Power amplification (PA) cascades. What is an output transistor? Quiescent current and cascaded amplifiers Low frequency amplifier output stage

In automation devices, the load of the output stage of a low-frequency amplifier can be an electromagnetic relay, an electric motor or some other actuator. In a radio or record player, the load is the speaker winding.

The output stage is the same as the prestage. ULF can be assembled on a transistor using a circuit with a common emitter. It should be noted that since the load resistance R n usually much less than the internal resistance of the collector circuit R vn.k, the power that is released at the load connected directly to the collector circuit will be very small. In order for this power to be maximum possible, it is necessary to fulfill the condition R n=R vn.k, i.e. the load resistance must be equal to the internal resistance of the useful signal source. For this purpose, matching transformers are used in practice (Fig. 28). Similar circuits of a single-ended transistor power amplifier with a common emitter are used if the output power does not exceed 3 - 5 W. Load R n switched on via matching transformer Tr.

The essence of the matching is that the resistance introduced into the primary winding of the transformer from the secondary winding R n was equal to the internal resistance of the collector circuit R vn.k or comparable to it. Then for given R n And R vn.k the task comes down to determining the transformation ratio k.

It is known that U 2/U 1=W 2/W 1=k, A I 2/I 1=W 2/W 1=k. Thus, the resistance introduced into the primary circuit

If we accept , then the transformation coefficient

i.e. the transformer must be step-down, since R n<R vn.k.

The considered circuits of the preliminary and output stages of the ULF operate in mode A. In this mode, the initial position of the operating point O is chosen in the middle of the load line CD. The amplitude of the alternating component of the collector current is less than the quiescent collector current. Operation in mode A is characterized by minimal nonlinear distortion and low efficiency (about 40%). In this mode, all preliminary and low-power ULF output stages, assembled on one transistor or one vacuum tube, usually operate in this mode.

In the case when it is necessary to obtain an output power of more than 5 W, push-pull amplifiers are used, assembled on two transistors or two lamps.

Let's consider the operation of such an amplifier using transistors (Fig. 29). The amplifier consists of two identical halves, each of which is similar to the amplifier shown in Fig. 28. The peculiarity of the push-pull circuit is that it can be used in a mode when the quiescent current of the collector circuits is close to zero. This mode is called mode B. When operating in this mode, the efficiency of the amplifier can reach 70%.

Operating point 0" on the input characteristic should be located in the region of base currents close to zero (Fig. 30, a). As a result, both halves of the circuit operate alternately, each opening during positive half-cycles of the input voltages u in 1 and uin 2, since they are out of phase by 180˚. The current pulses of the bases and collectors are also shifted by 180˚ (Fig. 30, b, c). At the same time, in the magnetic circuit Tr 2 a magnetic flux is formed that is close to sinusoidal, since current passes through the primary winding of the transformer (Fig. 30, d).

In multistage amplifiers, the last (output or final) stage is the stage for amplifying the power delivered to the payload. In this case, the output power of the PA cascade must be sufficient to drive the load connected to the entire output circuit. The output stage of the PA must maximize the power of the amplified signal with an acceptable nonlinear distortion factor and higher efficiency.

There are single-cycle or push-pull PA output stages, which can be assembled using powerful amplification tubes, or transistors, or gas-discharge thyratrons.

Single-ended power amplification stages. Such PAs operating in class A mode make it possible to select the output power of the useful signal from fractions of a watt to 3 ÷ 5 W to the load with an electrical efficiency of up to 10 ÷ 30% and minimal permissible levels of nonlinear distortion in a given frequency band.

In this case, the optimal value of the load resistance connected directly to the output circuit of the powerful stage is selected based on the relations Ra = R n= (2÷ 4) * Ri - for triode circuits and Rн = Ra ≈ (0.1 ÷ 0.5) * Ri - for PA stages on. high-power pentode or beam tetrode. Moreover, the circuits of such PA cascades and the methods of their graphical-analytical calculation are similar to the previously given circuits of voltage amplifier cascades (see Fig. 5, 7 and 8). Such simple PA stages make it possible to amplify the signal in terms of power with minimal nonlinear distortion in a wide frequency range.

