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Homework Help: Differential Eqn. of a 3rd order Filter

  1. Aug 2, 2012 #1
    Hi guys :) I'm going through some prep. work for my modules in Fall and I've come across a question I'm having some difficulty with.

    1. The problem statement, all variables and given/known data
    The question has a transfer function for which I need to find the differential equation relating the output voltage and the input voltage.

    2. Relevant equations
    Transfer function: H(s) = 15/(s3 + 6s2 + 15s + 15)

    3. The attempt at a solution
    I created the above circuit using a 3rd order sallen-key lowpass filter as in this website:http://sim.okawa-denshi.jp/en/Sallenkey3Lowkeisan.htm. I then split the circuit into 3 parts - A,B,C.

    I then deduced 3 voltage equations for the 3 nodes using kirchoffs circuit laws as follows;

    Va = (Va-Vin)/Z1 + (Va-Vb)/R2 + (Va-Vout)/Z2 = 0

    Vb = (Vb-Va)/R2 + (Vb-Vout)/Z2 + (Vb-Vc)/R3 = 0

    Vc = (Vc-Vb)/R3 + (Vc-0)/Z3 = 0

    where Vc = Vout.

    Are the above equations correct? I'm not very comfortable with 3rd order circuits as of yet, and am not sure if I got the equations right. I've gotten to this point, but after this I'm not sure what I need to do. If I simply the above equations I will get the transfer function, but that's not what I want to find, how do I proceed from here?
  2. jcsd
  3. Aug 2, 2012 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    If you're trying to derive the diff. eq. for the given transfer function, what are you doing with Sallen-Key filters???

    A transfer function is readily transposed to a diff. eq. since s → d/dt. Take it from there ....
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