What is a Schmitt Trigger Circuit?

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Editorial Team - everything PE

Feb 9, 2024

A Schmitt trigger or regenerative comparator is a type of electronic circuit that incorporates positive feedback to create hysteresis. Hysteresis means that the input voltage required to change the output state is different depending on whether the circuit is transitioning from a high to low state or from a low to high state. This hysteresis property makes the Schmitt trigger particularly useful in applications where noise or signal fluctuations may cause undesired state changes in standard comparator circuits.

Here's a simplified explanation of how a Schmitt trigger works:

• Initial State: Assume the output is initially in a low state.
• Input Rises: As the input voltage rises, the output stays low until it reaches a certain threshold called the upper threshold (high-level threshold). When this threshold is crossed, the output quickly switches to a high state.
• Positive Feedback: The positive feedback comes into play here. As the output goes high, it is fed back to the inverting or non-inverting input, reinforcing the high state.
• Input Falls: Now, as the input voltage falls, the output remains high until it reaches a lower threshold (low-level threshold). Once this lower threshold is crossed, the output quickly switches back to a low state.
• Positive Feedback Again: The positive feedback now reinforces the low state, and the process repeats.

A Schmitt trigger circuit can be designed using op-amps in two ways. If the input signal is connected to the inverting terminal of the op-amp, it is known as the inverting Schmitt trigger. If the input signal is connected to the non-inverting terminal of the op-amp, it is known as a non-inverting Schmitt trigger.

Inverting Schmitt Trigger: The input is applied to the inverting terminal of the op-amp and positive feedback is applied from the output to the input.If the applied voltage Vin is greater than V, the output of the circuit will be low. And if the applied voltage Vin is less than V, the output of the circuit will be high. To determine the voltage V, Kirchoff’s current law (KCL) is applied.

Inverting Schmitt Trigger Circuit

If the applied voltage Vin is greater than V, the output of the circuit will be low. And if the applied voltage Vin is less than V, the output of the circuit will be high. To determine the voltage V, Kirchoff’s current law (KCL) is applied.

Simplifying,

With the assumption that the output of the Schmitt trigger is high, Vout = Vand V=V1,

When the input signal is greater than the V1, the output of the Schmitt trigger will become low. Hence, V1 is an upper threshold voltage (VUT).

The output will stay low until the input signal is less than V. At this state,

The output will stay high until the input signal is less than V2. Hence V2 is known as lower threshold voltage(VLT).

Non-Inverting Schmitt Trigger: The input is applied to the non-inverting terminal of the op-amp and positive feedback from the output to the input is applied. The inverting terminal of the op-amp is connected to the ground terminal.

Non-Inverting Schmitt Trigger Circuit

Simplifying,

In the above Schmitt trigger circuit, the output will be high when the voltage V is greater than zero and the output will be low when voltage V is less than zero. To determine the voltage V, Kirchoff’s current law (KCL) is applied at node V.

With the assumption that the output of the Schmitt trigger is low, Vout = Vand V=V1,

When the voltage V1 is greater than zero, the output will be high, and hence

When the above condition is satisfied, the output will be high and this equation gives the value of upper threshold voltage (VUT).

Now, assuming that the output of the Schmitt trigger is high and voltage V is equal to V2

When the voltage V1 is less than zero, the output will be low, and hence

This equation gives the value of the lower threshold voltage (VLT).