A significant disadvantage of such transformerless PA circuits is that not only the useful alternating component of the anode current passes through the load, but also its direct component, significantly reducing the efficiency of the cascade and requiring a higher voltage power source E A. In addition, to maximize the use of useful output power that can be transferred by a transformerless final stage to an external load, it is necessary to maintain the equality of the optimal value of the output resistance of the output circuit of the PA cascade with the value of the external load resistance R n, included directly in this circuit, that is, R out= R n.

However, in practice, in most cases the load resistance R n may be less than the above-mentioned optimal value of anode resistance R a. This is explained by the fact that the winding of an electrodynamic loudspeaker, an electromagnetic relay, an electric motor, an electric contactor, a step finder, a recorder, a sound recording and sound reproducing head, a two-wire subscriber or feeder line, etc., which have small resistance (units, tens, hundreds of Ohms, units of kOhms).

Therefore, if R n< R вых к-да , то внешняя нагрузка включается в выходную цепь каскада УМ при помощи выходного трансформатора, согласующего величину Rн с оптимальной величиной выходного сопротивления каскада R вых к-да . При этом сопротивление внешней нагрузки, включенной во вторичную обмотку трансформатора, перерсчитывается в приведенное сопротивление его первичной обмотки, включенной в выходную цепь каскада, по следующей формуле:

where is the transformation ratio

More precisely, the optimal value of the equivalent resistance of the PA cascade can be determined graphically, using the most appropriate load line on the family of anode characteristics (Fig. 14) of the selected high-power amplification lamp, that is, the segments about and oa in the appropriate units of measurement:

Thus, according to the variable component of the anode current, the optimal value of the reduced resistance of the anode load Rout k-da can reach from units to tens and hundreds of kilo-ohms.

Using the same graph, using the ABC triangle you can determine the useful power in the load

The efficiency of powerful transformer cascades of the PA is higher than that of transformerless ones, since the quiescent current I a0 flows only through the low active resistance of the primary winding, bypassing Rн. Wherein

where Po = I a0 * E a - total power in class A mode consumed from the power source.

It should be borne in mind that single-cycle PA transformer cascades have a narrower frequency band, larger dimensions, weight, and higher cost, which reflects their disadvantages.

In Fig. Figure 15 shows typical circuits of single-ended transformer stages of a PA based on a powerful triode (a) and a beam tetrode (b), operating in class A mode with automatic shifting of the operating point.

In these circuits, the purpose of each element of the PA cascade is similar to the previously discussed circuits of voltage amplifier cascades with an anode load (Fig. 6 and 8).

As can be seen from the graphs in Fig. 16, to obtain the optimal value of useful output power

it is necessary to apply an input voltage with amplitude |±U to the input of the PA cascade max| ≈ |-U c 0 |, taken from the pre-amplifier stage or from the input signal sensor. In this case, the load line should pass almost tangentially to the permissible power curve P and additional, without crossing it.

Since in class A mode the operating point is in the middle of the straight section of the input dynamic characteristic of the cascade, this ensures the condition of operation with minimal nonlinear signal distortion.

Triode PA cascades have less nonlinear distortion than PA cascades based on pentodes or beam tetrodes.

However, in most cases, the electrical efficiency of the PA cascade in class A mode practically exceeds 10 ÷ 15% for triode circuits and 15 ÷ 30% for high-power pentode and beam tetrode circuits.

It must be borne in mind that in PA cascades with a transformer output, with a low active resistance of its primary winding (r 1tr = tens ÷ hundreds of Ohms) the anode voltage in rest mode is only slightly less than the power source voltage E a, that is

For triode circuits,

For circuits using pentodes or beam tetrodes with an additional screen grid circuit.

Therefore, the DC load line on the family of static anode characteristics (Fig. 16) goes very steeply, at a large angle

In the dynamic mode of operation, when a sinusoidal (harmonic) input signal is supplied to the input of the transformer stage of the PA at the optimal value of the reduced load R eq, the highest voltage E a the maximum between the output electrodes increases almost twice (and sometimes more) compared to U a0. This phenomenon is explained by the fact that when the output current decreases, the back EMF of the inductance of the primary winding of the transformer is added to the value of E a, which delays the process of decreasing the anode current. Therefore, in the dynamic operating mode of such a PA cascade, the load line for the alternating component of the anode current is determined by the value of R eq and E a max > Ea and, passing through the non-working point through which the DC load line passes, has a significantly smaller angle of inclination (Fig. 16)

