A Schmitt trigger is a type of electronic circuit used in digital electronics and signal processing applications. Its primary purpose is to convert an analog input signal into a digital output signal with improved noise immunity and stability. One of the key features of a Schmitt trigger is its ability to provide hysteresis.
Hysteresis is the phenomenon where the output of a system depends not only on the current input but also on the history of the input. In the context of a Schmitt trigger, hysteresis helps to prevent rapid switching of the output state in response to small fluctuations or noise in the input signal. This makes the Schmitt trigger more immune to signal noise and helps ensure a clean, stable output signal.
The Schmitt trigger achieves hysteresis through positive feedback. Positive feedback means that a portion of the output is fed back to the input, reinforcing the original input signal. In the case of a Schmitt trigger, there are two threshold voltage levels: a higher threshold voltage (Vhigh) and a lower threshold voltage (Vlow).
Here's how a Schmitt trigger works and provides hysteresis:
Initial State: Let's assume the Schmitt trigger's output is initially in the high state (logic '1'). The input voltage is below the lower threshold (Vlow) at this point.
Input Rising: As the input voltage increases, it eventually reaches the higher threshold (Vhigh). Once the input voltage surpasses Vhigh, the output of the Schmitt trigger switches to the low state (logic '0').
Hysteresis on Falling Edge: Now, even if the input voltage starts to decrease, it won't immediately cause the output to switch back to the high state. The output will remain in the low state until the input voltage drops below the lower threshold (Vlow). This is where hysteresis comes into play. The difference between Vhigh and Vlow is known as the "hysteresis voltage."
Output Stays Low: As long as the input voltage remains below Vlow, the output will stay in the low state, even if the input fluctuates slightly above Vlow.
Input Falling: To switch the output back to the high state, the input voltage must drop significantly below Vlow.
By providing this hysteresis, the Schmitt trigger ensures that the output state remains stable even in the presence of noise or rapidly changing input signals. It prevents the output from toggling rapidly near the threshold levels, which could lead to instability and unintended behavior in digital circuits.
Schmitt triggers are commonly used in applications such as signal conditioning, noise filtering, and debouncing in digital systems, where clean and reliable digital signals are required.