The Common-Mode Rejection of Transistor Differential Amplifiers

The primary aim of differential amplifiers is amplification of differential-mode input voltages and the suppression of common-mode input voltages. The problem of common-mode rejection of transistor-differential amplifiers is investigated. It is shown that in addition to the known possibilities of biasing differential stage from a constant current source and of providing common-mode negative feedback, increase of the internal colletector resistance, and the gain of the differential stage permit increasing the common-mode rejection to practically arbitrarily large values A transistor circuit is shown with a common-mode rejection of more than 140 dB.