Hello Ayumi,
In control systems, the Nyquist plot determines closed-loop stability by checking if the plot encircles the critical point (-1 + j0).
A positive gain margin usually indicates that the system is stable, while a negative gain margin suggests potential instability. However, a negative gain margin does not automatically mean the system is unstable; it depends on how the Nyquist plot encircles the (-1 + j0) point.
Even if the gain margin is negative, the closed-loop system can still be stable if the Nyquist plot does not encircle the (-1 + j0) point. The system's stability is ultimately determined by the Nyquist encirclement criterion, not just by the gain or phase margins alone.
