Variable gain amplifier with high linearity and low noise

Abstract

A variable gain amplifier that has a high linearity gain stage coupled with a low noise gain stage, and a steering circuit that steers the input signal between these two stages depending on the gain requirement. The preferred embodiment employs steering of the input signal and extensive linearization (in dB) of the control signal to provide stable and linear gain control and high dynamic range. One embodiment includes a first differential amplifier stage having no emitter degeneration and configured to receive a differential input signal and contribute to an output signal current; a second differential amplifier stage having emitter degeneration and higher linearity than the first differential amplifier stage, and configured to receive the differential input signal and contribute to the output signal current; a first current source coupled to the first differential amplifier stage and regulated by a control signal X according to the formula I*(K−x), where K is a constant and x is a variable ranging from 0 to K; a second current source coupled to the second differential amplifier stage and regulated by a control signal X according to the formula I*(x); and a signal steering circuit, coupled to the first and second differential amplifier stages, for steering the output signal current from the first and second differential amplifier stages either to a voltage source or to a corresponding load resistor.

Dependencies are inferred from the claim text.