When calculating the maximum output power of the transformer cascade of the PA, taking into account the efficiency of the transformer, the required value of the output power of the cascade is determined from the given value of the required useful power in the load Pnet, namely:

Then select an amplification tube whose permissible power dissipated by the anode, P and extra ≥ 6P out yes for triode and a P and extra ≥ 4P out to -yes for.pentode or beam tetrode. In this case, the voltage on the anode in quiescent mode is taken equal to Uа0 = (0.7 ÷ 0.8) * Ua add, and the quiescent current value is taken equal to

The useful power released in the load will be equal to P useful = η tr* P output k-da = 0.5 η tr* I ma*U ma =0,5 η tr* I 2 ma R eq.

From here you can determine the transformation coefficient

Voltage gain of the PA cascade

To take into account useful power losses in the output transformer, take the value of its efficiency within the limits specified in table. 1.

V. Mayorov, S. Mayorov - Amplifier devices based on lamps, transistors and microcircuits

Transcript

1 Lecture 7 Topic: Special amplifiers 1.1 Power amplifiers (output stages) Power amplification stages are usually output (final) stages to which an external load is connected, and are designed to obtain the required power in the load. The energy performance of these cascades is very significant and when analyzing amplifiers they are given the main attention. Power amplification stages are very diverse. They can be performed on bipolar and field-effect transistors connected according to the OB, OE (OI) or OK (OS) circuit. Depending on the method of connecting the load, amplification stages can be transformer-based or transformerless. The output stages of amplifiers are designed to obtain the required signal power in a low-impedance load and therefore they are characterized by a number of energy parameters: output power, efficiency, power gain and level of nonlinear distortion. To ensure high energy performance of the power amplifier, the amplitudes of the output voltages and currents, as well as the output power of the amplified signal, must be close to the corresponding maximum permissible parameters of the transistor used. Based on the method of connecting the load, the output stages are divided into transformer and transformerless. Transformer stages are practically not used in modern amplification devices. In transformerless output stages, transistors of the same and different types are used, which are connected in a push-pull circuit with direct connection of the load. In this case, transistors of different types have identical parameters and are called complementary. Currently, circuits based on the same type of transistors are used extremely rarely. The main disadvantage of such circuits is that one transistor is connected according to the OE circuit, and the other according to the OK circuit, which requires artificial equalization of the gain factors of the arms. The main requirement for power amplification cascades is to ensure that the maximum possible or specified value of signal power is provided in a given load resistance. This power must be supplied at an acceptable level of nonlinear and frequency distortions, as well as with the lowest possible power consumption from the power source. Therefore, the main initial data when calculating the cascade are: power P N delivered to the load; level of frequency and nonlinear distortions; working frequency band(); efficiency factor - N V 1

2nd cascade. A power amplifier is usually the output stage of an amplification device. The load resistance of a power amplifier, as a rule, does not exceed several tens or hundreds of ohms. If a low-resistance load is connected directly to the output circuit of the output stage transistor, which usually has a high output resistance, then the signal power in the load will be very small. In this case, matching the output resistance of the amplifier stage and the load resistance is carried out using an output transformer. If the load is sufficiently high-impedance, then it can be connected directly to the output circuit of the final amplifier stage. Classes of amplification in power amplifiers Depending on the position of the rest point on the DC load line, three main modes (early name class) of operation of transistors in power amplifiers are distinguished: A; B and AB. Specific modes C are also used; D (close to key) and key pulse mode. In mode A, the rest point of the transistor on the output characteristics is selected so that the operating point, when moving along the load line, does not fall into the area of ​​distortion of the output signal shape. Thus, all the amplifier stages discussed above operate in mode A. The energy parameters of the power amplifier are determined from graphical constructions: power in the collector circuit of the OE cascade: P, 5 U I; K OUT m K m power consumed from the power source: R EK I KP; Efficiency of the collector circuit: P U OUT m I K K m, 5. R EK I KP As follows from the formulas at maximum amplitudes of voltage and current (U OUT m EK and U K m I KP) the efficiency of a transistor power amplifier operating in mode A , does not exceed 5%. In class A mode, the quiescent point is selected so that the operating point, when moving along the load line, does not enter the nonlinear initial region of the collector characteristics and the collector current cutoff region, i.e., in the region of output signal distortion. In other words, all the stages considered operate in class A amplification mode. Class A mode is used in the so-called single-ended power amplification stages. Class A power amplification stages provide the least nonlinear distortion of the output signal, but have a minimum efficiency of 2

