Q.With neat circuit diagrams and waveforms explain the applications of comparator.

When used in open loop (no negative feedback), an op-amp comparator saturates its output; two important applications are the zero crossing detector and the Schmitt trigger, both of which convert a varying signal into a square wave. …

[!TLDR]

An open-loop op-amp comparator saturates to $\pm V_{sat}$; its two key applications are the zero crossing detector (switches at every zero crossing) and the Schmitt trigger (switches at $\pm V_{ref}$ with positive feedback, giving noise immunity) — both convert a varying signal into a square wave.

Comparator basics: Without negative feedback the op-amp works in open loop, where one input is compared with the other. The output $V_O = A(V_2 - V_1)$, and since the open-loop gain $A$ is of the order of $10^5$ or more, even a few millivolts of difference saturates the output to $+V_{CC}=+V_{sat}$ or $-V_{EE}=-V_{sat}$. Two important applications follow.

(1) Zero Crossing Detector (Fig 5.9.6):

• Circuit: the time-varying signal $V_{in}$ is applied to the non-inverting ($+$) input and the inverting ($-$) input is grounded.
• Operation:

• Every time $V_{in}$ crosses the zero level, the output $V_O$ switches state — hence the name Zero Crossing Detector.
• Waveform: a sine-wave input produces a square-wave output whose transitions occur exactly at the zero crossings, so the circuit converts a sine wave into a square wave.
• Drawback: it changes state for small noise signals near zero.

(2) Schmitt Trigger (Fig 5.9.7):

• Circuit: a comparator with the input $V_{in}$ on the inverting ($-$) input and a positive-feedback resistor divider $R_1$-$R_2$ from the output to the non-inverting ($+$) input, whose junction sets the reference voltage $V_{ref}$.
• Operation:

where the trip (reference) voltage is fed back positively:

$V_{ref} = \beta V_O = \frac{R_2}{R_1 + R_2}\,V_O \ \Rightarrow\ V_{ref} = +\beta V_{sat} \text{ or } -\beta V_{sat}.$ …