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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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A feature of the amplifier is the synchronous (with a signal) power supply of the stage on the operational amplifier with varying voltages and the use of mode B in the output stage.
The main parameters:
- Rated frequency range, Hz.......10...50000
- Rated output power at a load with a resistance of 8 ohms (with a harmonic coefficient of 0,2%), W.......10
- Rated input voltage, V ....... 1
- Input impedance, kOhm.......5
- Quiescent current, mA.......15
The amplifier contains a signal voltage amplification stage on the operational amplifier A1 and an output stage on transistors V5-V8.
In the absence of a signal, transistors V5-V8 are closed, which is the reason for the low current consumed by the amplifier. In this case, the voltage at the connection point of the zener diodes V3, V4 and the resistor R10 * is zero, and at the power supply terminals of the operational amplifier it is maintained equal to the nominal supply voltage by the stabilizers on the transistors V1, V2.
With the appearance of a signal at the input, the voltage at the connection point of the zener diodes changes by the value of the output signal. As a result, it is possible to obtain a significantly higher signal voltage from the operational amplifier than with a conventional power supply. Resistor R10 *, connecting the zener diodes to the output of the amplifier, reduces the maximum voltage at the inputs of the operational amplifier (for direct current, they are connected to a common wire, as can be seen from the diagram, so a common-mode change in supply voltages is equivalent to connecting a common-mode signal source to them).
The amplifier is not afraid of input overloads and short circuits in the load. If this happens, then the gain of the operational amplifier will decrease sharply (as a result, the output voltage will also decrease), and its output current (and therefore the output current of the entire amplifier) will be limited by resistor R6.
The gain of the amplifier is determined by the ratio of the resistances of the resistors R3 and R2 of the OOS circuit (K = 1 + R3 / R2). The latter reduces the output impedance of the amplifier, improving its performance in the low-frequency region, and stabilizes the voltage at the load - loudspeaker B1.
Setting up the amplifier consists in checking the constant voltage at the load (it should be close to zero) and selecting the resistor R10 *. For the time of adjustment, it is convenient to replace it with a variable one (with a resistance of 2,7 ... 3,3 kOhm). By applying an alternating voltage from the audio frequency signal generator to the input of the amplifier, the resistance of the resistor is reduced until the maximum possible undistorted output signal is obtained on the oscilloscope screen. It is not recommended to excessively reduce the resistance of the resistor R10 *, as this will lead to a decrease in the stability of the amplifier (it can go into trigger mode). With a significant (more than ± 0,1 V) difference in the output voltage from zero, resistor R1 should be selected.
See other articles Section Transistor power amplifiers.
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