Radio Experimenter Theory Notes

Automatic Gain Control (AGC)

Automatic Gain Control is the control of the gain of the receiver in sympathy with the strength of the received signal. The object is to ensure that the audio output (volume) remains constant when the signal may be subject to fading or to restrivct the output should the received signal be very strong.

The system operates as follows ;

The AGC samples the signal out of the last IF amplifier, just before it enters the detector stage. Hegative feedback is applied from D2 and by pulling the base of TR1 more negative, the gain of the transistor is reduced.