Effect of Cut-in Voltage of Diode

Parallel Clippers : Part2

       Let us use linear piecewise model of diode, in which diode conducts when potential of node A becomes just equal to cut-in voltage of the diode. And once diode starts conducting, it acts as a voltage source of value 0.7 V for silicon, neglecting internal resistance r_f of diode. Hence V_A = 0.7 V for the conducting diode.

Note : As V_o = V_A , the output voltage is 0.7 V till V_in >0.7 V.
       This is shown in the Fig. 1.

Fig. 1  Basic parallel clipper (Positive half cycle)

       Thus almost entire positive half cycle gets clipped off.

       In the negative half cycle of the input voltage V_in, when V_in is less than 0.7 V, the diode becomes reverse biased and acts as an open circuit. Hence current I flows through  R₁ and R_L. The output voltage is negative part of the input voltage whose magnitude is given by,

 Note : Thus output follows the input and negative cycle of V_in gets reproduced at the output.
       This is shown in the Fig. 2.

Fig. 2  Basic parallel clipper (Negative half cycle)

       The overall input-output waveforms can be shown as in the Fig. 3(a) using linear piecewise diode model.

Fig. 3  Piecewise model

Transfer characteristics :

       It is known that the transfer characteristics is the graph of output voltage against input voltage with time t as an implicate variable.

       For the basic parallel clipper circuit we can write,

       Using the equations the transfer characteristics can be obtained as shown in the Fig. 3.

Fig. 4  Transfer characteristics

Note : The portion for which V_in < V_γ is called the transmitting region while the portion for which V_in > V_γ is called limiting region or clipping region.

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