Q.How can you use an op amp as a comparator?

A comparator uses an op-amp in open loop (no feedback) to compare two input voltages. Because the open-loop gain is enormous, the tiniest difference between the inputs drives the output hard to one of its two saturation levels: the output goes to positive saturation $+V_{sat}$ when the non-inverting input exceeds the inverting input, and to negative saturation $-V_{sat}$ when it is lower. The comparator therefore gives a two-state (digital-like) output that reports which input is larger — the classic open-loop application of an op-amp.

Two important circuits are built on this action:

• A Zero Crossing Detector (ZCD) is a comparator with one input tied to ground (zero reference). Its output switches every time the input signal crosses zero volts, so a sine-wave input is converted into a square wave. This is used to detect zero crossings for timing, frequency measurement and waveform conversion. …