Monolithic op amps are inexpensive, versatile, and reliable. They can
be used not only for linear circuits like voltage amplifiers, current
sources, and active filters but also for nonlinear circuits such as
comparators, waveshapers, and active-diode circuits. The output of a
nonlinear op-amp circuit usually has a different shape from the input
signal because the op amp saturates during part of the input cycle.
Because of this, we have to analyze two different modes of operation
to see what happens during an entire cycle.
