In order to achieve the above requirements, it is necessary to have a control voltage or current signal that can change with the strength of the external signal, and use this signal to automatically control the gain of the amplifier. From the above analysis, it can be seen that after the amplitude detection of the amplitude modulation IF signal, in its output in addition to the audio signal, there are DC components. The size of the DC component is proportional to the amplitude of the intermediate frequency carrier, that is, to the external high frequency signal. Therefore, the DC component output by the detector can be used as the AGC control signal.

    AGC circuit working principle: can be divided into gain controlled amplifier circuit and control voltage formation circuit. The gain-controlled amplifier circuit is located in the forward amplification path, and its gain changes with the control voltage U0. The basic components of the control voltage formation circuit are the AGC rectifier and low-pass smoothing filter, and sometimes also include components such as gate circuits and DC amplifiers.

Amplifier and AGC circuit