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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

For a symmetrical square wave input of pulse duration 100 uS the circuit now acts as a long CR. The time constant is:

CR = 0.005 x 10-6 x 106 = 5,000 uS.

This is greater than ten times the pulse duration.

When the diode is conducting, R is shunted by the very low impedance of the conducting diode and the effective resistance is merely that of the diode RD, in this example 1kW. For the same square wave input the circuit now acts as a short CR where the time constant is:

CRD = 0.005 x 10-6 x l03 = 5 uS.

This is less than one-tenth of the pulse duration of the input.

Thus a clamping circuit acts either as a long CR or as a short CR depending upon whether the diode is cut off or conducting.

Positive Clamping to Zero Volts

Fig 4a illustrates a positive clamping circuit. In this the anode of the diode is earthed and the bottom of the output waveform is clamped to zero volts. The output waveform thus varies between zero and a positive value (Fig 4b). Note that since R and the diode are in parallel the output voltage always equals the voltage VR developed across R.

In addition, as in any CR circuit, the input voltage Vin = VC + VR at all times. The waveforms of Fig 4b are obtained as follows:-

A to B. The input rises by 100V from zero. The capacitor C is initially uncharged and cannot change its charge immediately. Hence VR rises instantly to +100V and since this voltage is applied to the cathode of the diode, the anode being earthed, the diode is cut off.

B to C. With the diode cut off C charges on a long time constant of CR seconds and VC rises by a small amount. Thus VR falls by the same amount. 


 

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