3 They have found application with a load power of no more than several tens of milliwatts. In Class B mode, the quiescent point is located at the far right of the stage's DC load line. The rest mode corresponds to the voltage U BE. In the presence of an input signal, the transistor's collector current flows only during one half-cycle, and during the other, the transistor operates in current cutoff mode. In class B mode, the power amplifier is implemented in a push-pull circuit using two transistors. Each of the transistors serves to amplify the corresponding half-wave of the input signal. The output stage has a higher efficiency and is used at higher powers than a single-cycle one. Class AB mode is intermediate between modes of classes A and B. It allows you to significantly reduce nonlinear distortions of the output signal, which are strongly manifested in class B mode due to the nonlinearity of the initial section of the input characteristic of the transistors. This is achieved by slightly shifting the rest point upward. Power amplification stages are considered on bipolar transistors, connected primarily according to the OE circuit. On field-effect transistors, these cascades are performed in a similar way. 1.3 Transformerless power amplifier on complementary transistors A single-phase amplified voltage is supplied to the input of the transformerless output stage on complementary transistors (Figure 1, a)). Both transistors are used according to the emitter follower circuit and are usually powered from two oppositely polarized identical power sources EK1 EK 2. The load in the amplifier stage is connected to the common connection point of the transistor emitters. T 1 T 2 i K1 R N u OUT i K 2 E K 1 E K 2 i K1 i K 2 u OUT 2 3 t t t t 3

4 a) c) Figure 1 Transformerless power amplifier on complementary transistors: a) circuit; b) timing diagrams of currents and voltages. A single-phase amplified voltage is supplied to the input of the transformerless output stage on complementary transistors (Figure 1, a)). Both transistors are used according to the emitter follower circuit and are usually powered from two oppositely polarized identical power sources EK1 EK 2. The load in the amplifier stage is connected to the common connection point of the transistor emitters. Using time diagrams of currents and voltages (Figure 1, b)), we will consider the principle of operation of a transformerless output stage. The transistors in the circuit operate alternately in the so-called B mode. For example, at an angular interval with a positive half-wave of the input harmonic voltage, transistor T 1 (n p n type) opens, passing a pulse of collector current i K1 into the load. In this case, a positive half-wave of the output voltage u OUT is released at the load. At interval 2, when a negative half-wave of the input voltage, voltage, is received at the input of the cascade, transistor T 2 (p n p type) opens and a current pulse i K 2 flows through the load, creating a negative half-wave of the output amplified voltage u OUT. The power amplifier stage is calculated by the graphic-analytical method, using the static characteristics of any transistor, for example T 1. The efficiency of the collector circuit increases with increasing amplitude of the output voltage and at values ​​of U Km EK reaches the limit value of 78.5%. Since both transistors are connected in an emitter follower circuit (the circuit is often called a push-pull emitter follower), matching the output impedance of the amplifier with a low-impedance load is greatly simplified. However, in this case, the output voltage does not exceed the input voltage, and power amplification is provided only by current amplification. The main disadvantage of push-pull power amplifiers operating in mode B is nonlinear distortion of the output signal due to the nonlinearity of the initial sections of the input characteristics of the transistors. The combined input characteristic of the two transistors has a kink near zero (Figure 2, a)). As can be seen from the diagrams, this nonlinearity distorts the base currents i B1 and i B 2, as a result of which the shapes of the collector currents of the transistors and the output voltage are distorted. This drawback is eliminated by introducing - 4

