Schmitt Trigger Threshold Helper

Compute switching thresholds VT+ and VT- (and hysteresis) from resistor ratios and output swing. Includes Show Work and share links.

How to Use

  1. Select a topology (inverting or non-inverting).
  2. Enter your resistor values and reference level (if used).
  3. Enter output swing (VOH / VOL) and (optional) input reference (VREF).
  4. Review thresholds, hysteresis width, and Show Work.
Threshold Readout
Upper/Lower thresholds and hysteresis width.
VT+
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VT-
—
ΔV
—
Mode
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Status: —
Input vs Output (conceptual) V_in V_out V_T+ V_T- Markers move based on computed thresholds (not a circuit simulator).
Inputs & Settings
Enter resistor values and swing. Tool computes thresholds and hysteresis.
Choose based on your comparator/op-amp configuration.
Outputs are shown in this unit (calcs in volts internally).
Typical: 1k–1M (avoid 0 or negative)
Ratio R1:R2 controls hysteresis
Use real swing if not rail-to-rail (e.g., 4.2V).
Often near 0V but not always (e.g., 0.1V).
If unused, leave blank to assume 0V reference in the equations used by this helper.
Optional adjustment added to thresholds (for quick “what-if”).
Show Work (step-by-step)
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Work is shown in base units (volts and ohms). This helper uses idealized threshold equations; real devices may vary with input bias, propagation delay, and output swing.

Reference Formulas

A Schmitt trigger uses positive feedback to create two switching thresholds: VT+ (rising) and VT- (falling). The difference is the hysteresis width ΔV.

  • β (feedback factor): depends on resistor ratio (tool shows the computed value).
  • Thresholds: computed from VOH, VOL, VREF, and resistor ratio per selected topology.
  • Hysteresis: ΔV = VT+ − VT-
Tip: Use measured VOH/VOL (not just supply rails) for best results.

FAQ

What’s the difference between VT+ and VT-?

VT+ is the input level where the output switches when the input is rising. VT- is where it switches when the input is falling. Their separation prevents chatter from noise.

Why do my thresholds exceed the supply rails?

It can happen in ideal math if the resistor ratio and swing values imply it. In real circuits, thresholds are effectively limited by input common-mode range, output swing, and reference constraints. Use the “Clamp thresholds” option if you want bounded readouts.

Should I use real VOH/VOL?

Yes. Comparators and op-amps often don’t swing exactly to the rails, especially under load. Real swing produces more accurate thresholds.

What resistor ranges are reasonable?

Common designs use 1k–1M depending on input bias current, noise, and power. Too low wastes power; too high can increase error from bias currents and leakage.

Tool Info

Last updated:

Updates may include additional topologies, unit polish, and edge-case handling.