High fidelity umzc circuitry. News and analytical portal "electronics time" Circuit design of power amplifier output stages

Output stages based on "twos"

As a signal source we will use an alternating current generator with a tunable output resistance (from 100 Ohms to 10.1 kOhms) in steps of 2 kOhms (Fig. 3). Thus, when testing VCs at the maximum output resistance of the generator (10.1 kOhm), we will to some extent bring the operating mode of the tested VCs closer to a circuit with an open feedback loop, and in another (100 Ohm) - to a circuit with a closed feedback loop.

The main types of composite bipolar transistors (BTs) are shown in Fig. 4. Most often in VC, a composite Darlington transistor is used (Fig. 4a) based on two transistors of the same conductivity (Darlington “double”), less often - a composite Szyklai transistor (Fig. 4b) of two transistors of different conductivity with a current negative OS, and even less often - a composite Bryston transistor (Bryston, Fig. 4 c).
The "diamond" transistor, a type of Sziklai compound transistor, is shown in Fig. 4 g. Unlike the Szyklai transistor, in this transistor, thanks to the “current mirror”, the collector current of both transistors VT 2 and VT 3 is almost the same. Sometimes the Shiklai transistor is used with a transmission coefficient greater than 1 (Fig. 4 d). In this case, K P =1+ R 2/ R 1. Similar circuits can be obtained using field-effect transistors (FETs).

1.1. Output stages based on "twos". "Deuka" is a push-pull output stage with transistors connected according to a Darlington, Szyklai circuit or a combination of them (quasi-complementary stage, Bryston, etc.). A typical push-pull output stage based on a Darlington deuce is shown in Fig. 5. If emitter resistors R3, R4 (Fig. 10) of input transistors VT 1, VT 2 are connected to opposite power buses, then these transistors will operate without current cut-off, i.e. in class A mode.

Let's see what pairing the output transistors will give for the two "Darlingt she" (Fig. 13).

In Fig. Figure 15 shows a VK circuit used in one of the professional and onal amplifiers.

The Siklai scheme is less popular in VK (Fig. 18). At the early stages of the development of circuit design for transistor UMZCHs, quasi-complementary output stages were popular, when the upper arm was performed according to the Darlington circuit, and the lower one according to the Sziklai circuit. However, in the original version, the input impedance of the VC arms is asymmetrical, which leads to additional distortion. A modified version of such a VC with a Baxandall diode, for which the base-emitter junction of the VT 3 transistor is used, is shown in Fig. 20.

In addition to the considered “twos,” there is a modification of the Bryston VC, in which the input transistors control transistors of one conductivity with the emitter current, and the collector current controls transistors of a different conductivity (Fig. 22). A similar cascade can be implemented on field-effect transistors, for example, Lateral MOSFET (Fig. 24).

The hybrid output stage according to the Sziklai circuit with field-effect transistors as outputs is shown in Fig. 28. Let's consider the circuit of a parallel amplifier using field-effect transistors (Fig. 30).

As an effective way to increase and stabilize the input resistance of a “two”, it is proposed to use a buffer at its input, for example, an emitter follower with a current generator in the emitter circuit (Fig. 32).

Of the “twos” considered, the worst in terms of phase deviation and bandwidth was VK Siklai. Let's see what using a buffer can do for such a cascade. If instead of one buffer you use two on transistors of different conductivities connected in parallel (Fig. 35), then you can expect further improvement in parameters and an increase in input resistance. Of all the considered two-stage circuits, the Szyklai circuit with field-effect transistors showed itself to be the best in terms of nonlinear distortions. Let's see what installing a parallel buffer at its input will do (Fig. 37).

The parameters of the studied output stages are summarized in Table. 1.

Analysis of the table allows us to draw the following conclusions:
- any VC from the “twos” on the BT as a UN load is poorly suited for work in a high-fidelity UMZCH;
- the characteristics of a VC with a DC at the output depend little on the resistance of the signal source;
- a buffer stage at the input of any of the “twos” on the BT increases the input impedance, reduces the inductive component of the output, expands the bandwidth and makes the parameters independent of the output impedance of the signal source;
- VK Siklai with a DC output and a parallel buffer at the input (Fig. 37) has the highest characteristics (minimum distortion, maximum bandwidth, zero phase deviation in the audio range).

Output stages based on "triples"

In high-quality UMZCHs, three-stage structures are more often used: Darlington triplets, Shiklai with Darlington output transistors, Shiklai with Bryston output transistors and other combinations. One of the most popular output stages at present is a VC based on a composite Darlington transistor of three transistors (Fig. 39). In Fig. Figure 41 shows a VC with cascade branching: the input repeaters simultaneously operate on two stages, which, in turn, also operate on two stages each, and the third stage is connected to the common output. As a result, quad transistors operate at the output of such a VC.

The VC circuit, in which composite Darlington transistors are used as output transistors, is shown in Fig. 43. The parameters of the VC in Fig. 43 can be significantly improved if you include at its input a parallel buffer cascade that has proven itself well with “twos” (Fig. 44).

Variant of VK Siklai according to the diagram in Fig. 4 g using composite Bryston transistors is shown in Fig. 46. In Fig. Figure 48 shows a variant of the VC on Sziklai transistors (Fig. 4e) with a transmission coefficient of about 5, in which the input transistors operate in class A (thermostatization circuits are not shown).

In Fig. Figure 51 shows the VC according to the structure of the previous circuit with only a unit transmission coefficient. The review will be incomplete if we do not dwell on the output stage circuit with Hawksford nonlinearity correction, shown in Fig. 53. Transistors VT 5 and VT 6 are composite Darlington transistors.

Let's replace the output transistors with field-effect transistors of the Lateral type (Fig. 57

Anti-saturation circuits of output transistors contribute to increasing the reliability of amplifiers by eliminating through currents, which are especially dangerous when clipping high-frequency signals. Variants of such solutions are shown in Fig. 58. Through the upper diodes, excess base current is discharged into the collector of the transistor when approaching the saturation voltage. The saturation voltage of power transistors is usually in the range of 0.5...1.5 V, which approximately coincides with the voltage drop across the base-emitter junction. In the first option (Fig. 58 a), due to the additional diode in the base circuit, the emitter-collector voltage does not reach the saturation voltage by about 0.6 V (voltage drop across the diode). The second circuit (Fig. 58b) requires the selection of resistors R 1 and R 2. The lower diodes in the circuits are designed to quickly turn off the transistors during pulse signals. Similar solutions are used in power switches.

