A transistor automatic gain control system for audio frequencies

Automatic gain control (AGC) circuits are useful in obtaining relatively constant output levels where the input signal may vary over a wide range. Several articles have been written covering methods of controlling transistor gain in intermediate radio-frequency circuits.1,2 The usual method employed is to change one of the transistor parameters by the application of an external signal. The two basic methods of gain control indicated are: 1. Emitter current (Ie) control. 2. Collector voltage (Vc) control.