What is the trip-point formula for a Schmitt trigger?

With the feedback resistor R1 between the output and the switching node, and the reference resistor R2 between that node and ground, the feedback fraction is beta = R2 / (R1 + R2). Each trip point equals beta times the output voltage at that saturation state: UTP = Vout(high) x beta and LTP = Vout(low) x beta. Hysteresis is the difference between them.

What is the difference between single-supply and dual-supply mode?

In single-supply mode the output swings from 0 V to Vcc, so the lower trip point sits at 0 V and the upper trip point is Vcc x R2/(R1+R2). In dual-supply mode the output swings symmetrically between -Vsat and +Vsat, so the trip points are placed symmetrically above and below 0 V and the hysteresis band is twice as wide for the same resistor ratio.

Why use a Schmitt trigger instead of a plain comparator?

A plain comparator has one threshold, so any noise riding on a signal near that threshold makes the output switch back and forth rapidly. A Schmitt trigger has two thresholds (UTP and LTP) separated by the hysteresis band, so the signal has to move past the opposite threshold before the output switches again, which suppresses chatter.

How do I widen or narrow the hysteresis band?

Hysteresis is proportional to R2 / (R1 + R2), so increasing R2 relative to R1 widens the band and increasing R1 relative to R2 narrows it. The supply or saturation voltage scales the whole band linearly as well.

Should I enter the supply voltage or the actual saturation voltage?

Most op-amps do not swing all the way to the rails. If you know the real output saturation voltage of your comparator, enter that for the most accurate trip points. If you only know the supply rails and are using a rail-to-rail output part, entering the supply voltage is a reasonable approximation.

Schmitt Trigger Calculator

Enter the feedback resistor R1, the reference resistor R2, and the supply voltage to calculate the upper trip point, lower trip point, and hysteresis band of an op-amp Schmitt trigger.

Schmitt Trigger Configuration

Supply Mode

Single Supply Dual Supply

Output swings 0 V to Vcc, reference resistor R2 goes to ground. Lower trip point is 0 V.

Feedback Resistor (R1)

Connects the comparator output to the switching node.

Reference Resistor (R2)

Connects the switching node to ground.

Supply Voltage (Vcc)

Assumes a rail-to-rail output swinging from 0 V to Vcc. Use the real output saturation voltage for a non rail-to-rail comparator.

Results

Enter R1, R2, and the supply voltage, then click Calculate

Summary

An op-amp Schmitt trigger uses positive feedback through two resistors to give a comparator two switching thresholds instead of one, so small amounts of noise near the trip point cannot cause the output to chatter. This calculator takes the feedback resistor (R1, between the output and the switching node) and the reference resistor (R2, between that node and ground) and computes the upper trip point (UTP), lower trip point (LTP), and the hysteresis band between them. Choose single-supply mode for an output that swings between 0 V and Vcc, or dual-supply mode for an output that swings symmetrically between -Vsat and +Vsat.

How it works

Choose single-supply mode (output swings 0 V to Vcc, referenced to ground) or dual-supply mode (output swings symmetrically between -Vsat and +Vsat).
Enter the feedback resistor R1 and select its unit (Ω, kΩ, or MΩ).
Enter the reference resistor R2, the one connecting the switching node to ground, and select its unit.
Enter the supply voltage (single-supply mode) or the saturation voltage (dual-supply mode).
Click Calculate to get the upper trip point, lower trip point, hysteresis band, and feedback fraction.

Use cases

Debounce a noisy mechanical switch or push-button input.
Turn a slow-ramping analog sensor signal into a clean digital edge.
Size a hysteresis band for a threshold detector, such as a temperature or light sensor trip circuit.
Check resistor values before building a comparator or relaxation-oscillator prototype.
Compare how much a real op-amp saturation voltage narrows the trip points versus an ideal rail-to-rail assumption.
Learn or teach comparator hysteresis and trip-point theory with instant numbers.

Frequently Asked Questions

Last updated: 2026-09-09 · Reviewed by Nham Vu