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Phase margin

Discussion in 'Electronic Basics' started by [email protected], Sep 11, 2008.

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  1. Guest

    If i consider the open loop gain of an opamp with negative feedback
    composed of 2 poles (with negative real part) and one zero (with
    positive real part) and if i want a phase margin of 45°, from its
    definition
    360°-tan^-1[-A(jwo)F(jwo)]=45°
    i have
    +180°-tan^-1[A(jwo)F(jwo)]=+180°-[-tan^-1(w/p1)-tan^-1(w/p2) -tan^-1(w/
    z)]=45°
    where +180 is obtained from 360°-tan^-1[negative constant]=360°-
    tan^-1[-1]=360°-180°
    Instead i've found as solution
    +-180°-tan^-1(w/p1)-tan^-1(w/p2) -tan^-1(w/z) =45°
    what's wrong?
    thanks
     
  2. Eeyore

    Eeyore Guest

    Are you trying to do one of those cute pole-zero compensations with the
    extra pole inside the op-amp itself.

    Fun isn't it ? I just hack it with simulations. I know why the lecturers at
    Uni said they didn't fancy getting into it !

    Graham
     
  3. If i consider the open loop gain of an opamp with negative feedback
    composed of 2 poles (with negative real part) and one zero (with
    positive real part) and if i want a phase margin of 45°, from its
    definition
    360°-tan^-1[-A(jwo)F(jwo)]=45°
    i have
    +180°-tan^-1[A(jwo)F(jwo)]=+180°-[-tan^-1(w/p1)-tan^-1(w/p2) -tan^-1(w/
    z)]=45°
    where +180 is obtained from 360°-tan^-1[negative constant]=360°-
    tan^-1[-1]=360°-180°
    Instead i've found as solution
    +-180°-tan^-1(w/p1)-tan^-1(w/p2) -tan^-1(w/z) =45°
    what's wrong?
    thanks
    =================

    wtf is all that?

    maybe rewrite them using better notation tan^-1 is also known as arctan,
    A(jwo) looks like a function such as f(x) and jwo looks like a single
    object.

    Also define all your symbols so it will be easier to decipher.


    in anycase maybe http://www.intersil.com/data/an/an9415.pdf will help
     
  4. Andrew Holme

    Andrew Holme Guest

    This appears to say:

    360 - arctan(-k) = 360 - arctan(-1) = 360 - 180 = 180

    Surely arctan(-1) = -45 and arctan(-k) <> arctan(-1) unless k=1

    No?
     
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