Derive transfer function for LP filter

Discussion in 'Homework Help' started by tblues87, Apr 9, 2013.

  1. tblues87

    Thread Starter New Member

    Apr 9, 2013
    11
    0
    Hi

    can someone help me derive transfer function of low pass biquad 2nd order filter. The transfer function should be Vout/Vin but my problem is if all the positive inputs to the opAmps are grounded the Vout is then 0.

    Example, for the first amp
    V3 = A(V+ - V3)
    V3/A = V+ - V3
    lim(A) -inf
    V+ = V3
    if V+ is grounded then is 0 and V3 = 0.

    Thanks
     
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
    326
    The fact that the + inputs are grounded does not imply that Vout is zero.

    You have 7 numbered nodes, so you will have to write at least 7 equations if you use nodal analysis. The transfer function will be very complicated.

    Do you really want to derive a complete transfer function in terms of the individual components, or will a generalized function as a ratio of a quadratic numerator and quadratic denominator be adequate?
     
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