What outcome does a negative Phase Margin indicate?

• A. The system is stable
• B. The system is unstable
• C. The system is critically damped
• D. The system has increased overshoot

Answer: (B)

A negative Phase Margin indicates that the system is unstable because it suggests that the system's phase response reaches a critical point where the feedback causes an oscillatory response that can grow rather than decay. In control theory, the phase margin is the amount of additional phase lag at which the system would be unstable. When the phase margin is negative, it implies that the system will likely experience sustained oscillations or diverge in response to disturbances or changes in input, leading to instability.

In a stable system, the phase margin is positive, indicating that there is enough buffering against potential oscillatory behavior before reaching the critical point of instability. Therefore, a negative phase margin clearly indicates that the system is not responding in a predictable or stable manner, resulting in instability.