The earliest definite long-tailed pair circuit appears in a patent submitted by Alan Blumlein in 1936. An early circuit which closely resembles a long-tailed pair was published by British neurophysiologist Bryan Matthews in 1934, and it seems likely that this was intended to be a true long-tailed pair but was published with a drawing error. The long-tailed pair was developed from earlier knowledge of push–pull circuit techniques and measurement bridges. The bias points of “long-tail” resistor circuit are largely determined by Ohm's law and less so by active-component characteristics. The circuit works the same way for all three-terminal devices with current gain. This circuit was originally implemented using a pair of vacuum tubes. Modern differential amplifiers are usually implemented with a basic two-transistor circuit called a “long-tailed” pair or differential pair.

Note that a differential amplifier is a more general form of amplifier than one with a single input by grounding one input of a differential amplifier, a single-ended amplifier results.

It is an analog circuit with two inputs V in − is zero, and the CMRR is infinite. A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs.

Differential amplifier