This chapter deals with a basic analysis of the differential pair, using bipolar junction transistors (BJT) or metal–oxide–semiconductor field-effect transistors (MOSFET), and examines its essential features, both in terms of the transfer function between input and output and in small-signal operation. It considers two types of circuits in which op amps play an essential role, namely, switched-capacitor circuits and digital/analog converters. Differential amplifiers are a special and important class of amplifiers, particularly in the form of integrated circuit (IC) operational amplifiers, because they inherently respond to the difference between two signals applied to their two inputs. The BJT differential pair, or emitter-coupled pair, is an important building block in both analog ICs, where it is extensively used in operational amplifiers, and in digital ICs in the form of emitter-coupled logic. An important advantage of the folded cascode is a wider common-mode input range, which is relevant, for example, to a unity-gain amplifier.