phase margin
Phase margin is the safety cushion that keeps a feedback amplifier from oscillating — measured in degrees of phase. Recall that negative feedback turns positive (and the circuit oscillates) when the loop picks up a full 180 degrees of extra phase lag while still having a loop gain of 1. Phase margin tells you how many of those 180 degrees you still have in hand before disaster. A big cushion means a calm, well-damped amplifier; a thin cushion means ringing; none means oscillation.
To read it, find the gain-crossover frequency — where the loop gain falls to exactly 1 (0 dB) — and look at how much phase lag the loop has there. Phase margin is 180 degrees minus that lag. Example: if the loop's phase at crossover is -135 degrees, the phase margin is 180 - 135 = 45 degrees. Roughly: 0 degrees is on the verge of oscillation; 45 degrees gives noticeable overshoot and ringing; 60 to 70 degrees gives a clean, well-behaved response with little overshoot. Most designers aim for 45 to 60 degrees.
Why this matters: phase margin is the single most useful number for predicting whether a circuit will be stable and how it will settle. Capacitive loads, fast op-amps run at low gain, and layout parasitics all eat phase margin, so you check it whenever those are in play. The honest tradeoff: more phase margin means rock-solid stability but a slower, more sluggish step response, while less margin gives a snappier response that risks ringing. Stability and speed pull in opposite directions.
A 60-degree phase margin gives a step response that overshoots only a few percent and settles cleanly. Drop the margin to 30 degrees (say by adding a capacitive load) and the same step overshoots by a quarter and rings several cycles before settling.
Phase margin = 180 degrees minus the loop's phase lag at the gain-crossover frequency.
A textbook 45 to 60 degrees is measured at the op-amp pin. Real layout adds parasitic phase lag the datasheet never saw, so leave extra margin — design for 60 and you may measure 45.