below is the transfer function of twin-t filter. in actual design, it can also be used for bandpass filter if connected between negative input and output of opamp. now, the twin-t network is a three order system, so, how to get the expression of center frequency f0 and quality factor Q? for two order system, we use 's^2+(2*pi*f0/Q)*s+(2*pi*f0)^2' to calculate f0 and Q, but for the three order system, how to calculate the parameter f0 and Q?
thanks all.

raja.cedt wrote on Jan 19^th, 2013, 10:56pm:


It is.
Ihlbluesky spoke about a twin-T section in the feedback path of an opamp.

If you choose C3=C1+C2  and at the same time R3=R1||R2 you have a pole-zero pair that cancels each other.
Thus, there is a reduction in order from n=3 to n=2.
Therefore, the 2nd order denominator allows to calculate the Q and wp expressions.