The chapter explains the operation of the bipolar junction transistor (BJT). It shows that the BJT is an amplifying device that basically amplifies power and voltage, albeit in a manner that is quite different from that of field-effect transistors. The general approach in analyzing the behavior of the BJT is the same as that applied to the metal–oxide–semiconductor field-effect transistors, namely, to consider first the behavior of an idealized device based on physical principles, and derive its voltage–current relations and small-signal equivalent circuit. High-frequency performance is limited by the effects of stored charges, which are modelled by incremental capacitances, and by physical considerations, such as the time it takes current carriers to move through the base of the BJT. The small-signal equivalent circuit can be readily derived from the preceding description of operation of the BJT. However, in the BJT, base-width modulation has a small secondary effect that makes the output voltage affect the input side.