Define phase margin and explain its importance for feedback amplifier stability.

• A. Phase margin is the delay between input and output at DC.
• B. Phase margin is the total phase shift at the unity-gain frequency.
• C. Phase margin is the extra phase shift before the loop reaches -180° at the gain crossover; larger margin indicates greater stability.
• D. Phase margin is the inverse of the gain margin.

Answer: (C)

Phase margin tells you how much extra phase lag the feedback loop can tolerate before the system becomes unstable. It looks at the loop gain at the gain crossover frequency—the point where the loop’s magnitude is 1 (0 dB). At that frequency, you measure how far the loop’s phase is from -180°. The phase margin is 180° plus the actual phase shift there. If the loop phase at unity gain is, for example, -135°, the phase margin is 45°. A larger margin means the system is more robust to model errors, time delays, and component variations, so the closed-loop response stays well damped and stable. Conversely, a phase that is too close to -180° at the gain crossover leads to poor damping or oscillations.

This is not about a delay at DC, nor the total phase shift at unity gain by itself, and it isn’t the inverse of the gain margin.