The GATE Grind

GATE 2023 EC – Question 22

Analog Circuits · BJT and MOSFET Amplifiers · 1 mark · Multiple choice

A cascade of common-source amplifiers in a unity gain feedback configuration oscillates when

  1. the closed loop gain is less than 1 and the phase shift is less than 180°.
  2. the closed loop gain is greater than 1 and the phase shift is less than 180°.
  3. the closed loop gain is less than 1 and the phase shift is greater than 180°.
  4. the closed loop gain is greater than 1 and the phase shift is greater than 180°.

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Show answer and explanation

Correct answer: (D) the closed loop gain is greater than 1 and the phase shift is greater than 180°.

Explanation

Sustained oscillation needs the Barkhausen condition: a loop gain of at least 1 at the frequency where the loop phase shift reaches 180° (which together with the inverting negative feedback gives positive feedback). With the gain above 1 and the phase shift beyond 180°, the amplitude of the oscillation builds up. So the closed loop gain must be greater than 1 and the phase shift greater than 180°.