How does a Bode plot help assess stability of a feedback system?

• A. It plots time-domain impulse response.
• B. It shows the noise spectrum.
• C. It shows magnitude and phase vs frequency; stability is inferred from the gain crossover and phase margin.
• D. It shows current versus voltage.

Answer: (C)

A Bode plot helps assess stability by showing how the loop gain and phase change with frequency, so you can read stability margins directly from the plot. The key idea is the gain crossover frequency, where the magnitude equals 1 (0 dB), and the phase at that frequency. The phase margin—the difference between that phase and -180 degrees—tells you how much extra phase lag the system can tolerate before becoming unstable. A positive phase margin generally indicates a stable negative-feedback system, with larger margins reflecting greater robustness. If there’s no gain crossover or if the phase margin is negative, the system may be unstable or marginally stable. The other options don’t provide these stability margins from frequency response: impulse response is a time-domain measure, the noise spectrum relates to noise rather than stability margins, and current vs. voltage is an I–V characteristic, not a stability assessment.