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Op Amp Schmitt Trigger

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A Comparator with Different On and Off Voltages

Uses Positive Feedback

To fully undertand Schmitt triggers you first need to understand ...

Here is a problem that the Schmitt trigger solves. You have built a circuit to turn on some lights when it gets dark. But when the lights turn on, it gets brighter so the lights go off again. But then it is too dark so they come on again - flashing on and off rapidly.

The Schmitt trigger solves this problem by having different threshold voltages for turning on and off.

When it gets dark, the lights come on. This makes it lighter but not light enough to turn the lights off again. Later when it gets light again, the lights go off. This makes it darker but not dark enough to turn the lights on again.

This behaviour is called hysteresis. The Schmitt trigger uses positive feedback to achieve this effect.

Simple Inverting Circuit

The input signal is connected to the inverting input.
This is the simplest Schmitt trigger with positive feedback.
It has the advantage of near infinite input impedance.
The switching levels are symetrical because R1 is connected to ground.
For non symetrucal levels, connect R1 to a fixed reference voltage other that zero.

Op Amp Inverting Schmitt Trigger Simple Version

Alternate Simple Non Inverting Circuit

Alternate Simple Inverting Schmitt Trigger

A More Versatile Inverting Circuit

This is a comparator converted into a Schmitt trigger by adding one positive feedback resistor to give different on and off switching levels.

Op Amp Inverting Schmitt Trigger Comparator

Oscilloscope plot showing the Voltage Transfer Characteristic of the Schmitt Trigger

Op Amp Inverting Schmitt Oscilloscope View

Noise Clean Up

As this is an inverting Schmitt Trigger, the output pulse is inverted relative to the input. Here you can see a very noisy input that would produce errors in four or five places with a simple comparator using the grey line reference voltage. The Schmitt trigger has two reference voltages. The input must go above the red line for a low output and below the blue line for a high output. This removes all noise unless the noise signal is so large that it exceeds the difference between the reference voltages. Although the amplitude noise is removed, there is still some timing noise called Jitter. The pulses might switch slightly too soon or too late. This can be removed with other digital techniques beyond A Level scope.

Inverting Schmitt Trigger used to Regenerate a Noisy Signal

Single power rail simple Inverting Schmitt Trigger

Single Power Rail Simple Schmitt Trigger

This uses a voltage divider to produce a six volt level.
The upper and lower switching levels in this exmple are 3V and 9V.
The feedback resistors need to be much larger than the voltage divider resistors for this circuit to behave well.

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