The Differential Amplifier

Differential amplifiers are everywhere: input stages of Op Amps; comparator inputs; some video amps; balanced line receivers for digital data transmission ; etc...

A differential amplifier is an amplifier that has two inputs, each of which is sensitive to the opposite polarity of the other, i.e., if the inverting input has a positive going signal, and the non-inverting input has the negative version, then there is an output equal their difference (multiplied by some gain, Gv). Conversely, if both inputs happen to be at the same value, then there is no output signal: they cancel one another, i.e., both signals (being the same polarity and amplitude) make no change is the shared emitter resistor's current, therefore, neither signal affects the other: there is "cancellation," otherwise known as Common Mode Rejection, CMR. Another way of saying the same thing is: if both inputs have the opposite polarity (or phase) signal, the shared emitter resistor draws current equal to the algebraic summation of both transistors.

The configuration of the transistors in a differential amplifier are a combination of common emitter and emitter follower.

• A signal into either input's base, causes an inverted signal at its collector, and simultaneously, a smaller, non-inverted output at the (shared) emitter resistor.
• Any signal at the emitter will appear at the collector as a non-inverted version of this signal--but amplified
• Therefore, any signal at one transistor's input is not only seen at its collector, but is also seen at the other transistor's collector, enabled by the action of the shared emitter resistor

This amplifier consists of two or three transistors (two in the simple version, three or more in the more precision version). These two input transistors are coupled to each other, via each emitter, and share the same emitter resistor . At this common connection each input transistor affects the output of itself, as well as, the other transistor's output.

As the two input transistors share the same emitter resistor and leaving the impression that a signal voltage was at the junction of the emitters and Re, when one transistor is increasing in current, e.g., positive alternation of a sine wave; the other transistor is decreasing in current, by an equal amount, for the negative alternation. Since the pair is sharing the one resistor, one can deduce that, ideally, there is always a constant current in that resistor. Ideally, it is desired that the emitters transfer all of their signal to the other transistor's emitter.