Often, to improve the quality, UMZCHs are equipped with separate power supply, increased by 10...15 V for the input stage and voltage amplifier and decreased for the output stage. In this case, in order to avoid failure of the output transistors and reduce the overload of the pre-output transistors, it is necessary to use protective diodes. Let's consider this option using the example of modification of the circuit in Fig. 39. If the input voltage increases above the supply voltage of the output transistors, additional diodes VD 1, VD 2 open (Fig. 59), and the excess base current of transistors VT 1, VT 2 is dumped onto the power buses of the final transistors. In this case, the input voltage is not allowed to increase above the supply levels for the output stage of the VC and the collector current of transistors VT 1, VT 2 is reduced.

Bias circuits

Previously, for the purpose of simplicity, instead of a bias circuit in the UMZCH, a separate voltage source was used. Many of the considered circuits, in particular, output stages with a parallel follower at the input, do not require bias circuits, which is their additional advantage. Now let's look at typical displacement schemes, which are shown in Fig. 60, 61.

Stable current generators. A number of standard circuits are widely used in modern UMZCHs: a differential cascade (DC), a current reflector ("current mirror"), a level shift circuit, a cascode (with serial and parallel power supply, the latter is also called a "broken cascode"), a stable generator current (GST), etc. Their correct use can significantly improve the technical characteristics of UMZCH. We will estimate the parameters of the main GTS circuits (Fig. 62 - 6 6) using modeling. We will assume that the GTS is a load of the UN and is connected in parallel with the VC. We study its properties using a technique similar to the study of VC.

Current reflectors

The considered GST circuits are a variant of a dynamic load for a single-cycle UN. In an UMZCH with one differential cascade (DC), to organize a counter dynamic load in the UN, they use the structure of a “current mirror” or, as it is also called, a “current reflector” (OT). This structure of the UMZCH was characteristic of the amplifiers of Holton, Hafler, and others. The main circuits of the current reflectors are shown in Fig. 67. They can be either with a unity transmission coefficient (more precisely, close to 1), or with a greater or lesser unit (scale current reflectors). In a voltage amplifier, the OT current is in the range of 3...20 mA: Therefore, we will test all OTs at a current of, for example, about 10 mA according to the diagram in Fig. 68.

The test results are given in table. 3.

As an example of a real amplifier, the S. BOCK power amplifier circuit, published in the journal Radiomir, 201 1, No. 1, p. 5 - 7; No. 2, p. 5 - 7 Radiotechnika No. 11, 12/06

The author's goal was to build a power amplifier suitable for both sounding "space" during festive events and for discos. Of course, I wanted it to fit in a relatively small-sized case and be easily transported. Another requirement for it is the easy availability of components. In an effort to achieve Hi-Fi quality, I chose a complementary-symmetrical output stage circuit. The maximum output power of the amplifier was set at 300 W (into a 4 ohm load). With this power, the output voltage is approximately 35 V. Therefore, the UMZCH requires a bipolar supply voltage within 2x60 V. The amplifier circuit is shown in Fig. 1. The UMZCH has an asymmetrical input. The input stage is formed by two differential amplifiers.

A. PETROV, Radiomir, 201 1, No. 4 - 12

Preface
Chapter 1. General information about UMZCH
Chapter 2. History, architecture and negative feedback
Chapter 3. Introduction to Power Amplifiers
Chapter 4: Small Signal Pre-Amp Stages
Chapter 5. Final Stage I
Chapter 6. Output stage II
Chapter 7. Correction, slew rate and stability
Chapter 8: Power Supplies and Power Supply Rejection Ratio (PSRR)
Chapter 9. Class A Power Amplifiers
Chapter 10. Class G Power Amplifiers
Chapter 11. Output stages based on field-effect transistors
Chapter 12. Thermal compensation and heat transfer dynamics
Chapter 13: Amplifier and Speaker Protection
Chapter 14. Grounding and some other practical issues
Chapter 15. Testing, safety requirements

Publisher: DMK Press
Year: 2011
Pages: 528
ISBN: 978-5-94074-702-4
Format: PDF
Language: Russian
Size: 13 MB
Download: Douglas S. Circuit design of modern amplifiers
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Circuit design of low-frequency power amplifiers

Low-frequency power amplifiers have as their main task amplification of a signal with a frequency from 10 Hz to 20,000 Hz. Such amplifiers are used both in industrial projects and devices, and in everyday life. Many electronics enthusiasts create ULFs on their own at home, using ready-made circuits. But drawing up a circuit diagram for such a device will be very difficult, since the circuitry of amplifiers is very specific and requires certain knowledge. The design features and operating principle are described in detail in the textbook by the authors S. A. Zavyalov and K. V. Murasov, entitled “Circuit design of low-frequency power amplifiers”
This manual is still relevant today, despite the fact that it was published back in 2010. It contains not only theoretical knowledge about the basic concepts of how amplifiers work, but also practical implementations, which will help consolidate all the information in your head and translate it into a real device.

Audio frequency amplification can be transformer and transformerless, single-cycle and push-pull or bridge. Previously, almost all circuits were transformer. One of them is presented below.

Rice. 1. Transformer circuit

The most significant drawback was the volume of the finished device and its weight. Naturally, the transformer needed to be wound independently, which not everyone could do. Therefore, transformerless circuits and circuits assembled with transistors began to gain great popularity.

The circuit of a simple ULF on transistors is significantly distinguished by its compactness and ease of assembly.

Rice. 2. Circuit of a simple ULF transistor

The entire circuit is powered from the “Krona” or a 9V constant voltage source.

The next circuit is also without a transformer, but with a large number of electronic components.

Rice. 3. Transformerless circuit

It is recognized by many radio amateurs and is quite easy to assemble. The output power is quite significant, starting from 100 W, which is already quite enough for a serious amplifier. By connecting such an amplifier in a bridge circuit, you can easily expect a power of up to 500 W. The input voltage is bipolar and is about 45 - 50 Volts. If you want to power such a circuit from the network, you will need to assemble an additional simple bipolar rectifier.

