What is a Schmitt trigger and how does hysteresis improve noise immunity?

• A. It prevents false triggering by requiring a larger input to switch states; reduces noise
• B. It speeds up switching by eliminating any threshold
• C. It increases power consumption
• D. It reduces input impedance

Answer: (A)

A Schmitt trigger is a comparator with hysteresis, meaning it has two different switching thresholds. As the input rises, the output doesn’t change until the input crosses an upper threshold; as the input falls, the output doesn’t switch back until the input drops below a lower threshold. This creates a hysteresis band that makes the circuit resistant to noise. Small fluctuations or noise near a single threshold won’t cause the output to chatter, because you must push past the higher threshold to switch in one direction and past the lower threshold to switch back in the other. That’s why it prevents false triggering and reduces the impact of noise. It’s not about eliminating a threshold, speeding up switching, increasing power consumption, or altering input impedance—the main benefit is stable switching in the presence of noisy or slowly varying signals.