Reshetnikov power amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Power amplifiers reduce distortion in several ways. Usually, a deep OOS is introduced, covering the amplifier with the output stage operating in mode B. However, this is fraught with an increase in dynamic distortion. Therefore, more often in the output stage of a high-quality amplifier, mode A is set with a shallow OOS. But this requires thermal stabilization of a large quiescent current of the output stage. The principle of constructing audio-frequency amplifiers with distortion correction in bridge circuits of amplifiers with an economical mode B is known.

The figure shows a functional diagram of an amplifying device, consisting of a preamplifier A1, an output stage on transistors V1 and V2 operating in mode B, and bridge elements R1C1, R2 and L1. The relationship between the output voltage at the load Un and the signal voltage Uc is expressed for this device by the equality

Ua=AUe+Bi,

where i is the base current of the output transistors, A and B are coefficients whose numerical values ​​depend on the parameters of the elements.

It follows from the formula that the main source of distortion can only be the base current of the output transistors, therefore, if you build an amplifier in which the coefficient B is equal to zero, you can eliminate the main source of distortion. The non-linear distortion compensation condition coincides with the bridge balance condition when L1 = R1R2C1.

The described method of non-linear distortion compensation is applied in the power amplifier proposed by O. Reshetnikov.

(click to enlarge)

The amplifier operates in the frequency range of 20...20000 Hz with a frequency response unevenness of ±1 dB. The output power of the amplifier is 30 W at a load of 8 ohms and a harmonic coefficient of less than 0,02%. With a load of 4 ohms, the output power reaches 40 watts. Amplifier sensitivity 0,2 V. Noise level 75 dB.

The four-stage preamplifier operates in mode A (A1, V3 ... V6 and V9), the output stage (V12, V15, V16) operates in mode B. The amplifier is equipped with output stage overload and short circuit protection (V7 and V13). In the first stage, a low-noise operational amplifier on a K140UD8B chip is used, covered by the OOS (C1R2R5C3) so that the gain at frequencies above 20 Hz is constant and approximately equal to 15. In the second stage, a low-noise transistor V4 is used, in the collector circuit of which a source current on transistor V3. The third stage - a double emitter follower on transistors V5, V6 serves to match the input resistance of the fourth stage, made on the transistor V9, with the output resistance of the second stage (V4). The R5C6R29L1 bridge is RF balanced.

At low frequencies, a low level of non-linear distortion is provided by a deep (50 ... 70 dB) OOS, the voltage of which enters the emitter circuit of the V4 transistor from the amplifier output through the divider R12 and R15. The amplifier can be used transistors V3, V5, V6, V7, V13 - KT3107B, KT3107D, (somewhat worse than KT361V, KT361D), V4 - KT3102A, KT342G; V9, V15- KT626A, KT626B, KT616V, KT814V, KT814G; coils L1 and L2 are wound turn to turn in two layers with PEV-2 1,0 wire on frames 7 mm in diameter, 28 mm long and contain 30 and 46 turns, respectively.

If the amplifier is properly assembled, no adjustment is required. Bridge R15C6R29L1 is configured to minimize output distortion. To do this, a sinusoidal signal with a frequency of 50 ... 100 kHz is fed to the input of the amplifier. By selecting the capacitance of the capacitor C6, observing the output signal on the oscilloscope screen, minimal distortion of the waveform is achieved.

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