– The neighbor stopped knocking on the radiator. I turned the music up so I couldn't hear him.
(From audiophile folklore).

The epigraph is ironic, but the audiophile is not necessarily “sick in the head” with the face of Josh Ernest at a briefing on relations with the Russian Federation, who is “thrilled” because his neighbors are “happy.” Someone wants to listen to serious music at home as in the hall. For this purpose, the quality of the equipment is needed, which among lovers of decibel volume as such simply does not fit where sane people have a mind, but for the latter it goes beyond reason 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 - sound reproduction technology and electronics in general. Which in the age of digital technology are inextricably linked and can become a highly profitable and prestigious profession. The optimal first step in this matter in all respects is to make an amplifier with your own hands: It is UMZCH that allows, with initial training on the basis of school physics on the same table, to go from the simplest designs 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 highlight the first stages of this path for beginners and, perhaps, convey something new to those with experience.

Protozoa

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

Note: If you haven’t soldered electronics yet, keep in mind that its components cannot be overheated! Soldering iron - up to 40 W (preferably 25 W), maximum allowable soldering time without interruption - 10 s. The soldered pin for the heat sink is held 0.5-3 cm from the soldering point on the side of the device body with medical tweezers. Acid and other active fluxes cannot be used! Solder - POS-61.

On the left in Fig.- the simplest UMZCH, “which just works.” It can be assembled using both germanium and silicon transistors.

On this baby it is convenient to learn the basics of setting up an UMZCH with direct connections between cascades that give the clearest sound:

• Before turning on the power for the first time, turn off the load (speaker);
• Instead of R1, we solder a chain of a constant resistor of 33 kOhm and a variable resistor (potentiometer) of 270 kOhm, i.e. first note four times less, and the second approx. twice the denomination compared to the original according to the scheme;
• We supply power and, by rotating the potentiometer, at the point marked with a cross, we set the indicated collector current VT1;
• We remove the power, unsolder the temporary resistors and measure their total resistance;
• As R1 we set a resistor with a value from the standard series closest to the measured one;
• We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
• Same as according to paragraphs. 3-5, V. And we set the voltage equal to half the supply voltage.

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

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

In the center of the same fig.- a simple UMZCH on transistors, already developing 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 these class D amplifiers (see below) in cheap Chinese computer speakers, their sound improves noticeably. Here we learn another trick: powerful output transistors need to be placed on radiators. Components that require additional cooling are outlined in dotted lines in the diagrams; however, not always; sometimes - indicating the required dissipative area of ​​the heat sink. Setting up this UMZCH is balancing using 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 amplifier with 100 W transistors. You can listen to music through it, but not Hi-Fi, operating class is AB2. However, it is quite suitable for scoring a picnic area or an outdoor meeting, a school assembly hall or a small shopping hall. An amateur rock band, having such a UMZCH per instrument, can perform successfully.

There are 2 more tricks in this UMZCH: firstly, in very powerful amplifiers, the drive stage of the powerful output also needs to be cooled, so VT3 is placed on a radiator of 100 kW or more. see. For output VT4 and VT5 radiators from 400 sq.m. are needed. see. Secondly, UMZCHs with bipolar power supply are not balanced at all without load. First one or the other output transistor goes into cutoff, and the associated one goes into saturation. Then, at full supply voltage, current surges during balancing can damage the output transistors. Therefore, for balancing (R6, guessed it?), the amplifier is powered from +/–24 V, and instead of a load, a wirewound resistor of 100...200 Ohms is switched on. By the way, the squiggles in some resistors in the diagram are Roman numerals, indicating their required heat dissipation power.

Note: A power source for this UMZCH needs a power of 600 W or more. Anti-aliasing filter capacitors - from 6800 µF at 160 V. In parallel with the electrolytic capacitors of the IP, 0.01 µF ceramic capacitors are included to prevent self-excitation at ultrasonic frequencies, which can instantly burn out the output transistors.

On the field workers

On to the next 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 meets the requirements for entry-level Hi-Fi (if, of course, the UMZCH works on the corresponding acoustic systems, speakers). Powerful field drivers do not require a lot of power to drive, so there is no pre-power cascade. Even more powerful field-effect transistors do not burn out the speakers in the event of any malfunction - they themselves burn out faster. Also unpleasant, but still cheaper than replacing an expensive loudspeaker bass head (GB). This UMZCH does not require balancing or adjustment in general. As a design for beginners, it has only one drawback: powerful field-effect transistors are much more expensive than bipolar transistors for an amplifier with the same parameters. Requirements for individual entrepreneurs are similar to previous ones. case, but its power is needed from 450 W. Radiators – from 200 sq. cm.

Note: there is no need to build powerful UMZCHs on field-effect transistors for switching power supplies, for example. computer When trying to “drive” them into the active mode required for UMZCH, they either simply burn out, or the sound is weak and “no quality at all.” The same applies to powerful high-voltage bipolar transistors, for example. from line scan of old TVs.

Straight up

If you have already taken the first steps, then it is quite natural to want to build Hi-Fi class UMZCH, without going too deep into the theoretical jungle. To do this, you will have to expand your instrument fleet - you need an oscilloscope, an audio frequency generator (AFG) and an AC millivoltmeter with the ability to measure the DC component. It is better to take E. Gumeli’s UMZCH as a prototype for repetition, described in detail in “Radio” No. 1 for 1989. To build it you will need a few inexpensive available components, but the quality meets very high requirements: power up to 60 W, band 20-20,000 Hz, frequency response unevenness 2 dB, nonlinear 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 currently known circumstances greatly simplify the establishment of this UMZCH, see below. Bearing in mind this and the fact that not everyone is able to get into the Radio archives, it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

The Gumeli UMZCH circuits and specifications for them are shown in the illustration. Radiators of output transistors – from 250 sq. see for UMZCH in Fig. 1 and from 150 sq. see for option according to fig. 3 (original numbering). Transistors of the pre-output stage (KT814/KT815) are installed on radiators bent from 75x35 mm aluminum plates with a thickness of 3 mm. There is no need to replace KT814/KT815 with KT626/KT961; the sound does not noticeably improve, but setup becomes seriously difficult.

