Op-amps are incredibly versatile tools used mainly in analog electronics circuits. I have written about op-amps and their applications in filter design, amplifier etc. Here I wanted to write a note on op-amp based Schmitt trigger circuit. While we often use them with negative feedback for linear amplification, there's a powerful behavior that emerges when we modify the feedback path—positive feedback—especially when connected to the non-inverting terminal.
By introducing positive feedback, we can turn a basic op-amp comparator into something more robust—specifically, a comparator with hysteresis. This kind of configuration is widely used in ON/OFF relay control systems where hysteresis is essential, such as in thermostat-based temperature regulation for homes. The resulting circuit is commonly known as the Schmitt trigger.
There are two primary types of Schmitt triggers:
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Inverting Schmitt trigger
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Non-inverting Schmitt trigger
Each behaves slightly differently, but both rely on the same core principle: the output is fed back positively to the input, causing a clear hysteresis window between the high and low thresholds.
Below are shown non-inverting Schmitt trigger circuits:
and inverting Schmitt trigger is shown below:
The beauty of hysteresis is its noise immunity. If the incoming signal has noise—small unwanted variations or ripple—the circuit ignores those minor fluctuations as long as the noise magnitude is less than the hysteresis band. This makes the Schmitt trigger not just a control mechanism but also an excellent signal cleaner.
Some op-amp-based Schmitt trigger designs go a step further, allowing adjustment of both the hysteresis magnitude and the central threshold voltage. This flexibility lets designers fine-tune the response of the circuit to the exact specifications needed. Such circuits are called Adjustable hysteresis Schmitt trigger.
Interestingly, Schmitt triggers are also at home in digital electronics. For instance, integrated circuits like the LM7414 package six Schmitt triggers in one chip—an essential component in many digital signal processing tasks. One of their most valuable roles in digital applications is de-bouncing.
When a mechanical switch is pressed, it doesn't create a clean, single transition. Instead, it rapidly toggles between high and low states before settling. This "bouncing" can confuse digital circuits. A Schmitt trigger smooths out this behavior, ensuring that only a clean signal is passed forward.
So, whether you're working with analog control systems or digital logic inputs, positive feedback with op-amps—via Schmitt trigger configurations—brings precision and reliability to your designs. It's a classic technique with timeless relevance.
