A precision control circuit for variable-gain amplifiers

The author describes a circuit that precisely controls the gain of one or more single-ended amplifiers by means of a corresponding number of Gilbert quads. The circuit employs a configuration consisting of a differential pair with two negative feedback loops and is used in conjunction with a precision potentiometer to generate a DC gain-control voltage as a function of its setting. The gain-control characteristic accurately mimics the potentiometer's function of resistance versus wiper position to achieve a control range in excess of 55 dB. Experimental results are provided for the frequency response and the gain-control characteristic of the amplifiers.