Op amp Schmitt trigger circuit

- a simple op amp or comparator based circuit and circuit diagram for a Schmitt trigger to remove multiple transitions on slow input signals

The Schmitt trigger is a very useful circuit to be able to design.

The Schmitt trigger is often used when interfacing an analogue signal to digital circuitry. Here it fires at a given voltage and then gives an output that is either one level or another dependent upon the input.

Why a Schmitt trigger?

A the simple comparator circuit using either an ordinary operational amplifier ( op-amp ) or a special comparator chip is often adequate.

However if the input waveform is slow or has noise on 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.

Schmitt trigger applications

A Schmitt trigger is used in most applications where a level needs to be sensed. Even if only a small amount of hysteresis is used, it reduces the multiple transitions that can occur around.

As such the Schmitt trigger applications include many different areas of electronics.

  • Digital to analogue conversion:   The Schmitt trigger is effectively a one bit digital to analogue converter. When the signal reaches a given level it switches from its low to high state.
  • Level detection:   The Schmitt trigger circuit is able to provide level detection. When undertaking this application, it is necessary that the hysteresis voltage is taken into account so that the circuit switches on the required voltage.

  • Line reception:   When running a data line that may have picked up noise into a logic gate it is necessary to ensure that a logic output level is only changed as the data changed and not as a result of spurious noise that may have been picked up. Using a Schmitt trigger broadly enables the peak to peak noise to reach the level of the hysteresis before spurious triggering may occur.

Op amp Schmitt trigger circuit

A basic comparator can be converted into a Schmitt trigger 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.

An op amp Schmitt trigger circuit diagram showing the three resistors that give the hysteresis on the positive feedback
Operational amplifier Schmitt trigger circuit diagram

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.

Schmitt trigger precautions

When using an op amp as a comparator, care must be taken. The op amp chip itself is optimised for closed loop operation with negative feedback. As a result, op amp manufacturers do not guarantee their op amps for use in circuits with no feedback, or with positive feedback as in the case of the Schmitt trigger.

One of the issues is that when an op-amp is used instead of a comparator, the switching speed will not be nearly as high, and it is also unlikely to hit the rails as hard.

Typically comparators are designed for open loop circuit conditions or even use with positive feedback in the case of the Schmitt trigger. They also have an open collector style circuit configuration that is intended for switching hard to the voltages rails as required for logic circuits. For this and many other reasons, comparators will provide much better switching characteristics than an op amp ever could.

Further problems that can arise on some occasions are that when an op amp is driven hard into the rails, it will consume more power than it normally would. An additional problem that can arise is in the form of latching, where the op amp will latch to the voltage rail and remain there not switching regardless of the input levels.

As a result it is often best to use a specific comparator in switching circuits such as these. The circuit diagram remains the same - the only change is that a comparator chip is used instead of an op amp.

By Ian Poole

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