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Homework Help: Filter circuit impedance?

  1. Mar 12, 2008 #1
    1. The problem statement, all variables and given/known data
    2. Relevant equations
    3. The attempt at a solution

    Hi all, i have some doubts on how to find the impedances for active filter circuits.

    Let's start with a simple low pass filter :

    http://img85.imageshack.us/img85/6484/52244578oo4.th.jpg [Broken]

    The input impedance should be R + Xc; and the output impedance should be ZL || (RF+R1) once i short the input.

    However, a same filter but at 2nd order configuration is confusing me. Assuming the circuit diagram is given as below :

    http://img85.imageshack.us/img85/4529/lp2ia8.th.jpg [Broken]

    Will be output impedance be solely ZL || (RF+R1) if seen from port A-B ? Or will it have to include the feedback capacitor and the input side with the source short-circuited as well?

    As for the input impedance, is this derivation method along the right line?

    applying KCL at red node V1

    Ii = I1 + I2
    (Vi-V1)/R1 = (V1-Vo)/XC2 + (V1-0)/(R2+XC2)

    where vo=KVx ; K is the DC voltage gain of the amplifier.

    Vx = (V1*XC1)/(R2+XC1)

    After that, from definition of Zi = Vi/Ii; try to arrange the equation to fit Vi/Ii?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Apr 1, 2009 #2
    can someone please help answer this?
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