Diode Clampers Lecture Notes PDF

Lecture 12: Diode Clampers Diode clampers are widely used in television receiver and FM (frequency modulation) transmitters, where the variable frequency interference can be removed by using clamping method. Diode Clampers A clamper adds a dc level to an ac voltage. Clampers are sometimes known as d...

Lecture 12: Diode Clampers Diode clampers are widely used in television receiver and FM (frequency modulation) transmitters, where the variable frequency interference can be removed by using clamping method. Diode Clampers A clamper adds a dc level to an ac voltage. Clampers are sometimes known as dc restorers. Figure 46 shows a diode clamper that inserts a positive dc level in the output waveform. 1  Figure 46: Positive clamper operation. The operation of this circuit can be seen by considering the first negative half-cycle of the input voltage. When the input voltage initially goes negative, the diode is forward biased, allowing the capacitor to charge to near the peak of the input (VP(in) - 0.7 V), as shown in Figure 46(a). Just after the negative peak, the diode is reverse- biased. This is because the cathode is held near (VP(in) - 0.7 V) by the charge on the capacitor. The capacitor can only discharge through the high resistance of RL. For a good clamping action the RC time constant should be at least 10 times of the period of the input frequency. The capacitor voltage acts as a battery in series with the input voltage, the dc voltage of the capacitor adds to the input voltage. 2 The shape of the input signal of a clamper is not changed. If the diode is turned around, a negative dc voltage is added to the input voltage to produce the output voltage as shown in Figure 47. Example: What is the output voltage that you would expect to observe across RL in the clamping circuit of Figure below? Assume that RC is large enough to prevent significant capacitor discharge. 3  Solution Ideally, a negative dc value equal to the input peak less the diode drop is inserted by the clamping circuit, VDC= - (Vp(l>o - 0.7 V) = -(24 V - 0.7 V) = -23.3 V Actually, the capacitor will discharge slightly between peaks, and, as a result, the output voltage will have an average value of slightly less than that calculated above. The output waveform goes to approximately +0.7 V. as shown in Figure below Review Questions: 1. Discuss how diode clampers operate. 2. What is the difference between a positive clamper and a negative clamper? 3. What component in a clamping circuit effectively acts as a battery? 4