This UMZCH is very critical to power supply, installation topology and general, so it needs to be installed in a structurally complete form and only with a standard power source. When trying to power it from a stabilized power supply, the output transistors burn out immediately. Therefore, in Fig. Drawings of original printed circuit boards and setup instructions are provided. We can add to them that, firstly, if “excitement” is noticeable when you first turn it on, they fight it by changing the inductance L1. Secondly, the leads of parts installed on boards should be no longer than 10 mm. Thirdly, it is extremely undesirable to change the installation topology, but if it is really necessary, there must be a frame shield on the side of the conductors (ground loop, highlighted in color 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, for adjustment, after which they are sealed with drops of solder.

Setting up this UMZCH is greatly simplified, and the risk of encountering “excitement” during use is reduced to zero if:

• Minimize interconnect installation by placing the boards on heatsinks of powerful transistors.
• Completely abandon the connectors inside, performing all installation only by soldering. Then there will be no need for R12, R13 in a powerful version or R10 R11 in a less powerful version (they are dotted in the diagrams).
• Use oxygen-free copper audio wires of minimum length for internal installation.

If these conditions are met, there are no problems with excitation, and setting up the UMZCH comes down to the routine procedure described in Fig.

Wires for sound

Audio wires are not an idle invention. The need for their use at present is undeniable. In copper with an admixture of oxygen, a thin 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 (apparently due to quantum uncertainties) remains. Sufficient to be noticed by discerning listeners against the background of the purest sound of modern UMZCH.

Manufacturers and traders shamelessly substitute ordinary electrical copper instead of oxygen-free copper - it is impossible to distinguish one from the other by eye. However, there is an area of ​​application where counterfeiting is not clear: twisted pair cable for computer networks. If you put a grid with long segments on the left, it will either not start at all, or will constantly glitch. Momentum dispersion, you know.

The author, when there was just talk about audio wires, realized that, in principle, this was not idle chatter, especially since oxygen-free wires by that time had long been used in special-purpose equipment, with which he was well acquainted by his line of work. Then I took and replaced the standard cord of my TDS-7 headphones with a homemade one made from “vitukha” with flexible multi-core wires. The sound, aurally, has steadily improved for end-to-end analogue tracks, i.e. on the way from the studio microphone to the disc, never digitized. Vinyl recordings made using DMM (Direct Metal Mastering) technology sounded especially bright. After this, the interconnect installation of all home audio was converted to “vitushka”. Then completely random people, indifferent to the music and not notified in advance, began to notice the improvement in sound.

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

Video: do-it-yourself twisted pair interconnect wires

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

Theoretical Interlude

As we can see, already in the early stages of mastering audio technology, we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity sound reproduction. Hi-Fi comes in different levels, which are ranked according to the following. main parameters:

1. Reproducible frequency band.
2. Dynamic range - the ratio in decibels (dB) of the maximum (peak) output power to the noise level.
3. Self-noise level in dB.
4. Nonlinear distortion factor (THD) at rated (long-term) output power. The SOI at peak power is assumed to be 1% or 2% depending on the measurement technique.
5. Unevenness of the amplitude-frequency response (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) sound frequencies.

Note: the ratio of absolute levels of any values ​​of I in (dB) is defined as P(dB) = 20log(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 homemade Hi-Fi UMZCH for the home, before moving on to these, you need to clearly understand the requirements for their power required to sound a given room, dynamic range (dynamics), noise level and SOI. It is not very difficult to achieve a frequency band of 20-20,000 Hz from the UMZCH with a roll off at the edges of 3 dB and an uneven frequency response in the midrange of 2 dB on a modern element base.

Volume

The power of the UMZCH is not an end in itself; it must provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. There are no natural noises in residential areas quieter than 20 dB; 20 dB is the wilderness in complete calm. A volume level of 20 dB relative to the threshold of audibility is the threshold of intelligibility - a whisper can still be heard, but music is perceived only as the fact of its presence. An experienced musician can tell which instrument is being played, but not what exactly.

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

90 dB is the volume level of a symphony orchestra in a very good concert hall. 110 dB can be produced by an extended 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 still perceived as distinguishable in meaning with an effort of will, but already annoying noise. The volume zone in residential premises of 20-110 dB constitutes the zone of complete audibility, and 40-90 dB is the zone of best audibility, in which untrained and inexperienced listeners fully perceive the meaning of the sound. If, of course, he is in it.

Power

Calculating the power of equipment at 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) speaker. In this case, the UMZCH will not noticeably add its distortions to those of the speakers; they are already the main source of nonlinearity in the sound path. But the UMZCH should not be made too powerful: in this case, the level of its own noise may be higher than the threshold of audibility, because It is calculated based on the voltage level of the output signal at maximum power. If we consider it very simply, then for a room in an ordinary apartment or house and speakers with normal characteristic sensitivity (sound output) we can take the 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 W.
• 70-100 sq. m – 100-150 W.
• 100-120 sq. m – 150-200 W.
• More than 120 sq. m – determined by calculation based on on-site acoustic measurements.

Dynamics

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

1. Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. No expert can distinguish a sound with a dynamics of 80-85 dB in a city apartment from ideal.
2. Other serious music genres – 75 dB excellent, 80 dB “through the roof”.
3. Pop music of any kind and movie soundtracks - 66 dB is enough for the eyes, because... These opuses are already compressed during recording to levels of up to 66 dB and even up to 40 dB, so that you can listen to them on anything.

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

SOI

Nonlinear distortions (ND) of UMZCH are components of the output signal spectrum that were not present in the input signal. 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. At 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 find it difficult to assess the timbre of the sound. In people without an ear for music, the masking effect is observed already at 45-40 dB of volume. Therefore, an UMZCH with a THD of 0.1% (–60 dB from a volume level of 110 dB) will be assessed as Hi-Fi by the average listener, and with a THD of 0.01% (–80 dB) can be considered not distorting the sound.

