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Zero-crossing detector

Analog Circuits Lab · free online experiment, runs in your browser with no install or sign-up.

Open this experiment in the circuit simulator

Objective

Turn a sine into a square wave with an open-loop op-amp comparator that switches at 0 V.

Procedure

Run and compare the sine with the square output.
Move the reference input from 0 V to 1 V: the switching point moves.

Theory

With no feedback the gain is huge, so the output sits at +Vsat when Vin > 0 and at −Vsat when Vin < 0.

Questions to answer

What happens with noise near the crossing?

Components used

2 × Battery / DC source, Function generator (sine), Op-Amp LM741, Oscilloscope (2-channel)

About the model

Time scale: for the browser simulator the source frequencies are divided by 100 and capacitors / inductors multiplied by 100; the waveforms keep their shape but run 100 times slower. Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.