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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 5

Clamping Circuits

Summary of Diode Clamping Circuits

A waveform is positively clamped if it is positive-going from the clamping level. It is nega-tively clamped if negative-going from the clamping level. A simple rule for deciding quickly if a circuit is a positive or a negative clamp is to imagine the anode of the diode as a piston about to begin a stroke towards the cathode, pushing the waveform in the process. Then if the piston stroke is 'down' towards the cathode the waveform is being pushed downwards and the circuit is a negative clamp (Fig. 9a). If the movement is upward, as in Fig. 9b., the circuit is a positive clamp. The clamping level in each case is the voltage to which the resistor R is returned.

Although all the square wave inputs considered have been symmetrical the clamping circuits will work equally well with asymmetrical inputs provided the short CR circuit formed by the conducting diode and C has a time constant less than one-tenth of the pulse duration of the narrow pulse.

In addition, note that sine wave inputs may be clamped to a required voltage level in exactly the same way as square wave inputs.

Grid Current Clamping

Clamping may occur in the grid circuit of a triode or pentode amplifier if grid current is allowed to flow, because the grid-cathode path acts as a diode in the same way as in the grid current limiter (see p. 88). This clamping at the grid may be wanted or it may be unwanted.

If a waveform is applied through a long CR circuit to the grid of an amplifier operating at zero bias (Fig. 10a), grid current flows and the coupling capacitor is charged in the same way as that of the capacitor in a diode clamping circuit. Thus, with no bias, the positive peak of an incoming signal is clamped to zero volts and the input waveform is said to be negatively clamped to zero volts (Fig. 10b). The action is very similar to that for obtaining automatic grid leak bias in an oscillator.

Note that grid current clamping does not occur in a normal Class A amplifier because grid current is not normally allowed to flow. Even if grid current did flow to cause clamping at the grid the amplified output is taken from a different part of the circuit so that it has its own d.c. level. It is only the input waveform which may be clamped to zero volts.

If the cathode follower amplifier shown in Fig. 10c is used, positive clamping to a variable bias level + V may be obtained. The negative peak of an incoming signal is now clamped to the voltage V.


 

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