Hysteresis refers to the phenomenon where the output state of a system depends not only on its current input, but also on its previous states or history. In other words, the system exhibits memory or persistence in its response to changes in input. Hysteresis can be observed in various physical systems and devices, where the system's behavior depends not only on its present conditions but also on its past conditions.
In electronics, hysteresis is commonly utilized in circuits to create stable and noise-resistant switching behavior. One classic application of hysteresis is in Schmitt trigger circuits.
A Schmitt trigger is a type of comparator circuit that converts an analog input signal into a digital output signal with well-defined high and low voltage levels. It introduces hysteresis to prevent the output from rapidly toggling back and forth when the input signal is near the threshold voltage. The hysteresis in a Schmitt trigger is achieved through positive feedback, where a portion of the output voltage is fed back to the input.
Here's how hysteresis works in a Schmitt trigger circuit:
Upper Threshold (High-Level Triggering): As the input voltage rises, the output remains low until it crosses a certain threshold voltage (Vhigh). Once the input voltage surpasses this threshold, the output quickly switches to a high state.
Lower Threshold (Low-Level Triggering): However, when the input voltage starts to decrease, the output remains high even after the input voltage goes below the threshold. The output only switches to a low state once the input voltage drops below a lower threshold voltage (Vlow).
This behavior introduces a range between the upper and lower threshold voltages where the output remains stable, preventing rapid switching due to noise or fluctuations near the threshold. This range, defined by the difference between Vhigh and Vlow, is the hysteresis window. The Schmitt trigger's output remains in its current state until the input voltage crosses one of the threshold levels, ensuring a more stable and noise-immune response.
The importance of hysteresis in Schmitt trigger circuits lies in its ability to eliminate the effects of input noise and signal fluctuations that might otherwise cause the output to oscillate between high and low rapidly. By providing a clear transition region between the two states, hysteresis ensures that the circuit only switches its output state when the input signal has significantly moved beyond the noise margin, leading to more reliable and predictable behavior.