Automatic Volume Control (AVC)
The AVC circuit in a radio automatically adjusts the gain of the i-f and r-f stages based on the strength of the carrier signal being received. Less gain is needed when tuned to a strong station than a weak station. A simple AVC circuit is shown below.
The gain of the i-f and r-f amplifiers is controlled by applying a negative AVC bias voltage to each control grid. Therefore the AVC circuit reduces the gain by increasing the negative bias on the i-f and r-f amplifiers.
This voltage is produced by the V3 diode circuit, typically found in the detector stage of the radio. Resistor RL is the diode plate load resistor. The voltage across RL is directly proportional to the signal strength. Strong signals produce more voltage, weaker signals produce less voltage.
R1-C1 form the AVC filter to eliminate r-f and audio variations resulting in a steady bias voltage. It has a relatively long time constant so that the d-c voltage across C1 cannot vary at the audio frequency.
The i-f and r-f amps receiving AVC bias return to C1 through the AVC line, so the voltage across C1 is the controlling bias of each stage. R2-C2 and R4-C4 are decoupling filters that isolate each control grid from the common AVC line.
In operation, suppose that a strong station is being received and generating -5 volts of AVC bias, resulting in a 3 volt audio signal from the second detector. When the receiver is tuned to a weaker station, less AVC bias will be produced causing the gain of the i-f and r-f stages to increase. This gain increase results in about the same level of audio output from the detector stage. So regardless of signal strength, the audio output will be about the same.