Negative feedback gain control for common electrode transistor

Abstract

The unwanted variation in operating point associated with varying the gain of a common input/output electrode transistor, such as a common emitter bipolar transistor, is obviated by coupling an electronically controllable conductance in a negative feedback path between a first input/output electrode (collector) and the control electrode (base) of the transistor. In a first embodiment applied to a bipolar device, the electronically controlled feedback element comprises a diode, having its forward conductance varied by adjusting current flow through a controllable current source. This controls the amount of feedback from the collector to the base and thereby the forward loop gain of the common emitter transistor. In a second embodiment for a bipolar device, the controlled feedback element comprises an emitter-follower transistor, with its forward gain controlled by varying the current drawn through its emitter by a controllable current source/sink. This, in turn, varies the amount of voltage fed back from the collector to the base and thus the gain of the common emitter transistor. Although varying the current drawn by the current source varies the operating point of the feedback element, the bias conditions for the common emitter transistor remain unaffected, so that its operating point and linearity remain constant.