The automatic gain control circuit AGC provides a signal with less harmonics and additional amplification, between certain limits.
As a result, it eliminates those intensity differences, annoying, and of speech music, which sometimes appear on radio and television.
Field effect transistors T1 is used as a variable resistance. It’s value can vary from infinity to about 150Ω. It is in parallel with R3 and R4 together with, inducing the amplification of A1. Without FET, gain
of A1 is about 20 dB.
Amplification of A1 and A2 compensates damper losses: total agc circuit gain, with T1 blocked is 0 dB. Signals with a level lower than that set by P1 are amplified by a factor of maximum of 6.9 (= 17 dB amplification).
D1 = 1N4148
T1 = BF245C
IC1 = TL072
IC2 = TI071