High quality power amplifier. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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High quality power amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The high quality power amplifier is designed to work with a 2V preamplifier. Distinctive features of the amplifier - high resistance to self-excitation and very low non-linear distortion.

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The main parameters:

Rated frequency range, Hz.......20...20 000
Rated output power, W, at a load with a resistance of 8 Ohm.......35
Harmonic coefficient, %, no more, in the nominal frequency range at output voltage, V 2.......0,002
3......0,005
4......0,007
8......0,02
17......0,05
Relative level of noise and background with a short-circuited input, dB.......-104

The amplifier contains two signal voltage amplification stages. The first of them is differential, on transistors V1, V2 with a current source in the emitter circuits on transistor V5. The second stage is made on transistors V7, V8: The output stage is quasi-complementary, on transistors V15-V18. The bias voltage necessary for the operation of the output stage is created by a device assembled on. transistor V14 and diodes V9-VII Temperature stabilization of the quiescent current of transistors V17, V18 (220 ... 250 mA) is carried out due to thermal coupling between the case of one of them and the case of transistor V14. Thermistor R20 is used for temperature compensation of zero drift of the output voltage of the amplifier; for this, the thermistor is installed on the heat sink of the transistor V8.

Small non-linear distortion and high stability of the amplifier are mainly due to the OOS, covering the output stage. The feedback voltage is removed from the divider R25R26 and fed to the input of the cascade through the transistor V13. This requires increasing the quiescent current of transistors V8, V13 to 25 mA and installing them, like transistors V15-V18, on a heat sink. Circuits L4R48 and C12R49 load-balance the output stage at high frequencies.

Another local negative feedback (about 15 dB deep covers the output stage and the preceding amplification stage on transistors V7, V8 The feedback voltage is supplied from the output of the amplifier through the resistor R23 to the base of the transistor V7. The C8R27C9 circuit weakens this connection at high frequencies, preventing self-excitation covered by its cascades and narrows the bandwidth to 300-350 kHz.

The bandwidth of the first stage (approximately 30 kHz) and the depth of feedback (approximately 30 dB in the audio frequency range) covering the entire power amplifier through the RJ3RJ4 voltage divider are chosen so that the overall feedback at frequencies above 1 MHz does not work. the bandwidth of the first stage was achieved by including coil L2 in the emitter circuits of transistors VI, V1 and separating the base of the second of these transistors from the amplifier output by resistors R12, R15 (they increase the frequency response of the cascade with increasing frequency). .

Instead of those indicated in the diagram in the amplifier, transistors KT644A, KT644V (instead of KT626B) can be used; KT805B, KT903A, KT908A (instead of KT805A) KT3102A, KT3102V/KT3102D (instead of KT3102B): KT373G (instead of KT3107B) Diodes D104 can be replaced by diodes D219A, D219B, D220A, D220B.

for example, it can be wound with a PEV-2 wire with a diameter of 0,15 mm (35 turns) on a unified three-section frame placed in an armature from the IF filter of the Sokol transistor receiver, the second and third - with a mounting wire with a cross section of 0,1 mm2 (12 15 turns on ferrite (400НН) rings of size K10 X 6 X 4. Unified chokes D-2, D-3 can be used as coils L0,1, L0,2. Coil L4 is wound in 4 layers on a frame with a diameter of 15 mm and contains 40 turns wire * PEV-2- 1,0.

To power the output stage of the amplifier, a conventional bipolar rectifier with filter capacitors with a capacity of at least 10000 μF and high-speed overcurrent protection is suitable. The power supply of the signal voltage amplification stages must be stabilized. The ripple voltage must not exceed 2 mV.

Setting up the amplifier comes down to eliminating (by trimmer R17) the constant component of the output voltage, setting (by selecting resistor R35) the required quiescent current of the output stage and checking the stability of the amplifier (if other types of transistors are used instead of those indicated in the diagram). The criterion for stable operation is the absence of self-excitation with an increase in the depth of any OOS from the amplifier output by 10 dB. With insufficient stability, it is necessary to select capacitors C8, C9 and resistor R28.

The amplifier can drive a load with a resistance of 4 ohms. To do this, it is necessary to reduce the supply voltage of the output stage to + 24 V (the current consumption will increase by about 1,5 times), and the inductance of the L4 coil to 10 μH.

See other articles Section Transistor power amplifiers.

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