Schmitt Trigger Explained

Schmitt Trigger

A Schmitt Trigger is a comparator circuit with hysteresis, meaning it has two different threshold voltages.

• Upper Threshold (V_UT) → LOW to HIGH transition
• Lower Threshold (V_LT) → HIGH to LOW transition

This prevents noise from causing multiple unwanted transitions.

Hysteresis Concept

• Input rising → switches at V_UT
• Input falling → switches at V_LT

Why useful?

• Cleans noisy signals
• Converts analog to digital
• Waveform shaping

Input–Output Behavior

• If input > V_UT → Output = HIGH
• If input < V_LT → Output = LOW
• If V_LT < input < V_UT → Output remains unchanged

Mathematical Analysis (Op-Amp Schmitt Trigger)

Circuit parameters:

• R₁: from output to positive input
• R₂: from positive input to ground
• Output levels: +V_sat and −V_sat

Threshold Voltages

V_UT = +V_sat × (R₂ / (R₁ + R₂))

V_LT = −V_sat × (R₂ / (R₁ + R₂))

Hysteresis Width

V_H = V_UT − V_LT

V_H = 2 × V_sat × (R₂ / (R₁ + R₂))

Example

If:

• V_sat = 10V
• R₁ = R₂

Then:

• V_UT = +5V
• V_LT = −5V

Applications

• Noise filtering
• Signal conditioning
• Square wave generation
• Switch debouncing

Summary

Positive feedback shifts the threshold depending on output.

• Output HIGH → threshold increases
• Output LOW → threshold decreases

This creates a memory effect

• Comparator with hysteresis
• Uses positive feedback
• Two switching thresholds
• Improves signal stability