Undesirable oscillations occur at the critical frequency when there is a phase shift around the loop of _______ degrees?

Undesirable oscillations occur at the critical frequency when there is a phase shift around the loop of _______ degrees?

• A. 90
• B. 180
• C. 270
• D. 360

Answer: (D)

The important idea is that oscillations start when the loop gain equals one and the signal fed back around the loop is in phase with the input. That in-phase condition means the total phase shift around the loop is a multiple of 360 degrees. At the critical frequency, if the phase shift sums to 360 degrees, that’s effectively the same as 0 degrees, so the feedback reinforces itself each pass and sustains oscillations. Since 360 degrees equals a full cycle, it satisfies the condition for constructive feedback, making undesired oscillations likely. Choices like 90, 180, or 270 degrees don’t align the feedback in phase with the input at the unity-gain point, so they don’t produce the same self-reinforcing effect.

The important idea is that oscillations start when the loop gain equals one and the signal fed back around the loop is in phase with the input. That in-phase condition means the total phase shift around the loop is a multiple of 360 degrees. At the critical frequency, if the phase shift sums to 360 degrees, that’s effectively the same as 0 degrees, so the feedback reinforces itself each pass and sustains oscillations. Since 360 degrees equals a full cycle, it satisfies the condition for constructive feedback, making undesired oscillations likely. Choices like 90, 180, or 270 degrees don’t align the feedback in phase with the input at the unity-gain point, so they don’t produce the same self-reinforcing effect.