4.1 Introduction – Pulse and Digital Circuits


Diodes, transistors and field-effect transistors (FETs) can be used as linear circuit elements if the operation is restricted to that limited region in the characteristic in which it can be approximated to a straight-line characteristic (small-signal conditions). However, when the input is increased further (under large-signal conditions), these devices no longer behave as linear circuit elements; and the operation can go into the nonlinear region of the characteristic. When these non-linear circuit elements are used in waveshaping applications, the resultant process is termed non-linear waveshaping. In communication systems, sometimes it becomes necessary to eliminate a portion of the input signal, either at a single level or at two independent levels. The circuits that accomplish this task are called clipping circuits. These circuits can also be used to eliminate the noise associated with an input signal. Such clipping circuits are called noise clippers. The parameters of these devices can change with variations in temperature. Thus, in applications where precision is required, it becomes necessary to provide temperature compensation so that temperature variations do not influence the behaviour of the circuit.

Amplitude comparators (also called comparators) are circuits that compare an input with a reference signal and deliver a high output the moment the input reaches the reference level. A simple diode comparator circuit is discussed and possible techniques to improve the sharpness of the break region are presented in this chapter. A comparator circuit may be used to control other circuits with the help of its output; the instant the input reaches a predefined reference level.