Parallel Clipper Circuits with Reference Voltage VR

       If it is necessary to have reference voltage level for clipping, other than V_γ then an external battery can be used in the basic circuit.

Note : The introduction of battery in series with the diode is effective in clipping the a.c. voltage waveform above the certain reference voltage called bias voltage or reference voltage.

       Reference voltage is denoted as V_R. The Fig. 1 shows the basic parallel positive clipper circuit with reference voltage.

Fig. 1  Positive clipper with reference voltage

1.1 Operation

       The operation for positive half cycle of the input can be divided into two cases.
       Case 1 : V_i is positive but less than V_R

       When V_i is positive but less than V_R, the diode D is reverse biased and acts as open circuit. The equivalent circuit is as shown in the Fig. 2.

Fig. 2  V < V_i < V_R

       Hence the output voltage

      But selecting  R₁ < <R_L,
      V_o = V_i                                         .......... (1)
Case 2 : V_i positive and greater than V_R

Fig. 3   V_i  > V_R 

       Assuming ideal diode, when V_i becomes greater than V_R, the diode D becomes forward biased and acts as short circuit. This is shown in the Fig. 3. The output voltage now is same as voltage of node A which is V_R, as the drop across ideal ON diode is zero.

.^..              V_o =  V_R        for                V_i  > V_R  ............. (2)
       When V_o again becomes less than V_R, the diode D becomes OFF and V_o ≈ V_i .

       For the entire negative half cycle of the input, V_i < V_R  hence diode D remains reverse biased. It acts as open circuit.

Note : The entire negative half cycle is reproduced at the output.
       The input and output waveforms are shown in the Fig. 4.

Fig. 4  Waveforms for positive parallel clipper with V_R 

1.2  Transfer Characteristics

       The mathematical equations for transfer characteristics are,

       The transfer characteristics are shown in the Fig. 5

Fig. 5  Transfer characteristics

1.3  Effect of cut-in Voltage of Diode

       Let the diode is non-ideal with cut-in voltage V_γ = 0.7 V
Note : For diode D to be ON, the potential of node A must be 0.7 V more than V_R.
       So   V_A  = 0.7 +V_R  for diode ON

       During the positive half cycle of the input voltage V_in, as long as V_in < V_A, the diode will not conduct. But when V_in = V_A the diode starts conducting and the output voltage V_o = V_A = 0.7 + V_R. This is shown in the Fig. 6.

Fig. 6

       But when V_in < V_A and throughout negative half cycle the diode is reverse biased and acts as open circuit and hence V_o = V_in. The magnitude of V_o remains same as obtained before, for clipper without V_R.

        Thus entire negative half cycle is reproduced as it is, at the output. The circuit acts as positive clipper with the clipping level as V_A other than V_γ.

       The input-output waveforms are shown in the Fig. 7.

Fig. 7  Biased clipper waveforms with non-ideal diode

       The dashed portion is the clipping region of the circuit. For ideal diode, V_γ = 0 and hence clipping level is V_A = V_R.

Note : Varying the reference voltage up or down, the clipping level of the circuit can be changed, as per the requirement.

The transfer characteristics equations are,

       The transfer characteristics are shown in the Fig. 8

Fig. 8  Transfer characteristics

1.4 Parallel Negative Clipper With Reference Voltage V_R
       The Fig 9 shows parallel negative biased clipper with bias reference voltage Vr. This is obtained by reversing the direction of diode in a parallel positive clipper with reference voltage V_R.

Fig 9 parallel negative clipper with V_R

Operation : Assume Ideal Diode
       During positive half cycle of the input, when V_i= 0 V, due to Vr diode is forward biased immediately.
       The voltage of node A is V_A= V_R when diode is ON as shown in the Fig 10
                      .^.. V_o =V_A  =V_R ....... D ON
The condition exists as long as V_i ˂ V_R.

Fig. 10 ON, V_i ˂ V_R

       When increasing V_i becomes greater than V_R, the diode D becomes reverse biased. The circuit is as shown in the Fig 11.

Fig.11 D OFF, V_i ˃ V_R

      The output voltage follows the input voltage.

       when V_i becomes less than V_R, the diode D again becomes ON and V_o = V_R. The diode D remains ON for entire negative half cycle of the input and for which V_o = V_R.
Note : The entire negative half cycle of the  input is clipped off and the circuit acts as parallel negative clipper.
       The input and output waveforms are shown in the Fig 12.

Fig. 12 Waveforms for negative parallel clipper with V_R

       Transfer characteristics : the equations for the transfer characteristics are,

       The transfer characteristics are shown in the Fig 13

Fig. 13 Transfer characteristics for negative clipper with V_R

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