How it works
A comparator operates the op-amp open-loop — no feedback resistor. When V+ > V−, output saturates at +Vsat ≈ VCC − 1.5 V; when V+ < V−, output goes to −Vsat ≈ −VCC + 1.5 V. Switching happens at the threshold voltage. A zero-crossing detector connects Vin to V+ and V− to ground: output is high for positive Vin and low for negative. Noise near the threshold causes multiple false switching — solved by adding hysteresis. Schmitt trigger adds positive feedback: upper threshold VUT = Vref + (Rf/R1) × Vsat and lower threshold VLT = Vref − (Rf/R1) × Vsat. The gap between VUT and VLT is the hysteresis window. For Vref = 0, R1 = 10 kΩ, Rf = 100 kΩ, and Vsat = 13 V: VUT = +1.3 V, VLT = −1.3 V, window = 2.6 V.
Key points to remember
Key comparator types: zero-crossing detector (threshold at 0 V), non-inverting comparator (threshold = Vref on inverting input), and Schmitt trigger (with hysteresis). Hysteresis prevents output chattering near the threshold due to noise — essential in industrial sensor circuits. The hysteresis window width = 2 × (Rf/R1) × Vsat for a symmetric Schmitt trigger centred at zero. Dedicated comparators like LM339 and LM393 have open-collector outputs requiring a pull-up resistor (typically 4.7 kΩ to VCC) and switch far faster than op-amps. Using a μA741 as a comparator is poor practice because its slew rate of 0.5 V/μs causes slow transitions and potential latch-up near the threshold.
Exam tip
The examiner always asks you to calculate the upper and lower threshold voltages of a Schmitt trigger — use VUT = (R2/R1) × Vsat and VLT = −(R2/R1) × Vsat for the zero-centred case, and draw the transfer characteristic with hysteresis for full marks.