GATE EC 2023 Set 1 — Question 22
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Analog Circuits → Feedback & Oscillators → Barkhausen Criterion
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Question
A cascade of common-source amplifiers in a unity gain feedback configuration oscillates when
- A.the closed loop gain is less than and the phase shift is less than .
- B.the closed loop gain is greater than and the phase shift is less than .
- C.the closed loop gain is less than and the phase shift is greater than .
- D.the closed loop gain is greater than and the phase shift is greater than .
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Correct answer
(MTA)
Solution
For a feedback system to oscillate, it must satisfy the Barkhausen criterion. For a negative feedback system with loop gain , the condition for oscillation is and the phase shift of the loop gain must be (so that the total phase shift around the loop is , creating positive feedback). In a unity gain feedback configuration, . Thus, the system oscillates when the gain and the phase shift is at least . Option (D) correctly identifies that both gain must be greater than 1 and phase shift must be greater than for sustained oscillations/instability.
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