# 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

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2. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,720
496
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?