AP 3302 Pt. 3
Section 2
CHAPTER 5
Clamping Circuits
Introduction
We have seen that limiters prevent a waveform from rising above or falling below a pre-determined voltage by cutting off part of the waveform. However in radar there is also a need to change the reference level of a waveform without reducing its amplitude. Circuits which move waveforms 'up' or 'down' in this way are known as clamping circuits because their effect is to fix or clamp the top or bottom level of the waveform to a required voltage. The difference between limiters and clamping circuits is illustrated in Fig 1.
When a square wave is applied to a CR circuit the voltage developed across the resistor is such that its mean value is always zero, the d.c. component of the input being 'blocked' by the capacitor (Fig 2a). If we take the voltage across the resistor and apply it as the input to a clamp-ing circuit any of the waveforms shown in Fig 2b (and many more) may be obtained. The waveform may be made to vary with respect to any reference level, its amplitude remaining un-changed.
An alternative name for clamping circuits is 'd.c. restorers'. This name originates from the fact that any waveform, after passing through a capacitor, has its d.c. component removed and it is often necessary to 'restore' the d.c. component of the input signal in the output waveform.
