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Original question:

LM21215: Bode Plot -- Gain Margin and Phase Margin

LM21215: Can a control system with positive phase margin and NEGATIVE gain margin?

Victoria Qin
Intellectual 1820 points
Part Number: LM21215

Hi Team,

As we know, generally speaking, the premise of a stable system is that about 45° phase margin and 10dB gain margin. I noticed about some system is still stable even if when -180° with a negative gain margin. Why does system still keeps stable when a signal is reversed and is amplified over 1 time? Can a control system be stable with negative gain margin? 

Could u please kindly explain about the principle ?Any documents provided?

Thanks & Best regards!

Victoria 

  • 0
    TI__Intellectual 1420 points

    Hello Victoria,

    Thanks for your question. Could you share with us the plot you were talking about? How is this question related to the LM2125?

    The stability assessment can be done via phase and gain margin but there are methods which take into account the distance of the transfer function to the instability point. 

    Regards 

    Nelson

  • 0 in reply to Nelson
    TI__Intellectual 1820 points

    Hi Nelson,

    Thanks for your reply! I'm afraid I'm not able to provide an exact bode plot and this question is not related with this socket. I have seen a bode plot before but now I'm afraid I couldn't find it. Could u please let me describe? For example ,in a bode plot,  the amplitude plot is over the 0dB where the phase margin is -180° which means a negative gain margin. The system is right in positive feedback, and meanwhile this signal is being amplified. In theory, it's contradictory to the positive gain margin demand. But the conclusion is system is still stable. 

    About what you said 'the distance of the transfer function to the instability point' , could u please kindly make it clear? Thanks a lot! 

    Wish u a good holiday!

    Victoria

  • +1 in reply to Victoria Qin
    TI__Intellectual 1420 points

    Hi Victoria,

    The following article explains how to proceed when the bode plot does not provide clear stability indicators:

    How-to-use-nyquist-plots-to-assess-system-stability

    With the Nyquist method, stability is determined by looking at the number of encirclements of the point at (-1,0j).

    Let me know if this information clarifies your question.

    Best     regards

    Nelson

  • 0 in reply to Nelson
    TI__Intellectual 1820 points

    Hi Nelson,

    Thanks for recommending such an excellent article and giving me a new vision to judge system stability. I can't wait to study it right now!

    Victoria 

  • 0 in reply to Victoria Qin
    TI__Intellectual 1420 points

    Hi Victoria,

    I am glad this article helped you. Relieved

    Regards
    Nelson