Gain control circuit for communications amplifier - has RC circuit between first input of differential amplifier and output

摘要

The gain-control circuit has a differential amplifier (V2) with MISFETs in its input stage. The first MOSFET has an unprotected gate in its input circuit. This first input is connected via a reistor (R2) and a relay contact (k) to the circuit receiving the d.c. voltage (U1) proportional to the pilot signal. A reference voltage (Ur) is applied to the second input. A capacitor (C1) in sries with a resistor (R1) is connected between the differential amplifier's first input and output. The capacitor is also connected via an over-voltage protection circuit (2) to the supply poles such that it acts as a storage capacitor when the relay contact is open and as an integrating capacitor when it is closed.