The final stage of the servo controller circuit is a comparator (Comparator 2
in Figure 
).
The integrator output is compared to the (amplified) DC signal.
This comparator circuit utilizes the second 741 op-amp on the LM324
Quad Op-Amp chip shown in Figure .
Whenever the integrator signal amplitude is greater than the DC
input, the output of the comparator is high (+Vcc). Whenever it is less
than the DC input, the output of the comparator is low (0 V). This operation
is shown for an arbitrary signal in Figure . Consider
the time-varying periodic signal as compared to the DC
threshold level. When the threshold is set to level 1, the
time-varying signal exceeds the threshold only for a relatively short
duration near its peak. During this time, the output signal from the
comparator is a rectangular pulse train with a duty cycle as shown
in Figure . If the threshold is reduced to level 2, the
time-varying signal exceeds the threshold for a longer duration and the
duty cycle is increased.
It should now be apparent that raising the DC signal voltage, or threshold level, will decrease the duty cycle of the comparator's output and lowering the DC signal voltage will increase the duty cycle of the output.
