Schmitt trigger fundamentals: Identify the correct characteristics of a Schmitt trigger comparator stage used for noise-immune switching.
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Ahas two trip points
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Bis a zero crossing detector
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Chas positive feedback
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Dhas two trip points and positive feedback
Answer
Correct Answer: has two trip points and positive feedback
Explanation
Introduction / Context:A Schmitt trigger is a comparator circuit with hysteresis used for cleaning noisy or slow-ramping input signals. This question checks recognition of its defining traits.
Given Data / Assumptions:
- Noninverting or inverting Schmitt topologies exist.
- Two distinct thresholds: VT+ (rising) and VT− (falling).
- Feedback network sets hysteresis width.
Concept / Approach:Positive feedback shifts the effective switching threshold depending on output state, creating two trip points. This hysteresis rejects small noise around the threshold and eliminates chatter.
Step-by-Step Solution:
1) With output LOW, the input threshold is VT+ for rising transitions.2) After switching HIGH, the feedback moves the effective threshold down to VT− for falling transitions.3) The difference VT+ − VT− is the hysteresis voltage, giving noise immunity.Verification / Alternative check:Plot the transfer with arrowed path; observe different thresholds on up and down sweeps.
Why Other Options Are Wrong:
- Has two trip points: Incomplete—does not mention the cause (positive feedback).
- Is a zero crossing detector: A basic comparator without hysteresis fits zero-crossing; Schmitt does not switch exactly at zero.
- Has positive feedback: True but incomplete—must also mention two trip points.
Common Pitfalls:Confusing Schmitt triggers with simple comparators and assuming a single threshold.
Final Answer:has two trip points and positive feedback