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Waveform Generators

# Op-Amp Astable Multivibrator

This multivibrator provides a square-wave signal with an output voltage swing
of approximately 0 to *V*_{CC}. The multivibrator circuitry
consists of an op-amp,
*IC*_{1}, and the associated components shown in the figure above.
Since a single power supply (*V*_{CC}) is used for the op-amp,
equal value resistors, *R*_{1} and *R*_{2}, are used
to provide a bias voltage at the positive input of the op-amp equal to
one-half of the supply voltage.

The multivibrator circuitry waveforms are presented in the figure above.
Resistor *R*_{3} provides positive feedback and, in conjunction
with resistors *R*_{1} and *R*_{2}, determines the
swing of the square-wave voltage about the bias point at the positive input of
the op-amp. When *R*_{1}, *R*_{2}, and
*R*_{3} are equal, the swing of the voltage at the positive input
is from *V*_{CC}/3 to *V*_{CC}*2/3.
The charging and discharging of the R-C network,
*R*_{4} and *C*_{1}, produces an almost triangular
waveform at the negative input of the op-amp.
A change of state of the op-amp output occurs when the negative input voltage
level crosses the voltage at the positive input of the op-amp. Thus, the
op-amp functions as a comparator, operating in a saturated switching mode,
at a frequency *f* determined by the *R*_{4}*C*_{1}
time constant. When *R*_{1}, *R*_{2}, and
*R*_{3} are equal, the frequency can be calculated as