We put 5 transistors in the intermediate AB mode (Figure 2, b)). This is achieved by applying small negative bias voltages to their bases, equal to the trigger voltage. Typically, diodes, zener diodes, or diode-connected transistors serve as the source of base bias. I B I B i B1 i B1 u BE i t B 2 u BE i B 2 t t t a) b) Figure 2 Diagrams of operation of two transistors: a) in mode B; b) in AB mode 1.4 Selective amplifiers Selective amplifiers are designed to amplify narrowband signals. As a rule, the ratio of the boundary frequencies of the operating band of a selective amplifier does not exceed f / f 1, 11, 5. Their frequency response should have fairly sharp, close to rectangular, declines at the boundaries of the passband. Based on the frequency range used, selective amplifiers are divided into two classes: resonant and frequency-dependent. In one of the simplest circuits of a transistor resonant amplifier with a bipolar transistor with a common emitter, the load of the collector circuit is a parallel oscillatory LC circuit (Figure 3). Communication with the subsequent amplifier stage or load is most often carried out through a decoupling capacitor. High frequency transformer coupling can also be used. The gain of the resonant cascade with OE is determined by the formula K U h21r / h11, where R is the resonant resistance of the circuit, which replaces the load resistance R KN. The purpose of the elements in the circuit of the amplifier stage of Figure 3 is V N 5

6 is the same as in the circuit of an amplifier stage on a bipolar transistor connected according to a circuit with a common emitter. In the cascade, to improve the output characteristics, negative serial feedback for direct and alternating current is used, which is set by resistors R 1, R 2, R E. To eliminate negative serial feedback for alternating current, resistor R E is shunted with a large capacitor S E Separation capacitors C P1 and C P2 separate the alternating and direct voltage components in the circuit. Resistors R 1 and R 2 are called base bias resistors. With their help, the voltage offset at the input of the active element, in particular the transistor T, decreases. Resonant amplifiers are used at intermediate and high frequencies (over hundreds of kHz). They are usually performed on integrated circuits, which contain all the elements of the circuit diagram, except for the oscillatory circuit (at relatively low frequencies). In the frequency range up to several tens of kilohertz, resonant L circuits are not used due to the large dimensions of capacitors and inductors. Therefore, at sufficiently low frequencies, selective amplifiers with frequency-dependent feedback, consisting of RC circuits, are used. R 1 L C C P2 E K C P1 T u OUT R H R 2 R E C E Figure 3 Selective amplifier 1.5 Phase inverted stage Phase inverted stages are the pre-final stages of the amplifier if the final stage is a push-pull power amplifier. The phase inversion stage must provide two identical voltages at the input of the push-pull power amplifier, shifted in phase by 18. The easiest way to carry out inversion is using a stage with a transformer output. The secondary winding is made with the output of the middle 6

7 points (Figure 4). The calculation of such a stage does not differ from the calculation of the transformer stage of a power amplifier operating in mode A. The load of the secondary winding arm is the input resistance of one arm of the push-pull power amplifier, and the transformation ratio is defined as the ratio of the number of turns of half the secondary winding to the number of the primary. E K Tr u out 1 R 1 u out 2 C p1 T R G e G ~ u in R 2 R E C E Figure 4 Diagram of a phase inversion cascade with a transformer output The main disadvantages of a transformer inversion cascade are its large weight, dimensions and cost, as well as the availability additional nonlinear distortions. Therefore, a so-called phase-inverted stage with a divided load is often placed between the pre-final and final stages. The phase-inverted cascade (cascade with a divided load) is designed to obtain two output signals with a phase shift of 18. The phase-inverted cascade diagram is shown in Figure 4. It is obtained from the OE circuit by disconnecting the capacitor SE and connecting the second load R H 2 through C p3 to R E. The output signals are removed from the collector and emitter of the transistor. The signal u OUT 2, taken from the emitter, is in phase with the input signal (Figure 5), and the signal u, taken from the collector (Figure 5), is in antiphase with it. OUT 1 7

8 E K R 1 R K C p2 C p1 T R G C p3 R N 1 u out 1 e G ~ u in R 2 R E R N 2 u out 2 Figure 2.5 Diagram of a phase-inverted cascade 8

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Task 1 Let's determine the initial data: 1. Draw a circuit of a rectifier with a filter, on which we denote the voltages and currents in the transformer windings, valves and load. We indicate the polarity of the output terminals. 2.

Class A amplifier.

Operates in linear mode: both transistors operate in the same modes. This providesminimum distortion , but as a result of this low efficiency (15-30%), i.e. This class is uneconomical in terms of energy consumption and heating. Power consumption does not depend on the output power.

