Abstract: A high input impedance differential amplification circuit using two operational amplifiers with the input signals coupled to respective ones of the inverting input of one amplifier and the non-inverting input of the other, and with resistance networks of pre-set value relationships coupling the other amplifier inputs and their outputs.

Abstract: The drain-to-source resistance of a field-effect transistor (FET) can be used as a variable resistor controlling say the gain of an AGC amplifier, the AGC signal being applied to the gate of the FET. However, the resistance is dependent not only upon the gate drive but also on the device characteristics which vary even from one sample to another of the same device type. Thus, replacing the FET in say an AGC amplifier may well change the amplifier characteristics. Similarly, it may be difficult to ensure that multiple reproductions of the amplifier operate uniformly. Herein, an FET of which the drain-to-source resistance controls the gain of an amplifier is implemented as one of two matched FET's in a dual FET device. The same drive signal is applied to both FET's and this drive signal is formed by a feedback circuit which sets the drive signal to maintain the current through the second FET dependent upon a received input signal and to maintain the potential across the second FET constant.