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Homework Help: Identifying type of feedback by looking at bode plot

  1. Feb 14, 2016 #1
    1. The problem statement, all variables and given/known data
    Below is the bode plot of return difference, 1+C(S)P(S) in a feedback system. Identify the frequencies where the feedback is large, negative, positive and negligible.

    [tex]P(S) = \frac{100}{s^2+2s+1}[/tex]

    [tex]C(S) = \frac{220(s+50)}{s+200}[/tex]


    2. Relevant equations

    Bode Plot, Feedback

    3. The attempt at a solution

    Is there a way of identifying the frequencies where the feedback is large, negative, positive and negligible just by looking at the bode plot?

    So the positive feedback would be where we start gaining phase & magnitude, so it'd be approximately w = 10^2.

    Negligible gives you a gain of 1, so the frequency would be w = 5. Is my reasoning right?

    Also what characterizes a negative and large feedback?
  2. jcsd
  3. Feb 15, 2016 #2


    User Avatar

    For answering the question you should know the definitions for negative and positive feedback:
    * Region of negative feedback: Magnitude of return difference >0 dB (Transfer function gain decreases due to feedback)
    * Region of positive feedback: Magnitude of return difference < 0 dB (Transfer function gain increases due to feedback).
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