Comparators and Schmitt Triggers Questions and Answers

Q: Schmitt Trigger is a special configuration of :

A Schmitt trigger is a positive feedback op-amp circuit.Part of the output is fed back to the non-inverting input, so once the input crosses a threshold the output is driven hard to the opposite saturation level. The feedback shifts the threshold, giving two switching levels (upper and lower trip points) and hysteresis, which makes the circuit immune to noise on slowly varying inputs.An open loop comparator switches at a single threshold and chatters with noise. Negative feedback produces linear amplifiers, not switching circuits. 'Differential input' describes every op-amp and does not distinguish the configuration.

Q: An op-amp based comparator with positive feedback is called :

A comparator with positive feedback is a Schmitt trigger.Feeding part of the output back to the non-inverting input gives the comparator two switching thresholds, an upper and a lower trip point, instead of one. The gap between them, the hysteresis, stops the output from chattering when a noisy input hovers near the threshold, and turns slowly varying inputs into clean square waves.An astable multivibrator is a Schmitt trigger with an added RC timing network that makes it oscillate on its own, so it is more than a comparator with feedback. A pulse generator is a general instrument, and an integrator uses capacitive negative feedback.

Q: How does the Schmitt trigger eliminate the erratic operation in the op-amp comparator ?

The Schmitt trigger works by using positive feedback.A plain comparator switches whenever a noisy input crosses the single reference level, so noise riding on a slowly changing input makes the output chatter back and forth. The Schmitt trigger feeds a fraction of the output back to the non-inverting input. This creates two thresholds, an upper and a lower trip point, and the gap between them (the hysteresis) means that once the output has switched, the input must move all the way to the other threshold before it switches back. Small noise excursions are ignored.Negative feedback would make the circuit a linear amplifier, not a switch. The thresholds come from the feedback network, so simply raising the reference voltage or reducing sensitivity would not remove the chatter.

Schmitt Trigger is a special configuration of :

A.Open loop op-amp
B.Differential input op-amp
C.Positive feedback op-amp
D.Negative feedback op-amp

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

A Schmitt trigger is a positive feedback op-amp circuit.

Part of the output is fed back to the non-inverting input, so once the input crosses a threshold the output is driven hard to the opposite saturation level. The feedback shifts the threshold, giving two switching levels (upper and lower trip points) and hysteresis, which makes the circuit immune to noise on slowly varying inputs.

An open loop comparator switches at a single threshold and chatters with noise. Negative feedback produces linear amplifiers, not switching circuits. 'Differential input' describes every op-amp and does not distinguish the configuration.

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An op-amp based comparator with positive feedback is called :

A.Astable multivibrator
B.Schmitt trigger
C.Pulse generator
D.Integrator

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

A comparator with positive feedback is a Schmitt trigger.

Feeding part of the output back to the non-inverting input gives the comparator two switching thresholds, an upper and a lower trip point, instead of one. The gap between them, the hysteresis, stops the output from chattering when a noisy input hovers near the threshold, and turns slowly varying inputs into clean square waves.

An astable multivibrator is a Schmitt trigger with an added RC timing network that makes it oscillate on its own, so it is more than a comparator with feedback. A pulse generator is a general instrument, and an integrator uses capacitive negative feedback.

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How does the Schmitt trigger eliminate the erratic operation in the op-amp comparator ?

A.By reducing the sensitivity
B.By introducing negative feedback
C.By using positive feedback
D.By increasing the reference voltage

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The Schmitt trigger works by using positive feedback.

A plain comparator switches whenever a noisy input crosses the single reference level, so noise riding on a slowly changing input makes the output chatter back and forth. The Schmitt trigger feeds a fraction of the output back to the non-inverting input. This creates two thresholds, an upper and a lower trip point, and the gap between them (the hysteresis) means that once the output has switched, the input must move all the way to the other threshold before it switches back. Small noise excursions are ignored.

Negative feedback would make the circuit a linear amplifier, not a switch. The thresholds come from the feedback network, so simply raising the reference voltage or reducing sensitivity would not remove the chatter.

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Direction for [ Question No: 4 To 4 ] :

Directions : The following items consist of two statements, one labelled the "Assertion (A)" and the other labelled the "Reason (R)". You are to examine these two statements carefully and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer accordingly.

Assertion (A) : The fast changing inputs do not depend upon slew rate property of an OPAMP.

Reason (R) : Schmitt trigger does not work on the slew rate property of an OPAMP.

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true, but (R) is not correct explanation of (A).
C.(A) is true, but (R) is false.
D.(A) is false, but (R) is true.

Exam Year(s)

UGC NET - 2012,

Answer: D

The assertion is false and the reason is true (option D). Fast-changing inputs do depend on the slew rate of an op-amp: if the input changes faster than the maximum rate of change of the output (in volts per microsecond), the output is distorted.

The reason is true because the Schmitt trigger is a regenerative circuit that switches by positive feedback, so its switching does not rest on the slew rate property. Options A, B and C, which accept the assertion, therefore fail.

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Direction for [ Question No: 5 To 5 ] :

Assertion-Reason type questions : Q. 41 to 50 :

The following items consist of two statements, one labelled the ‘Assertion (A)’ and the other labelled the ‘Reason (R)’. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer sheet accordingly.

Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
(B) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(C) (A) is true and (R) is false.
(D) (A) is false and (R) is true.

Assertion (A) : Schmit trigger employs positive feedback and has hysteresis property that helps to compare two voltage levels.

Reason (R) : In some logic circuits rise and fall time play important role in switching. Schmit trigger improves the rise and fall time.

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true, but (R) is not the correct explanation of (A).
C.(A) is true and (R) is false.
D.(A) is false and (R) is true.

Exam Year(s)

UGC NET - 2012,

Answer: B

Both statements are true but the reason is not the correct explanation, option (B). A Schmitt trigger uses positive feedback, which gives it two threshold levels and hence a hysteresis, and this lets it compare an input with two voltage levels, so the assertion is true. The reason is also true, since rise and fall times matter in logic switching and a Schmitt trigger turns a slow edge into a sharp one. But the reason does not explain the assertion: positive feedback and hysteresis follow from the thresholds and not from the improvement of the edges. Option (A) says it is the explanation, and options (C) and (D) call one statement false.

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Comparators and Schmitt Triggers Questions and Answers

Direction for [ Question No: 4 To 4 ] :

Direction for [ Question No: 5 To 5 ] :

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