4. Reference voltage. The sinusoidal reference voltage must be generated
within the control circuit of the inverter or taken from an outside reference.
It may seem as though the function of the inverter bridge is unnecessary
because a sinusoidal voltage must be present before the bridge can operate
to produce a sinusoidal output. However, there is very little power required
from the reference signal. The power supplied to the load is provided by
the dc power source, and this is the intended purpose of the inverter. The
reference signal is not restricted to a sinusoid, and other waveshapes can
function as the reference signal.