transformer from the push-pull circuit by running it on two powerful complementary transistors of different types (n-p-n and p-n-p). Most modern power amplifiers use this principle and are designed to operate in class "B". A diagram of such a power amplifier is shown in the figure below.

Both of its transistors are connected according to a common collector (emitter follower) circuit. Therefore, the circuit transfers the input voltage to the output without amplification. If there is no input signal, then both transistors are on the border of the on state, but they are turned off.

When a harmonic signal is input, its positive half-wave opens TR1, but puts the p-n-p transistor TR2 in full cutoff mode. Thus, only the positive half-wave of the amplified current flows through the load. The negative half-wave of the input signal opens only TR2 and turns off TR1, so that the negative half-wave of amplified current is supplied to the load. As a result, a full power amplified (due to current amplification) sinusoidal signal is emitted at the load.

Single transistor amplifier

To assimilate the above, we will assemble a simple transistor amplifier with our own hands and figure out how it works.

As a load of a low-power transistor T of type BC107, we turn on headphones with a resistance of 2-3 kOhm, we apply the bias voltage to the base from a high-resistance resistor R* of 1 MΩ, which decouples an electrolytic capacitor C with a capacity of 10 μF to 100 μF, we include it in the base circuit T. Power the circuit we will be from a 4.5 V / 0.3 A battery.

If resistor R* is not connected, then there is neither base current Ib nor collector current Ic. If the resistor is connected, then the voltage at the base rises to 0.7 V and the current Ib = 4 μA flows through it. The current gain of the transistor is 250, which gives Ic = 250Ib = 1 mA.

Having assembled a simple transistor amplifier with our own hands, we can now test it. Connect the headphones and place your finger on point 1 of the diagram. You will hear a noise. Your body perceives the radiation of the mains at a frequency of 50 Hz. The noise you hear from the headphones is this radiation, only amplified by the transistor. Let us explain this process in more detail. An AC voltage of 50 Hz is connected to the base of the transistor through capacitor C. The voltage at the base is now equal to the sum of the DC bias voltage (approximately 0.7 V) coming from resistor R* and the AC finger voltage. As a result, the collector current receives an alternating component with a frequency of 50 Hz. This alternating current is used to move the diaphragm of the speakers back and forth at the same frequency, which means we can hear a 50Hz tone at the output.

Listening to the 50 Hz noise level is not very interesting, so you can connect low-frequency signal sources (CD player or microphone) to points 1 and 2 and hear amplified speech or music.