Lamps

The last statement will probably cause rejection, even fury, among adherents of tube circuitry: they say, real sound is produced only by tubes, and not just some, but certain types of octal ones. Calm down, gentlemen - the special tube sound is not a fiction. The reason is the fundamentally different distortion spectra of electronic tubes and transistors. Which, in turn, are due to the fact that in the lamp the flow of electrons moves in a vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minority charge carriers (electrons and holes) move in the crystal, which is completely impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly visible in it, and there are very few combinational components (sums and differences in the frequencies of the input signal and their harmonics). Therefore, in the days of vacuum circuitry, SOI was called harmonic distortion (CHD). In transistors, the spectrum of distortions (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, designers of transistor UMZCHs used the usual “tube” SOI of 1-2% for them; Sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as pure. By the way, the very concept of Hi-Fi did not yet exist. It turned out that they sound dull and dull. In the process of developing transistor technology, an understanding of what Hi-Fi is and what is needed for it was developed.

Currently, the growing pains of transistor technology have been successfully overcome and side frequencies at the output of a good UMZCH are difficult to detect using special measurement methods. And lamp circuitry can be considered to have become an art. Its basis can be anything, why can’t electronics go there? An analogy with photography would be appropriate here. No one can deny that a modern digital SLR camera produces an image that is immeasurably clearer, more detailed, and deeper in the range of brightness and color than a plywood box with an accordion. But someone, with the coolest Nikon, “clicks pictures” like “this is my fat cat, he got drunk like a bastard and is sleeping with his paws outstretched,” and someone, using Smena-8M, uses Svemov’s b/w film to take a picture in front of which there is a crowd of people at a prestigious exhibition.

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

Experimental stand

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

But enough of the lyrics, let's get to the point. For those who like to tinker with the lamps in Fig. – diagram of a bench lamp UMZCH, intended specifically for experiments: SA1 switches the operating mode of the output lamp, and SA2 switches the supply voltage. The circuit is well known in the Russian Federation, a minor modification affected only the output transformer: now you can not only “drive” the native 6P7S in different modes, but also select the screen grid switching factor 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. For the manufacture of the output transformer, see below.

Guitarists and rockers

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

An unpleasant circumstance soon emerged: the sound of an electric guitar with a fuser acquires full strength and brightness only at high volumes. This is especially true for guitars with a humbucker-type pickup, which gives the most “angry” sound. But what about a beginner who is forced to rehearse at home? You can’t go to the hall to perform without knowing exactly how the instrument will sound there. And rock fans just 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 shocking surroundings.

So, it turned out that the fatal sound appears at volume levels acceptable for residential premises, if the UMZCH is tube-based. The reason is the specific interaction of the signal spectrum from the fuser with the pure 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 outline and light for viewing.

Those who need a tube amplifier not for experiments, but due to technical necessity, do not have time to master the intricacies of tube electronics for a long time, they are passionate about something else. In this case, it is better to make the UMZCH transformerless. More precisely, with a single-ended matching output transformer that operates without constant magnetization. This approach greatly simplifies and speeds up the production of the most complex and critical component of a lamp UMZCH.

“Transformerless” tube output stage of the UMZCH and pre-amplifiers 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 pre-amplifier options for it. At the top - with a tone control according to the classic Baxandal scheme, which provides fairly deep adjustment, but introduces slight phase distortion into the signal, which can be significant when operating an UMZCH on a 2-way speaker. Below is a preamplifier with simpler tone control that does not distort the signal.

But let's get back to the end. In a number of foreign sources, this scheme is considered a revelation, but an identical one, with the exception of the capacitance of the electrolytic capacitors, is found in the Soviet “Radio Amateur Handbook” of 1966. A thick book of 1060 pages. There was no Internet and disk-based databases back then.

In the same place, on the right in the figure, the disadvantages of this scheme are briefly but clearly described. An improved one, from the same source, is given on the trail. rice. 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 is powered through the load. If, instead of high-impedance speakers, you turn on a matching transformer with regular speakers, as in the previous one. 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 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 leakage 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,” hums, or beeps. Foucault currents are combated by reducing the thickness of the magnetic circuit plates and additionally insulating them with varnish during assembly. For output transformers, the optimal plate thickness is 0.15 mm, the maximum allowable is 0.25 mm. You should not take thinner plates for the output transformer: the fill factor of the core (the central rod 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 distortions and losses in it.

In the core of an audio transformer operating with permanent magnetization (for example, the 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 magnetization; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a core with a larger cross-section. 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 (cut), pos. 1 in Fig. In them, a non-magnetic gap is formed during core cutting and is therefore 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 is closed are solid. Transformer cores without bias are often assembled from Shp plates, because Shp plates are made from high-quality transformer steel. In this case, the core is assembled across the roof (the plates are laid with a cut in one direction or the other), and its cross-section is increased by 10% compared to the calculated one.

It is better to wind transformers without bias on USH cores (reduced height with widened windows), pos. 2. In them, a decrease in 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 made from them. Then the core assembly is carried out cut to pieces: a package of W-plates is assembled, a strip of non-conducting non-magnetic material is placed with a thickness equal to the size of the non-magnetic gap, covered with a yoke from a package of jumpers and pulled together with a clip.

Note:“sound” signal magnetic circuits of the ShLM type are of little use for output transformers of high-quality tube amplifiers; they have a large stray field.

At pos. 3 shows a diagram of the core dimensions for calculating the transformer, at pos. 4 design of the winding frame, and at pos. 5 – patterns of its parts. As for the transformer for the “transformerless” output stage, it is better to make it on the ShLMm across the roof, because the bias is negligible (the bias current is equal to the screen grid current). 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 be much less than 800 Ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings are wound turn to turn (if there is no winding machine, this is terrible) from the thinnest possible wire; the laying coefficient of the anode winding for the mechanical calculation of the transformer is taken 0.6. The winding wire is PETV or PEMM, they have an oxygen-free core. There is no need to take PETV-2 or PEMM-2; due to double varnishing, they have an increased outer diameter and a larger scattering field. The primary winding is wound first, because it is its scattering field that most affects the sound.

You need to look for iron for this transformer with holes in the corners of the plates and clamping brackets (see figure on the right), because “for complete happiness,” the magnetic circuit is assembled as follows. order (of course, the windings with leads and external insulation should already be on the frame):

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

The core assembled using this technology has very good plate insulation and steel filling. Magnetostriction losses are not detected at all. But keep in mind that this technique is not applicable for permalloy cores, because Under strong mechanical influences, the magnetic properties of permalloy irreversibly deteriorate!

