where K is the gain.
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.
Differential amplifiers are found in any system that utilises negative feedback, where one input is used for the input signal, the other for the feedback signal. A common application is for the control of motors or servos, as well as for signal amplification applications. In discrete electronics, a common arrangement for implementing a differential amplifier is the long-tailed pair, which is also usually found as the differential element in most op-amp integrated circuits.