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.