On microcircuits

UMZCHs on integrated circuits (ICs) are most often made by those who are satisfied with the sound quality up to average Hi-Fi, but are more attracted by the low cost, speed, ease of assembly and the complete absence of any setup procedures that require special knowledge. Simply, an amplifier on microcircuits is the best option for dummies. The classic of the genre here is the UMZCH on the TDA2004 IC, which has been on the series, God willing, for about 20 years now, 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 with a very low-resistance, up to 1.6 Ohm, load, which allows you to extract full power when powered from a 12 V on-board network, and 7-8 W when supplied with a 6-volt power supply, for example, on a motorcycle. However, the output of the TDA2004 in class B is not complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 does not produce better sound, but is more powerful, up to 25 W, because The upper limit of the supply voltage has been increased to 25 V. The lower limit, 4.5 V, still allows it to be powered from a 6 V on-board network, i.e. The TDA7261 can be started from almost all on-board networks, except for the aircraft 27 V. Using attached components (strapping, on the right in the figure), the TDA7261 can operate in mutation mode and with the St-By (Stand By) function, which switches the UMZCH to the minimum power consumption mode when there is no input signal for a certain time. Convenience costs money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: If you are somehow 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 supply TDA7482, on the left in the figure, operating in the so-called. class D. Such UMZCHs are sometimes called digital amplifiers, which is incorrect. For real digitization, level samples are taken from an analog signal with a quantization frequency no less than twice the highest of the reproduced frequencies, the value of each sample is recorded in a noise-resistant code and stored for further use. UMZCH class D – pulse. In them, the analogue is directly converted into a sequence of high-frequency pulse-width modulated (PWM), which is fed to the speaker through a low-pass filter (LPF).

Class D sound has nothing in common with Hi-Fi: SOI of 2% and dynamics of 55 dB for class D UMZCH are considered very good indicators. And TDA7482 here, it must be said, is not the optimal choice: other companies specializing in class D produce UMZCH ICs that are cheaper and require less wiring, for example, D-UMZCH of the Paxx series, on the right in Fig.

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

UMZCH for subwoofer

A subwoofer, which can be translated as “subwoofer” or, literally, “boomer,” reproduces frequencies up to 150-200 Hz; in this range, human ears are practically unable to determine the direction of the sound source. In speakers with a subwoofer, the “sub-bass” speaker is placed in a separate acoustic design; this is the subwoofer as such. The subwoofer is placed, in principle, as conveniently as possible, 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. Experts agree that it is 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 among consumers with normal hearing and not particularly demanding ones.

The “leakage” of mid-high frequencies into the subwoofer, and from it into the air, greatly spoils the stereo, but if you sharply “cut off” the sub-bass, which, by the way, is very difficult and expensive, then a very unpleasant sound jumping effect will occur. Therefore, channels in subwoofer systems are filtered twice. At the input, electric filters highlight midrange-high frequencies with bass “tails” that do not overload the midrange-high frequency 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 additionally filtered so that the stereo does not deteriorate; in the subwoofer it is already acoustic: a sub-bass speaker 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 UMZCH for a subwoofer is subject to a number of specific requirements, of which “dummies” consider the most important to be as high a power as possible. This is completely wrong, if, say, the calculation of the acoustics for the room gave a peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if S-30 speakers are suitable for the room, then a subwoofer needs 1.6x30 = 48 W.

It is much more important to ensure the absence of phase and transient distortions: if they occur, there will definitely be a jump in the sound. As for SOI, it is permissible up to 1%. Intrinsic bass distortion of this level is not audible (see curves of equal volume), and the “tails” of their spectrum in the best audible midrange region will not come out of the subwoofer.

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 UMZCHs are switched on back-to-back through a speaker; signals to the inputs are supplied in antiphase. The absence of phase and transient distortions in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers forming the arms of the bridge is ensured by the use of paired UMZCHs 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 a 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 circuits R5C3 and R’5C’3. Radiator area TDA2030 – from 400 sq. see. Bridged UMZCHs with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current, which can damage the speaker, and the sub-bass protection circuits often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive oak bass head with non-polar batteries of electrolytic capacitors (highlighted in color, and the diagram of one battery is given in the inset.

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 fairly 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 chamber. For a specific loudspeaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, the least impact on the stereo effect. To tune the pipes, they take a pipe that is obviously longer and, by pushing it in and out, achieve the required sound. The protrusions of the pipes outward do not affect the sound; they are then cut off. The pipe settings are interdependent, so you will have to tinker.

Headphone amplifier

A headphone amplifier is most often made by hand for two 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 drive “buttons” or “burdocks”. The second is for high-end home headphones. A Hi-Fi UMZCH for an ordinary living room is needed with dynamics of 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 W per channel, which is too much for an ordinary apartment: listening at a power that is much lower than the rated power 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 UMZCHs with such an additional weight are clearly absurdly inflated.

The circuit of the simplest headphone amplifier using transistors is given in pos. 1 pic. The sound is only for Chinese “buttons”, it works in class B. It is also no different in terms of efficiency - 13 mm lithium batteries last for 3-4 hours at full volume. At pos. 2 – TDA’s classic for on-the-go headphones. The sound, however, is quite decent, up to average Hi-Fi depending on the track digitization parameters. There are countless amateur improvements to the TDA7050 harness, but no one has yet achieved the transition of sound to the next level of class: the “microphone” itself does not allow it. TDA7057 (item 3) is simply more functional; you can connect the volume control to a regular, not dual, potentiometer.

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

Power amplifiers (PA) are designed to transmit high signal powers without distortion to a low-impedance load. They are usually the output stages of multistage amplifiers. The main task of the PA is to allocate as much signal power as possible to the load; voltage amplification in it is a secondary factor.

The main tasks when designing a PA are:

◆ providing a mode for matching the output resistance of the PA with the load in order to transfer maximum power to the load;

◆ achieving minimal nonlinear signal distortion;

◆ obtaining maximum efficiency.

UMs are classified by:

◆ amplification method - single-cycle and push-pull;

◆ matching method - transformer and transformerless;

◆ amplification class - classes A, B, AB, C, D.

The following design methods can be used:

◆ graphic-analytical (construction of DH, etc.);

◆ based on average parameters.

