[sdiy] MS 20 FILTER

[Aaron Lanterman]

On Sun, 18 Dec 2005, harrybissell wrote:

> The two resistors that tune the S-K are replaced by OTA.  If you assume that the
> driving impedance is much lower than the first (input) resistor... it cannot work
>
> both ways, its essentially grounded.  The second resistor is terminated in a cap
> and a high impedance, so that one is essentially 'one way' also.  The MS-20
> feedback starts at a low impedance point, so that sould be pretty much 'one way'
> as well.

Ah, I think I found something describing what I'm thinking of.

Check out this paper, which I've temporarily put up at

users.ece.gatech.edu/~lanterma/00055014.pdf

Fig 1.b is a direct resistor-OTA replacement - three OTAs are used

Fig 1.c is a two-OTA version, which is what I think people on the list 
generally call an OTA Sallen Key.

According to Table 1, they have different transfer functions - and that's 
just for a version where the VCVS has unity gain...

I think the issue is the "pretty much" in the feedback path...

- Aaron

-----------------------------------------------------------------------------

Dr. Aaron Lanterman, Asst. Prof.       Voice:  404-385-2548
School of Electrical and Comp. Eng.    Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: GCATT 334B