Complementary transistor power amplifier with full arm symmetry for both half-waves of the amplified signal and with a double differential stage at the input. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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It has the following main specifications:

Rated output power ...... 60 W
Harmonic distortion .......... 0,04%
Operating frequency band .......... 20 ... 150 Hz
Signal-to-noise ratio .......... 88 dB
Supply voltage ........... ±40 V
Quiescent current .............. 60 mA

The amplifier is completely made on complementary transistors. It works in AB mode. The applied circuit solutions made it possible to reduce non-linear distortions to a minimum. The main feature of the amplifier is the symmetry of the arms for both half-waves of the amplified signal. This made it possible to reduce the non-linear distortion of the amplifier without the introduction of feedback. Another feature of co. stands in the output stage circuit, which allows amplifying the signal not only in current, but also in voltage. At the same time, the operating mode of the transistors of the preliminary stage is facilitated, since the required signal amplitude is significantly less than for a conventional output stage.

Rice. 1 (click to enlarge)

The circuit diagram of the amplifier is shown in fig. 1. It contains a differential complementary transistor stage (VT1, VT4, VT2, VT5), a voltage amplification stage [VT7, VT8], an output stage (VTIO-VT13, VT15, VT16), and an overcurrent protection device (VT14 , VT17). The differential input stage on complementary transistors has an additional advantage over the usual one: if the base currents of transistors VT1 and VT2 (VT4 and VT5) are equal, current may not flow at all through resistors R2 and R3 and through resistor R30. This allows, without violating the balancing of the cascade, to change the resistance of these resistors within a fairly large range. To increase the voltage transfer coefficient and improve linearity at high thermal stability, current sources on transistors VT3 and VT6 are included in the emitter circuits of the transistors of the differential stage. The voltage amplification stage is made on a complementary pair of transistors VT7 and VT8 operating in mode A.

The arms of the output stage contain three transistors VT10, VT12, VT15 (VT11, VT13, VT16) covered by local OOS through resistors R25 and R29 (and, accordingly, R26 and R31). In this case, the voltage gain of each trio of transistors approaches three. Local feedback also reduces the spread in the gains of the output stage arms, which reduces the requirement for the identical parameters of complementary transistors. Another feature of the output stage is as follows. The voltage of the local OOS, covering the triple of transistors, is removed from the resistors R34 and R35, the voltage on which is proportional to any changes in the current of the output transistors (including depending on temperature). This additionally stabilizes the quiescent current of the output transistors. The bias voltage of transistors VT10 and VT11 depends on the voltage drop in the emitter-collector section of the transistor VT9, set by the divider on the elements VD4, R20 - R22.

Parametric OOS through the diode VD4, located on a common heat sink with the output transistors, provides temperature stabilization of the quiescent current of transistors VT15, VT16. With increasing temperature, the voltage drop across the VD4 diode also decreases, while the emitter-collector voltage VT9 also decreases. All this allows you to maintain the quiescent current of the transistor outputs at a constant level at different power levels and fluctuations in ambient temperature. The entire amplifier is covered by a common OOS, the voltage of which is supplied from the output of the amplifier through the divider R30, RI5, C3 to the bases of transistors VT4, VT5.

Elements C1, C5, C6, C7, C8, C9, R44, L1 are designed to correct the frequency response at high frequencies. They also ensure the stability of the amplifier when covering its overall OOS and with possible load changes. Transistors VT14, VT17 shunt the emitter junction of the output transistors during overload. The amplifier is powered by an unstabilized bipolar ± 40 V source.

The diode VD4 is placed on the heat sink next to the transistors VT15 or VT16. The amplifier uses resistors MLT, SPZ-1K (R22), capacitors K50-6, KM. The LI coil is wound on a resistor R45 (MLT-2) and contains 10 turns of PEV-2 0,8 wire. To power the amplifier, a bipolar source is required that provides a current of at least 40 A at a voltage of ± 2 V.

The adjustment of the amplifier, assembled from serviceable elements, consists in checking the correct installation and setting the initial current of the collectors VT15 and VT16 (50 ... 70 mA) with resistor R22.

Fig. 2

The amplitude-frequency and phase-frequency characteristics of a normally operating amplifier are shown in fig. 2.

Publication: cxem.net

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