Voltage controlled variable gain amplifier - has three inverting operational amplifiers and variable resistance FET providing temp. independence

摘要

Variable gain amplifier whose amplification is controlled from a voltage source. The amplifier circuit is intended to allow this control to be independent of the variable characteristics, i.e. temperature etc, of the circuit components used to control the gain. The circuit consists in its basic form of three operational amplifiers (20, 30, 40) with resistive feedback. Each amplifier is fed from its negative or inverting input, the feedback setting it at virtual earth. Two of the amplifiers supply an FET (36) whose drain-source resistance varies with the input control voltage (14) to one amplifier and sets the gain of the third or output amplifier (30).