When devices such as diodes, bipolar junction transistors (BJTs) and field-effect transistors (FETs) are used in amplifiers, oscillators, rectifiers and other such applications, these devices are used either as linear or nonlinear circuit elements, for which they have to be used in a limited range of the transfer characteristic (defines the relation between the input and the output). If the operation goes beyond the linear region of the transfer characteristic, unwanted frequencies called harmonics—integer multiples of the fundamental frequency—appear in the output of the circuit. However, when the signal swing is large, as in power amplifiers, the output is invariably distorted. This distortion can be minimized using a push–pull configuration as this arrangement eliminates even harmonics. To analyse a given circuit comprising such devices, it is possible to replace the device by its equivalent circuit. To simplify the analysis, it is necessary, at times, to piece-wise linearize the transfer characteristic so that the behaviour of the device can be predicted in that limited region of operation.
These devices—diodes, transistors, FETs and so on—can also be used as switches in switching applications by driving the device into the OFF state in one case and by driving the device into the ON state in the other case. However, the inter-electrode capacitances limit the switching speed. Operational amplifiers and negative resistance devices also find applications in pulse and switching circuits. This chapter presents a brief overview of the fundamentals to facilitate comprehension of the principles of pulse and switching circuits.