Simple AGC circuit in transistor radio

Peak envelope detection requires that the input high-frequency signal is relatively large, and the amplitude should be greater than 0. 5v, so it is also called a large signal peak envelope detector . According to
the connection method of the diodes in the envelope detection circuit, the envelope detection circuit is divided into series peak envelope detector and parallel peak envelope detector.
    The series peak envelope detector
consists of an input loop, a diode VD and an RC low-pass filter.
The diode peak envelope detector with a simple AGC circuit is shown in the figure. Diode VD
adopts 2AI-9.
    RP1 and G form a low-pass filter of a simple AGC circuit
. The slowly varying DC component is taken out from the detected audio signal as a control
signal to directly control the gain of the transistor. It can be seen from the analysis
that the larger the signal, the larger the DC component after detection, and
the smaller the collector current of the IF amplifier transistor, so this automatic gain
control circuit should be a reverse AGC. Adjusting the variable resistor RP1 can
make the cutoff frequency of the low-pass filter lower than
the lowest frequency of the demodulated audio signal to avoid reverse modulation.