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Miller's thorem and negative feedback

Discussion in 'Electronic Basics' started by Jean-Pierre Trolet, Jul 1, 2007.

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  1. Hello,

    Just a question of pure theory . Why does the Miller's theorem not
    account for itself for the decreased output impedance when applying
    negative feedback ?
    To be clear :

    suppose an inverting amp with a Rf / R feedback network with a gain of
    - Rf/R. Rf is the feedback resistor joining the output to the
    inverting input ( = very classical !). Suppose also Zout to be the
    output impedance in open loop.
    Suppose Av is the gain in open loop.

    If we apply Miller's theorem on Rf it tells us that the effect on
    the output is equivalent to putting a resistor in parallel to the
    output that is worth = Rf * Av / ( Av -1 )
    It does not account for the fact that Zout also decreases by a factor
    1/ ( 1 + Av*beta).

    So I think that applying Miller's theorem in other conditions than
    calculating the parasite capacitors on the intput and output is
    misleading and even a trap.

    What do you think of it ?

  2. neon


    Oct 21, 2006
    this is active miller is inactive capisce
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