AP 3302 Pt. 3
Section 2
CHAPTER 5
Clamping Circuits
In Fig. 7b before the input is applied, C charges to the bias voltage VB (in this example - 25V) and remains steady at this value. VR is zero and Vout = + VB = + 25V. Now let us consider the action from this point:
A to B. The input rises by 100V from zero. Since C cannot charge immediately Vc remains at - 25V and the voltage step of 100V is developed across R. Thus VR rises immediately to + 100V, and the diode cuts on. The output Vout rises to 100 + 25 = 125V.
B to C. With the diode conducting, C charges on a short time constant of CRD seconds and Vc rises quickly to + 75V as VR falls to zero. When VR reaches zero the output voltage Vout has fallen to + 25V. The diode is now cut off
C to D. The input falls by 100V to zero. Since Vc cannot change instantly both VR and Vout fall by 100V, VR to - 100V and Vc to - 75V. The diode remains cut off and Vc remains at + 75V.
