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Schmitt trigger (hysteresis)

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Objective

Add positive feedback to a comparator so it switches at two different thresholds ±Vth.

Procedure

Run and read where the output switches on the input waveform.
Change R2 to 4 kΩ: the thresholds widen to ±2.6 V.

Theory

Feedback divider R2 = 9 kΩ (output → +) and R1 = 1 kΩ (+ → ground) puts the threshold at Vth = ±Vsat·R1/(R1+R2) = ±13 · 0.1 = ±1.3 V. The input must cross +1.3 V to switch low and −1.3 V to switch high.

Questions to answer

Why does hysteresis help with noisy signals?

Components used

2 × Battery / DC source, Function generator (sine), Op-Amp LM741, Oscilloscope (2-channel), 2 × Resistor 220 ohm 1/4W

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.