Electronic Design

Build A Voltage-Controlled Dual Clipper

In the clipper circuit shown, VIN is the input signal, V1 and V2 are the dc reference voltages (clipping levels), and VO2 is the output signal (Fig. 1). The Q1/Q2 npn transistor pair acts as a “High Level Signal” pass circuit.

For VIN > V1, transistor Q1 is in its active region, transistor Q2 is off, and VO1 = VIN. For VIN < V1, VO1 = V1. (VO1 will have a −0.6 V level shift due to the VBE of the Q1/Q2 transistor pair). Similarly, the Q3/Q4 pnp transistor pair produces an output signal at VO2, which is the lowest voltage level of the VO1 and V2A input signals.

Transistor Q5 acts as a level shifter to compensate for a VBE drop of Q4. VIN and V1 don’t experience any dc shift due to cancellation of VBE drops. If a 6-V p-p sine wave is applied at the VIN input, V1 = −2 V, and V2 = +2 V, the resulting output waveform is as shown in Figure 2.

Other reference polarities could be used. The circuit has reasonably highfrequency response and accuracy. The input and reference voltages should be such that base-emitter breakdown voltages are not exceeded.

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