Twin-T Lowpass Filter Analysis

[gmikol]

Okay--

I've racked my brain trying to figure this one out. I'm trying to figure out how to analyse and design filters of this topology. I understand how the "Twin-T" part of it works, but I can't figure out the interaction between Co and Ra/Rb. And I can't find a text that explains it. I'm getting the results out of a filter design tool, and it doesn't explain how it derives the results.

This is being used as a building block for a higher-order active Cauer-Elliptic filter.

Fc = 747.9366
Q = 0.7164
Fnotch=7557.0414

*Ra=1k
Rb=44.5k
R=2.11k
*C=10n
Co=505n

* = Values I provided to the calculator.

R and C work for the correct notch frequency in a Twin-T topology.

Any help that the experts out there can provide would be appreciated. There is a similar high-pass topology with Co replaced by a resistor, but I haven't even started playing with that.

--Greg

[gmikol]

Bump...

Anybody???

--Greg

[consort_ee_um]

The more usual way of doing a T filter is shown here:
http://focus.ti.com/lit/an/slyt235/slyt235.pdf
Not sure why you are interested in Co, are you trying to compensate for the input capacitance of the op-amp? It will be small enough to be ignored in most cases.

[gmikol]

consort_ee_um--

Thanks for the link to the App. Note. I found that earlier in my search. But I'm not looking for *just* a notch filter. This topology is a lowpass filter with notch. The circuit I've shown above provides the frequency response attached. Co and Ra/Rb interact to provide the lowpass functionality (everything else is just your basic Twin-T notch), but I can't figure out how they interact and how to derive the values.

--Greg

[forr]

Hi GMikol,
I suggest you have a look there :
The Best Active Crossover - Here!
there is a reference to Bainter's notch filter, I can send you his paper. Just email me.