AF power amplifier (80 watts). Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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The AF amplifier has very low coefficients of harmonic and intermodulation distortion, it is relatively simple, able to withstand a short-term short circuit in the load, and does not require external elements for thermal current stabilization of the output stage transistors.
Main Specifications
Maximum power at a load with a resistance of 4m, W. . . 0
Nominal frequency range, Hz. . . 20...20 000
Harmonic distortion at maximum
output power 80 W, %, at frequency:
1 kHz. . . 0,002
20 kHz. . . 0,004
Intermodulation distortion factor, % . . . 0,0015
The maximum frequency at which the maximum
power is reduced by 1 dB, kHz. . . fifty
Output slew rate
voltage (without capacitor C2), V / μs. . . 40
Fig. 1
The circuit diagram of the amplifier is shown in fig. 1. Changes have been made to the output stage. To increase its input resistance, transistors VT1, VT2 are introduced into the AF amplifier. This facilitated the operation of the op-amp DA1 and made it possible to ensure a stable base-emitter voltage of transistors VT3, VT4 when the temperature changes. In addition, the amplifier is supplemented with a cascade on transistors VT5, VT6, which, together with current sensors R33, R34 and output stages on transistors VT7-VT10, form two current generators in idle mode, which eliminates the cutoff of the emitter current of the transistors of the final stage and reduces switching distortion. The latter, as is known, favorably affects the spectrum of harmonics.
In addition to these changes, a deeper local OOS was introduced into each arm of the output stage by increasing the resistance of resistors in the emitter circuits of transistors VT3, VT4, which made the output stage more linear. Since the resistors R20, R21 are connected to the current sensors R33, R34, a rather rigid thermal stabilization of the quiescent current of the transistors of the final stage is obtained. . The presence of current sensors R20, R90, deep DC feedback and current-limiting resistors in the base circuits of transistors VT150, VT180 leads to the limitation of their collector currents to an acceptable value during short circuits in the load. Resistor R33 sets the symmetry of the shoulders of the output stage.
No other changes were made to the amplifier. Nonlinear distortions were measured with an S1-68 oscilloscope using a ZCh GZ-118 signal generator (Kg - about 0,002%) and a precision double T-bridge included in the generator set. The measurements were carried out according to the method described in the article by Yu. Mitrofanov "Economic mode A in a power amplifier" (see "Radio", 1986, No. 5, pp. 40-43). The coefficient of intermodulation distortion was measured according to the recommendations given in the article by V. Kostin "Psychoacoustic sound quality criteria and the choice of UMZCH parameters" (see "Radio", 1987, No. 12, p. 40-43). When testing the amplifier with a pulsed signal, no spikes were observed at the output voltage.
Author: G. Bragin; Publication: cxem.net
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