How is Gain Margin (GM) interpreted?

• A. A negative GM means the system is very stable
• B. A negative GM means the system is unstable
• C. GM = 0 dB means the system is highly robust
• D. Large GM means the system is close to instability

Answer: (B)

Gain Margin (GM) is a crucial concept in control systems that provides insight into the stability of a system. Specifically, a negative Gain Margin indicates that the system is unstable. In stable control systems, the Gain Margin is typically a positive value, which suggests that there is a buffer before reaching instability. When GM is negative, it signals that the system's gain at the phase crossover frequency has increased such that it is now above the threshold for stability, meaning that any additional gain could push the system into an unstable condition.

Understanding Gain Margin helps in analyzing how much gain can be added to the system before it becomes unstable, which is vital for system design and robustness testing. In practice, engineers often look for positive GM values during system design to ensure that the system can handle variations in system parameters or external disturbances without becoming unstable.