Hysteresis is a fundamental concept in many electronic circuits, including Schmitt trigger circuits. It refers to the behavior where a system's output response is not only determined by its current input value but also by its past history or the path it took to reach that value. In other words, hysteresis introduces memory or feedback into a system's response.
A Schmitt trigger is a type of comparator circuit that incorporates hysteresis to provide noise immunity and stable switching behavior. It's commonly used to convert a varying analog input signal into a clean digital output signal. The key characteristic of a Schmitt trigger is that it has two threshold voltage levels: one for the rising edge (positive-going threshold) and one for the falling edge (negative-going threshold) of the input signal.
Here's how hysteresis works in a Schmitt trigger circuit:
Positive-Going Threshold (Upper Threshold):
When the input voltage increases and crosses the positive-going threshold, the output of the Schmitt trigger switches to its high state (logic 1). However, this high output state is maintained even if the input voltage starts to decrease slightly. This is the hysteresis effect in action. The circuit's output will remain high until the input voltage drops below a lower threshold, which is lower than the upper threshold. This range between the upper and lower thresholds is known as the "hysteresis band."
Negative-Going Threshold (Lower Threshold):
Conversely, when the input voltage decreases and crosses the negative-going threshold, the output of the Schmitt trigger switches to its low state (logic 0). Similar to the positive-going threshold case, the output will remain low until the input voltage increases above an upper threshold, which is higher than the lower threshold.
The hysteresis in the Schmitt trigger prevents the output from rapidly toggling between high and low states when the input voltage is near the threshold value. This is particularly useful in scenarios where the input signal is noisy or exhibits small fluctuations. Without hysteresis, the output could jitter around the threshold due to noise, causing unwanted instability.
In summary, hysteresis in Schmitt trigger circuits ensures that the output state change occurs only after the input voltage crosses a threshold value by a significant margin, preventing rapid toggling around the threshold due to noise or small disturbances. This characteristic provides noise immunity and stability, making Schmitt triggers valuable in various applications, such as signal conditioning, debouncing switches, and generating square wave signals.