For all previously considered amplification stages it was assumed. That they operate in class A mode. The choice of the resting operating point, for example for BT, (see Figure 2.10) is made in such a way that the input signal is completely placed on the linear section of the input current-voltage characteristic of the transistor, and the value I b 0 was located in the middle of this linear section. On the output current-voltage characteristic of the transistor in class A mode, the operating point ( I to 0, U To 0) is located in the middle of the load line so that the amplitude values ​​of the signals do not go beyond those limits of the load line where changes in the collector current are directly proportional to changes in the base current. Since mode A is characterized by the operation of transistors in almost linear sections of their current-voltage characteristics, the PA in this mode will have minimal NI (usually K G≤1%).

When operating in class A mode, the transistor is in the open state all the time, therefore, the cutoff angle (half the time during the period during which the transistor is open) φ ost=180°. The power consumption of the power supply occurs at any moment, therefore cascades operating in class A mode are characterized by low efficiency (ideally - 50%, in reality - (35...45)%). Class A amplification mode in PAs is used in cases where minimal NI is required, and power and efficiency are not critical.

More powerful versions of the output stages operate in class B mode, characterized by φ ost=90° (Figure 4.1).

Figure 4.1. Class B mode

In rest mode, the transistor is closed and does not consume power from the power source, but opens only during half the period of the input signal. Relatively low power consumption makes it possible to obtain an efficiency value of up to 70% in class B PAs. Class B mode is usually used in push-pull PAs. The main disadvantage of class B minds is the high level of NI ( K G≤10%).

Class AB mode occupies an intermediate value between class A and B modes and is used in push-pull PAs. In quiescent mode, a small quiescent current flows through the transistor I to 0 (Figure 4.2), which outputs the main part of the working half-wave of the input harmonic signal to a section of the current-voltage characteristic with a relatively low nonlinearity.

Figure 4.2. Class AB mode

The cutoff angle in class AB mode reaches (120...130)°, efficiency and NI are average between the values ​​for class A and B modes.

In class C mode, the transistor is biased U cm(Figure 4.3), φ ost=90°, therefore class C PAs are more economical than class B PAs.

Figure 4.3. Class C mode

However, in class C mode the NI is large, so class C is used mainly in generators and resonant amplifiers, where higher harmonic components are filtered out by a resonant circuit in the load circuit.

In powerful amplifiers-converters, class D mode or the key mode of operation of amplifying elements is used. This mode, in combination with pulse-width modulation, allows powerful, economical PAs, incl. and for sound broadcast systems.

Thus, the active element in the PA can operate both without current cutoff (class A) and with cutoff (classes AB, B, C, D). The gain class is determined by the position of the operating point in rest mode.

As single-cycle transformerless PAs The already discussed cascades with OE (OI) and OK (OS), made on powerful BT or PT, can be used, and the emitter (source) follower is effective with a low-resistance (on the order of units of Ohms) load. The main disadvantage of such cascades is that in the mode of matching with the load, the efficiency is ≤25%.

Single-cycle transformer PAs have an efficiency of ≤50% due to optimal matching with the load using a transformer (Figure 4.4).

Figure 4.4. Single-cycle transformer PA

The AC load resistance is:

R n ≈ ≈ R n· n²,

where n is the transformation ratio, n=U 1 /U 2 .

This cascade finds limited use in modern PA circuitry due to a number of significant disadvantages:

◆ low efficiency;

◆ large frequency distortions due to the transformer;

◆ large NI due to the transformer bias current;

◆ impossibility of implementation in the form of an IC.

Transformer amplifiers are described in detail in classic textbooks on amplifiers, for example, in.

Push-pull PAs, due to the possibility of using modes AB, B, C and D, are characterized by better energy performance. Figure 4.5 shows the diagram push-pull PA with transformer coupling .

Figure 4.5. Push-pull transformer PA

When this PA operates in class B mode, there is no resistor circuit R b2. Transformer Tp 1 matches the PA input with the signal source, transformer Tp 2 matches the output resistance of the PA with the load resistance. Transformer Tp 1 also performs the functions of a phase inverter (see Figure 4.5 for the phasing of its windings).

The signal amplification in the PA under consideration occurs in two cycles of operation of the device. The first cycle is accompanied by amplification of the positive half-wave of the harmonic signal using transistor VT 2, the second by amplification of the negative half-wave of the harmonic signal using VT 1.

Graphic and energy calculations of a push-pull push-pull transformer PA are quite fully presented in classic textbooks on amplification devices, for example. Energy calculations show that the efficiency of such a PA actually reaches about 70%, which is about 1.5 times more than that of single-cycle PAs.

When choosing a type for a PA, you should take into account the fact that a voltage of approximately 2· is applied to the collector of a closed transistor E to, which is explained by the summation E to and voltage on the primary winding section Tp 2.

Due to the fact that each transistor passes current for only one half-wave of the harmonic signal, class B mode is characterized by better current utilization of the transistor.

As noted above, the absence of quiescent current in a class B PA leads to the appearance of significant NI. Due to the nonlinearity of the input current-voltage characteristics, the output signal in a push-pull class B PA has transient distortions of the “step” type (Figure 4.6).

Figure 4.6. Signal distortion in a push-pull transformer PA

Reducing NI is possible by switching to class AB mode (see Figures 4.2 and 4.6). Because Since the quiescent currents in class AB mode are small, they have virtually no effect on the energy performance of the PA.

Since the transformer is a very “inconvenient” element when making a PA in the form of an IC and introduces significant distortions into the output signal of the amplifier, PAs with transformers find limited use in modern PA circuitry.

The most widely used in modern electronics are transformerless push-pull PA . Such PAs have good weight and size parameters and are simply implemented in the form of an IC.

It is possible to construct push-pull transformerless PAs according to the block diagram shown in Figure 4.7.

Figure 4.7. Block diagram of the mind using FI

Here FI is a phase-inverted pre-amplification stage (driver), UM is a push-pull power amplification stage.

Can be used as a driver split cascade load (Figure 4.8).

Figure 4.8. Load split cascade

It can be shown that when , .

Despite such advantages as simplicity and low frequency and nonlinear distortions, a split-load cascade is of limited use due to its low K 0 and different R out, which leads to an asymmetrical frequency response of the outputs in the HF and LF regions.

