Electronic Design

One Pot Adjusts Polarity, Gain

By adjusting one potentiometer, this circuit's output can be varied from a positive-going version of the input signal, smoothly through zero output, and then to a negative-going version of the input (see the figure). If the input signal is a positive-going pulse of, say, +2 V peak, the output pulse amplitude can be smoothly varied from +2 V through ground (no output) to -2 V peak.

By choosing the appropriate resistor values, the full-scale output can vary from the millivolt range to slightly less than half of the total of the op-amp supply voltage. Consequently, the circuit creates signals useful for testing various transducers, analog-to-digital converters, and so on.

Taking a closer look at the setup, assume a +2 V peak input signal. The A section of the quad op amp is an input buffer, op amp C provides a fixed negative-going output of -4 V peak, and op amp B supplies a positive-going output that varies from +2 V to +6 V peak. The D section adds the B and C outputs. Thus, by varying the B output, the circuit output varies smoothly from -2 V to +2 V peak.

The circuit can, of course, also be used as a 0 /180 phase switcher. For instance, with a ground-centered sine-wave input of 4 V pk-pk, the output varies from 4 V pk-pk in phase with the input, smoothly through 0 V, to 4 V pk-pk 180 out of phase with the input.

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