Loop gain ($L(\omega )=A(\omega )\beta (\omega )$)

The loop gain is the product of the forward amplifier response and the return-network fraction, with the summing-junction minus sign kept explicit. Closed-loop poles are determined by $1+L=0$ in this convention. Large low-frequency magnitude controls gain error, while frequency-dependent phase affects stability.

The phase margin measures the remaining phase lag before a negative-feedback loop reaches the critical phase $-\pi$ at unity loop magnitude, using a continuous phase convention. Positive adequate margin is a useful stability and transient-response design diagnostic for standard single-crossover amplifier loops. More general unstable open-loop systems and multiple crossovers require the full stability criterion.