Much more often used PI based on differential cascade (DK) (Figure 4.9).

Figure 4.9. Phase inversion cascade based on DC

DC will be considered further, but for now we note that through R e double the quiescent current of transistors VT1 and VT2 will flow and, therefore, the value of the resistor R e in a phase-inverted cascade circuit is halved compared to the calculation of a cascade with an OE.

When examining, for example, the left half of the phase-inverted cascade, it is clear that in the emitter circuit of transistor VT1 (connected with the OE) there is R e and parallel to it the input resistance of transistor VT2 (connected with OB), R inb≈1/S 0 .

Usually they take R e>>R inb(or replace R e equivalent of a high-resistance resistance in the form of a stable current source, which will be considered later together with DC), so you can substitute R os into the expression for the depth of POOST (see subsection 3.2) R inb:

A = 1 + S 0 · R inb ≈ 1 + S 0 /S 0 = 2

Therefore, we can assume that the phase-inverted cascade contains a POOST with a depth of two. Taking into account that, relative to the emitter VT2, the transistor VT1 is connected according to the OK circuit, it is easy to show that if the parameters of the transistors are identical K 01 ≈K 02 ≈K 0 /2, i.e. The voltage transfer coefficients of the arms of a phase-inverted cascade based on a DC are equal to half the transmission coefficient of a cascade with an OE.

FI on complementary transistors is quite widely used, a circuit version of which is presented in Figure 4.10.

Figure 4.10. FI on complimentary BT

The use of a complementary pair of transistors VT1 and VT2, which have different conductivities, but the same parameters (for example, KT315-KT361, KT502-KT503, KT814-KT815, etc.) allows you to invert the phase of the input signal by 180° at the first output.

In addition to the cascades discussed above, cascades with OE are also used as phase inversion cascades, connected according to the block diagram shown in Figure 4.11. Note that the FI built according to this scheme has an imbalance in the frequency response and phase response of the outputs.

Figure 4.11. FI based on cascades with OE

As a PA output stage connected to the PI outputs, a cascade can be used, one of the varieties of which is shown in Figure 4.12.

Figure 4.12. Output stage PA with FI

In this cascade it is possible to use modes of classes B, AB, C. The advantages of the cascade include the possibility of using powerful transistors of the same conductivity type. When using a bipolar power supply, it is possible to directly connect the load, which makes it possible to do without an output coupling capacitor, which usually has a large capacitance and dimensions and, therefore, is difficult to implement in micro-design.

In general, in PAs made according to the structural diagram presented in Figure 4.7, high efficiency is not achievable due to the need to use class A mode in the FI.

Push-pull transformerless PAs made with complementary transistors have much better parameters. Such minds are usually called boosters . There are voltage and current boosters. Since voltage amplification is usually carried out by the preliminary stages of a multistage amplifier, and the PA load is usually low-impedance, output stages in the form of a current booster are most widespread.

Figure 4.13 shows a diagram of the simplest version of a class B current booster using complementary transistors and bipolar power supply.

Figure 4.13. Class B current booster

When a positive half-wave of the input harmonic signal is applied to the booster input, transistor VT1 opens and current flows through the load. When a negative half-wave of the input harmonic signal is applied to the booster input, transistor VT2 opens and current flows through the load in the opposite direction. Thus, on Rн an output signal will be generated.

The inclusion of transistors with OK allows you to obtain a low output resistance, which is necessary for matching with a low-resistance load in order to transfer maximum output power to it. The large input impedance allows the cascade to be well matched to the voltage preamplifier. Due to 100% POOSN K 0 ≈1.

Thanks to the use of a bipolar power supply, galvanic coupling of the cascade with the load is possible, which makes it possible to use current boosters in DC amplifiers. In addition, this circumstance is very favorable when implementing a booster in the form of an IC.

A significant disadvantage of the booster under consideration is the large NI ( K G>10%), which limits its practical use. Free from this drawback is the AB class current booster, the diagram of which is shown in Figure 4.14.

Figure 4.14. Class AB current booster

The initial quiescent currents of the transistor bases are set here using resistors R b1 and R b2, as well as diodes VD 1 and VD 2. In the integrated version, diode-connected transistors are used as diodes. Let us recall that the voltage drop across a forward-biased diode is Δφ≈0.7 V, and in silicon ICs parametric thermal stabilization is carried out using diodes (see subsection 2.6). Resistance R acc. is introduced for better matching with the previous amplifier stage.

With a positive half-wave of the input harmonic signal, the diode VD 1 is switched off and on the basis of VT 1 the input potential will be monitored, which will lead to its unlocking and the formation of a positive half-wave of the output harmonic signal at the load resistance. With a negative half-wave of the input harmonic signal, VD 2 and VT 2 operate, and a negative half-wave of the output harmonic signal is formed at the load.

To increase the output power, boosters can be used on composite transistors connected according to the Darlington circuit (Figure 4.15), in which the current transfer coefficient is equal to the product of the current transfer coefficients of the base of transistors VT 1 and VT 2, and a single-chip implementation of this structure is possible, for example, a composite transistor KT829.

Figure 4.15. Darlington circuit

Of the field-effect transistors in the PA, the most suitable are MOS transistors with induced n- and p-type channels, which have the same bias character in the gate-source circuit as bipolar ones, but have a more linear input current-voltage characteristic, leading to a lower level of current-voltage characteristic. The circuit diagram of a PA based on a PT of the specified type is shown in Figure 4.16.

Figure 4.16. UM on PT

In this cascade, positive feedback on power supply is introduced by connecting resistor Rst in series with Rc. To the point a the output voltage is supplied through a capacitor and serves as a “voltage booster” that increases the supply voltage of the pre-final stage during the half-cycle during which the current of transistor VT 1 decreases. This allows you to remove from it a sufficient voltage amplitude necessary to control the final source follower, and increases the output power and efficiency of the amplifier. A similar “voltage boost” circuit is used in the PA on the BT.

PAs that use operational amplifiers as preliminary stages are widely used. Figures 4.17a, b show the corresponding PA circuits of class B and AB modes.

Figure 4.17. PA based on operational amplifiers

These examples illustrate another direction in the development of UM - the use of general environmental protection, which serves, in particular, to reduce the level of NI.

A more detailed description of PA circuits is contained in.