Symmetrically gain controlled differential amplifier

摘要

An arrangement for symmetrically gain controlling a differential amplifier is disclosed. A control circuit including first and second similarly arranged transistors coupled between points of operating potential develops first and second control voltages of prescribed magnitude at collector outputs of the transistors. The first transistor is biased to provide the first control voltage at a fixed, prescribed level related to the gain control bias requirements of the amplifier. The second control voltage varies symmetrically with respect to the first control voltage in response to the setting of a potentiometer coupled to a bias circuit of the second transistor. The potentiometer is connected directly between points of operating potential such that the control voltages are equal for a center potentiometer setting, and the second control voltage varies symmetrically with respect to the first control voltage as the potentiometer is adjusted between minimum and maximum extremes to thereby produce symmetrical amplifier gain control. Symmetrical gain control is predictable and independent of potentiometer tolerance effects and variations of the operating potentials associated with the control circuit.