Clipper Circuits using Op-amp

Clipper Circuits

The circuits which are used to clip off the unwanted portions of the input voltage above or below certain levels, so as to produce limited required output are called limiting circuits. As some part of input gets clipped off to produce output, these circuits are commonly called clipping circuits. These circuits are classified as,

1. Positive clipper circuit and 2. Negative clipper circuit.

The positive clipper circuits remove some positive part from the input to produce the output. The negative clipper circuits remove some negative part from the input to produce the output.

1. Positive Clipper Circuit

A positive clipper circuit using op-amp is shown in the Fig. 3.21.1.

The clipping level is determined by the reference voltage V_ref. This reference voltage is obtained from the positive supply voltage + V_CC or negative supply voltage -V_EE. In the circuit shown the V_ref is determined by pot R_p, say 2 V. 

In the positive half cycle of the input, the diode D conducts till V_in = V_ref. When V_in is lee than V_ref, the diode D becomes forward biased and op-amp acts as a voltage follower. Hence the output voltage V_o is same as V_in.

But for V_in greater than V_ref, the diode D becomes reverse biased as V' is + V_sat and becomes open. This opens the feed back loop and due to this, output voltage V_o remains at V_ref and entire waveform above V_ref gets clipped off.

Thus the diode is ON when V_in < V_ref and OFF for V_in > V_ref. The output follows input when diode is on when it is alternates between closed loop and open loop operation and hence op-amp used must be high speed and compensated for unity gain. The high speed op-amp like HA 2500, L_M 310 and pA 318 can be used for such applications. The input and output waveforms are shown in the Fig. 3.21.2.

In the same circuit if the pot Rp is used with - V_EE to generate negative V_ref, instead of V_CC then the entire waveforms above - V_ref gets clipped off. Let V_ref = - 2 V_in the same circuit, the output follows input only when V_in < Vref i.e. V_in <-2V. The waveforms with V_ref = -2 V are shown in the Fig. 3.21.3. 

2. Negative Clipper Circuit

The negative clipper circuit can be obtained by reversing the connections of diode D and using pot R_P to generate negative reference voltage V_ref. This circuit is shown in the Fig. 3.21.4.

When V_in > - V_ref, then the diode D conducts and the output voltage follows the input voltage. But when Vin < Vref' D is off and the voltage below - Vref gets clipped off. The circuit hence is called as negative clipper circuit. The waveforms are shown in the Fig. 3.21.4.

If for the same circuit, reference voltage is generated using + V_CC i-e. + V_ref then diode D is ON for V_in > V_ref and V_o = while diode D is OFF for < V_ref and V_o = V_ref. The corresponding waveforms are shown in the Fig. 3.21.6.

Review Questions

1. Explain a positive clipper circuit using an op-amp and a diode with neat diagrams.

April-10, May-17, Marks 8

2. Explain a negative clipper circuit using an op-amp and a diode with neat diagrams.

3. Design a circuit of a clipper which will clip the input signal below a reference voltage.

Dec.-15, Marks 8