Twin-T Lowpass Filter Analysis - diyAudio
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Old 15th May 2007, 05:50 AM   #1
gmikol is offline gmikol  United States
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Default Twin-T Lowpass Filter Analysis

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
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File Type: png twin-t lpf.png (3.5 KB, 162 views)
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Old 17th May 2007, 03:11 AM   #2
gmikol is offline gmikol  United States
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Bump...

Anybody???

--Greg
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Old 17th May 2007, 07:24 PM   #3
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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.
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Old 17th May 2007, 08:21 PM   #4
gmikol is offline gmikol  United States
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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
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File Type: png twin-t lpf fr.png (25.4 KB, 129 views)
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Old 17th May 2007, 09:16 PM   #5
forr is offline forr  France
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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.
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