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Beginner's question of opamp stability

  1. Oct 26, 2016 #1
    I am embarrassed to ask this. I just want to make triple sure about this stability condition. In the attached plot, I have the open loop gain and phase plot in light green and closed loop gain and phase plot in dark blue. You can see the closed loop gain is 30dB which is about 22 or so.

    I want to confirm when looking at stability, I look at -3dB of the closed loop gain ( blue) which is -27dB as vertical red line drawn labeled -3dB. You can see the phase at that point is about -70deg.

    I want to verify: To look at stability, this is the only point I need to look at. This shows I have phase margin of 110deg( 180-70). So this circuit is stable given the open loop and closed loop gain phase plot. That I don't have to look at the 0dB crossover ( I circled "Don't care") at all.



    Attached Files:

  2. jcsd
  3. Oct 29, 2016 #2
    I have problems to distinguish between the curves; could you please post a more clear picture? Maybe change colors, at least.
  4. Oct 30, 2016 #3


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    Please note that for finding stability margins (phase and/or gain margin) you need to evaluate the LOOP GAIN (magnitude and phase plot) of a circuit with feedback.
    Only for the special case of 100% feedback (feedback factor "1") loop gain is identical to the open-loop gain of the opamp.
    The closed-loop gain plot contains no information on stability margins.
    More than that - even unstable systems show a closed-loop Bode plots that looks quite "normal".
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