Question

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

• A. the closed loop gain is less than 1 and the phase shift is less than $180^\circ$.
• B. the closed loop gain is greater than 1 and the phase shift is less than $180^\circ$.
• C. the closed loop gain is less than 1 and the phase shift is greater than $180^\circ$.
• D. the closed loop gain is greater than 1 and the phase shift is greater than $180^\circ$.

Hint:

Remember Barkhausen: unity (or greater) loop-gain magnitude and total loop phase $= 0^\circ \pmod{360^\circ}$. With inverting stages like common-source, watch for the phase crossing $180^\circ$ as frequency rises.

Answer 1

The Correct Option is D

For unity feedback, oscillation requires the loop gain to satisfy the Barkhausen conditions at some frequency: \(|A(j\omega)\beta| \ge 1\) and total phase shift \(= 360^\circ\) (i.e., $0^\circ$ modulo $360^\circ$).
A common-source stage contributes approximately \(180^\circ\) phase shift; a cascade plus frequency-dependent phase can push the total phase past \(180^\circ\), turning the feedback effectively positive when the magnitude exceeds unity.
Thus the relevant option is that the (so-called) closed-loop/loop gain is greater than 1 and the phase shift is greater than $180^\circ$.
\[ \boxed{\text{(D)}} \]