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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

C to D. The input falls by 100V to zero and since VC cannot change immediately VR also falls by 100V to a small negative potential. This causes the diode to cut on.

D to E. With the diode conducting, C discharges on a short time constant of CRD seconds. Both VC and VR quickly return to zero volts and the diode is cut off.

E to F. The input rises again by 100V from zero and the cycle is repeated.

Except for the small negative pips the output VR is clamped to a base level of zero volts and is positive-going from this level. DC restoration has taken place and the mean level of the output equals that of the input. It will be remembered that the mean value of voltage for VR in a long CR in the absence of clamping is zero.

For a negative-going square wave input the action is as follows (Fig 5):

A to B. The input falls by 100V from zero and, since VC cannot change instantly, VR falls to -100V. This causes the diode to conduct.

B to C. With the diode conducting, C charges on a short time constant of CRD seconds and VC quickly falls to -100V. VR returns to zero in the same time and the diode is cut off.

C to D. The input rises by 100V to zero. VC cannot change instantly and remains at -100V so VR rises by 100V from zero to +100V. The diode remains cut off.

D to E. With the diode cut off, C discharges on a long CR towards zero volts but rises by only a small amount in the time available. VR falls by the same amount.

Thereafter the action is as described for the positive-going input. Apart from the initial negative spike in Fig 5, the output VR is the same in both cases. In Fig 5, the mean d.c. level of -50V is restored to +50V in the output. No matter what values the input varies between, the output from the circuit described is clamped to zero volts and rises positively by an amount equal to the amplitude of the input.

Negative Clamping to Zero Volts

Fig 6a illustrates a negative clamping circuit. In this the cathode of the diode is earthed and the top of the output waveform is clamped to zero volts. The output waveform thus varies between zero and a negative value (Fig 6b). The waveforms of Fig 6b are obtained as follows:

A to B. The input rises by 50V from zero. C cannot change its charge instantly so VC remains at zero volts and VR rises to +50V, cutting on the diode.


 

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