Class B amplifier

This class mainly includes amplifiers with output transistors of the same conductivity. Each of the transistors operates in key mode, i.e. amplifies only its half-wave signal in linear mode (for example, positive if transistors with N-P-N conductivity are used). In order to amplify the negative half-wave of the signal, a phase inverter is used on another transistor. It's like two separate A classes (one for each half-wave). An amplifier of this class has a high efficiency (about 70%). The power consumption of the amplifier is proportional to the output power; in the absence of a signal at the input, it is zero. Amplifiers of this class are rare among modern amplifiers.

Class AB amplifier

The most common type of amplifier. This class combines the qualities of Class A and Class B amplifiers, i.e. high efficiency of class B and low level of nonlinear distortion of class A. A cutoff angle of more than 90 degrees is used here, i.e. the operating point is selected at the beginning of the linear section of the current-voltage characteristic. Due to this, in the absence of a signal at the inputthe amplifying elements are not switched off and some current flows through them (the so-called "quiescent current") , sometimes significant. And here there is a need to regulate and stabilize this current so that the transistors operate in the same modes without overloading each other. Incorrect setting of the quiescent current will lead to overheating of the transistors and their failure.

So: for the output stage there are two very important parameters (and especially for class AB):

quiescent current and quiescent voltage

If the transistors had an ideal characteristic (which in fact does not happen), then the quiescent current could be considered equal to zero. In reality, the collector current can increase both due to the scatter in the characteristics of the transistors and their temperature. Moreover: an increase in temperature can lead to avalanche-like overheating and thermal breakdown of the transistor. The fact is that as the temperature increases, the collector current only increases, and therefore the heating of the transistor increases.

resting voltage: constant voltage at the point of connection of the transistors (output to the load). It should be equal to "0" when the output stage is supplied bipolarly, or half the supply voltage when it is unipolarly supplied. In other words: both transistors of the output stage must have the same base bias, that is, they are open evenly, compensating each other.

These two parameters must be stabilized, and first of all, their temperature dependence must be eliminated.

For this purpose, amplifiers use an additional transistor, connected in a ballast manner to the base circuits of the output transistors (and most often it is placed directly on the radiator next to the output transistors, thereby controlling their temperature).

The distortion of the amplifier output stage (and this is where it is very significant, compared to the distortion of the preliminary stages) depends on the optimal choice quiescent current(operating point) of transistors. When moving away from the optimal operating point, the output stage begins to generate high order distortions, which are very negatively perceived by human hearing and are one of the reasons for the “transistor sound” of the amplifier.

Typically, to organize the bias of the output stage, it is used voltage generator. With the relative simplicity of the circuit, it provides easy adjustment of the operating point of the output stage. And somehow it just so happened that this node is not given much importance.
However...

However, for high-quality sound amplification, there are, alas, no secondary things.

The output stage bias generation circuit performs two functions:

1. provides the task optimal quiescent current output stage amplifier (AB mode). Usually, in order to reduce “step” distortion, the output stage is switched to “AB” mode, despite some loss of amplifier efficiency. In this case, the bias circuit sets the quiescent current of the output transistors to about 70-100 mA.

2. provides thermal compensation of the quiescent current when the temperature of the output transistors changes. In the “silent” mode, the current through the transistors of the output stage is small - it corresponds to the quiescent current, and the heating of the transistors is not strong. With high output power, the current through the transistors increases, and their temperature increases significantly.

At the same time, most transistors are characterized by positive thermal coefficient, i.e. When the transistor heats up, the current through it increases. As a result, it is possible avalanche self-heating transistor: as the current increases, the temperature rises, and if the temperature rises, then the current also increases.

The bias setting circuit should reduce the current of the output transistors when they heat up.

Let's consider what properties the output stage bias circuit should have.

1. Provide operating point stability during external disturbances: instability of supply voltage, changes in ambient temperature, etc.

2. Provide the necessary temperature compensation accuracy. For different stages: emitter followers, Sheklai stages, etc. The requirements for the accuracy of maintaining the bias voltage are different.

3. Provide high temperature compensation speed. When the transistors heat up, the circuit must quickly reduce the current through them, and when they cool down, it must also promptly return it to its previous value.

