Simple AGC - 'LO-FI' but useful

7-18 — elektor july/august 1982 This circuit will provide an output with a fairly constant amplitude of 4 V peak to peak, from an input that may vary between 100 mV to 2 V. There was no intention of achieving "hi-fi" performance as the distortion figures are not exactly in that league. "LO-F1" but useful Nevertheless, this automatic gain con- trol is ideal for use when recording computer programs onto cassette tape where a constant amplitude is more important than low distortion. Opamp Al provides an output impedance that is sufficiently low to drive the attenuator formed by diodes D1 and D2. Opamp A2 is a straight- forward amplifier with a gain of 100x but its DC setting is a little unusual in that it is derived from the average of the input signal via R5 and C4. The off-set voltage of A2 cannot escape being modified to some degree but, since this is relatively stable, it should not present too much of a problem. The output includes a peak detector consisting of D3 and C5. A pr...