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RC in opamp feedback loop

  1. Jul 19, 2011 #1
    I am trying to understand Type1, 2 and 3 opamp compensators used in DCDC converters.
    Type 1 is just an opamp integrator.

    Type 2 has a C in feedback loop with parallel RC. How does this boost the phase Margin?
    (page 307 - http://tinyurl.com/3qyt5dc)
    I understand phase and gain margin when I look at the bode plots. But I don't get it when I am designing a ckt.

    Also, how do I relate phase shift with phase margin.
    For example, the inverting integrator output has -180 phase shift, but phase margin is -90 degrees. Is there a way to relate the two?
     
  2. jcsd
  3. Jul 21, 2011 #2
  4. Jul 23, 2011 #3
    No replies because there are a lot of gory math details to explaining it generally. The following is pretty good in that respect:

    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.7235&rep=rep1&type=pdf

    Instead of that, however, the easy way is to always refer to the Op Amp manufacturer's documentation on how to compensate it. If they don't have such, it's internally compensated for any reasonable use already. If they do, then just use the design-specific formulae they provide and don't think about it too deeply.

    If you need to know this for an engineering school assignment, then the above PDF links is what you want. You have to grok it at that level to generalize and build an intuition for it. Sorry. Nature of the beast.
     
  5. Jul 23, 2011 #4
    No replies because there are a lot of gory math details to explaining it generally. The following is pretty good in that respect:

    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.7235&rep=rep1&type=pdf

    Instead of that, however, the easy way is to always refer to the Op Amp manufacturer's documentation on how to compensate it. If they don't have such, it's internally compensated for any reasonable use already. If they do, then just use the design-specific formulae they provide and don't think about it too deeply.

    If you need to know this for an engineering school assignment, then the above PDF links is what you want. You have to grok it at that level to generalize and build an intuition for it. Sorry. Nature of the beast.
     
  6. Jul 23, 2011 #5
    No replies because there are a lot of gory math details to explaining it generally. The following is pretty good in that respect:

    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.7235&rep=rep1&type=pdf

    Instead of that, however, the easy way is to always refer to the Op Amp manufacturer's documentation on how to compensate it. If they don't have such, it's internally compensated for any reasonable use already. If they do, then just use the design-specific formulae they provide and don't think about it too deeply.

    If you need to know this for an engineering school assignment, then the above PDF links is what you want. You have to grok it at that level to generalize and build an intuition for it. Sorry. Nature of the beast.
     
  7. Jul 25, 2011 #6
    I am puzzled by the integrator bode plot.
    High gain at zero and gain gradually reduces.

    Even though there is a resistor before the capacitor.

    Shouldn't it be like a low pass filter. Constant gain up to some freq and then gain gradually reduces?
     
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