The amplitude of vm(t) is set by an impulse amplitude modulator circuit.
The output square wave of the impulse duration modulator drives the output vm(t) of the switch SW to be equal to +vx during the time interval t1, and to –v during the time interval t2 (Figure 3.11(a)).
Then, after an initial transient, the output voltage v (t) of the low-pass filter (integrator) is the mean value of vm(t):