For more than 30 years, a voltage generator with thermal feedback has been used as a temperature compensation element. Its scheme is quite simple:

To provide thermal feedback, the transistor T1 itself is usually mounted on the heatsink of the output transistors.

I note that sometimes there are circuits where the bias voltage is adjusted resistor R1(this is what they propose to make tuning). This option is not exactly wrong, but quite dangerous. The mechanical contact of the trimming resistor is very unreliable. It can also fail due to mechanical reasons or due to oxidation.

If the trimmer resistor motor circuit breaks in the presented version, the output transistors of the amplifier will simply close, the amplifier will switch to mode “B” and this will not bring catastrophic consequences (except for increased distortion).

If you make resistor R1 a trimmer, then if its motor breaks, the current of the output transistors will increase as much as it can. It’s good if the protection circuit (if your amplifier has one) can limit this current in time. Otherwise, you will have to change the output transistors and everything that will burn out along with them.

To ensure stability of the operating point under various external disturbances, the bias circuit is powered from a current generator:

Here, transistor T6 is a voltage amplifier (pre-output stage), and a stable current source is assembled on transistor T7.

The circuit is quite simple, but it does not take into account “slow” disturbances due to temperature changes: in the room (in summer and winter the temperature can differ significantly), inside the amplifier case. After prolonged operation, due to heating of the output transistors inside the device, the temperature increases significantly, and this leads to a change in the current not only of the output transistors but also of the first stages of the output double/triple.

This temperature drift can be compensated in the following ways:

1. Douglas Self method using a diode:

2. Method of I. Pugachev. In amplifiers with relatively high output power, triple cascades are used. In this case, output transistors are often installed on radiators, pre-output transistors are installed with small heat sinks on a printed circuit board, the first transistors of the trio are usually installed simply on a printed circuit board without a heat sink. The power dissipation of the first transistors is usually small and here it is necessary to compensate only for the change in voltage Ube with changes in the ambient temperature.

To do this, you can use base-emitter junctions of similar transistors:

For temperature compensation, the transistors are combined in pairs (can be glued with the back walls) T1 with T4 and T3 with T5. Transistor T2 is attached to the output transistors (more on this below).

It is better to solve problems of accuracy of maintaining the operating point and speed of response together.

The ideal option would be sensors located directly on the output transistor crystals. Then both the accuracy of temperature measurement and the speed of response (there are no thermal resistances of radiators, etc.) would be extremely possible.

And today there is such a solution. These are transistor-diode assemblies from the company ThermalTrak:

Here, a powerful transistor and a diode are placed in one housing, which is used as a temperature sensor in the circuit for setting the bias of the output stage.

An example of a power amplifier circuit using the following assemblies:

Click to enlarge.

Unfortunately, in the vastness of the “Great Power” these assemblies are quite problematic to find, and they are a bit expensive. Therefore, a simple radio amateur has to use old-fashioned methods in his amplifiers - use a discrete transistor as a temperature sensor. But even here you need to approach it wisely!

For some reason, historically, the temperature sensor is most often mounted on the radiator between output transistors:

Moreover, in addition to the thermal resistance of the “transistor-radiator”, a very decent thermal resistance is added radiator section between the transistor and the temperature sensor. In this case, talk about accuracy And high speed Thermal compensation is somehow not logical.

As the practice and experiments of Douglas Self show, it heats up the hottest and cools down faster top surface transistors (the side on which the markings are usually applied). Therefore, it would be logical to mount the sensor directly on one of the output transistors:

If the transistors have insulated housings, then a washer between them is optional.

Many people probably have a question: which arm transistor is best to attach the sensor to? It is difficult to answer this question unequivocally. It all depends on whether your amplifier is inverting or non-inverting.

It is best to determine the optimal sensor mounting experimentally:

1. We attach the sensor according to the “standard” method between the transistors.

2. turn on some recording of the choir (Turetsky’s choir does not rule in this case)

3. when playing choral recordings, the transistors of one of the arms will definitely heat up much more than the transistors of the other arm. If you hate to burn your fingers, then even the cheapest Chinese multimeter comes with a temperature sensor. You can use it.

4. We fix the thermal sensor transistor on the hottest transistor.

Is the bias circuit of the output transistors in your amplifier done correctly???