Schmitt Trigger Circuit

- a tutorial, overview or summary of the basics of the Schmitt trigger circuit using and operational amplifier.

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A comparator circuit, whether using an operational amplifier or some other electronics technology will work well under some conditions but it is not always ideal. If there is a slow waveform, or one with some noise ion it, then there is the possibility that the output will switch back and forth several times during the switch over phase as only small levels of noise on the input will cause the output to change.

This may not be a problem in some circumstances, but if the output from the operational amplifier comparator is being fed into fast logic circuitry, then it can often give rise to problems as the circuit will see several low high or high low transitions and will respond to each one. This can easily cause many problems.

Under these circumstances circuits that combat this problem are require. One known as the Schmitt trigger has been in use for many years, having been originally invented by an American scientist named Otto Schmitt. The Schmitt trigger switches at different voltages depending upon whether it is moving from low to high or high to low, employing what is termed hysteresis.

In terms of the fact that the Schmitt trigger has hysteresis, the circuit symbol for one of these circuits incorporates the hysteresis symbol into it. Accordingly all Schmitt triggers use this symbol.

Schmitt Trigger Circuit Symbol
Schmitt Trigger Circuit Symbol

Schmitt trigger circuit

The problem can be solved very easily by adding some positive feedback to the operational amplifier or comparator circuit. This is provided by the addition of R3 in the circuit below and the circuit is known as a Schmitt trigger.

Operational amplifier Schmitt trigger circuit

Operational amplifier Schmitt trigger circuit

The effect of the new resistor (R3) is to give the circuit different switching thresholds dependent upon the output state of the comparator or operational amplifier. When the output of the comparator is high, this voltage is fed back to the non-inverting input of the operational amplifier of comparator. As a result the switching threshold becomes higher. When the output is switched in the opposite sense, the switching threshold is lowered. This gives the circuit what is termed hysteresis.

The fact that the positive feedback applied within the circuit ensures that there is effectively a higher gain and hence the switching is faster. This is particularly useful when the input waveform may be slow. However a speed up capacitor can be applied within the Schmitt trigger circuit to increase the switching speed still further. By placing a capacitor across the positive feedback resistor R3, the gain can be increased during the changeover, making the switching even faster. This capacitor , known as a speed up capacitor may be anywhere between 10 and 100 pF dependent upon the circuit.

It is quite easy to calculate the resistors needed in the Schmitt trigger circuit. The centre voltage about which the circuit should switch is determined by the potential divider chain consisting of R1 and R2. This should be chosen first. Then the feedback resistor R3 can be calculated. This will provide a level of hysteresis that is equal to the output swing of the circuit reduced by the potential divide formed as a result of R3 and the parallel combination of R1 and R2.

By Ian Poole

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