[sdiy] Serge Resonant Equalizer mystery solved

Eric Schlappi eric.schlappi at gmail.com
Mon Dec 19 10:09:10 CET 2022

For those also about to google, the TI app note referenced (I think) is

and it has some delightful illustrations in it
[image: image.png]

thanks for all the research Aaron, this is great stuff!

On Mon, Dec 19, 2022 at 9:27 AM Lanterman, Aaron D via Synth-diy <
synth-diy at synth-diy.org> wrote:

> So, yes, the top three filters on the CGS schematic are incorrect. And it
> looks like that error long predates Ken Stone’s schematic; Serge found a
> pencil sketch of the circuit that had the correct connection from the
> positive input of the 1st op amp to the input of the 2nd, but he had erased
> it and erroneously connected the output of the 1st op amp to the input of
> the second.
> I finally found it on the filter on this TI app note (have to take Figure
> 5-5 and substitute it into Figure 5-10 to see it; it’s also Circuit 12 on
> p. 78), where it goes by the name "INIC Band Pass Active Filter,” where
> INIC stands for "ideal current-inversion negative-immittance converter.”
> So it’s like the filter in a Wein bridge oscillator, but with this INIC
> thingamabob stuck in the middle.
> Someone else discovered it in Fig. 8.16 on p. 308 of “Burr-Brown
> Operational Amplifiers Design and Applications” by Graeme & Tobey. (The TI
> app note was originally a Burr-Brown app note, so that makes sense). Serge
> recalled having a copy of that book, and he’s pretty sure that’s where he
> must have seen it.
> Serge went to this topology for the higher bands since he had trouble with
> limitations of the gain bandwidth product of the op amps he was using from
> giving him the performance he wanted with the multiple feedback topology he
> used in the lower bands.
> Aaron Lanterman, Prof. of ECE, Georgia Tech
> My blog on Education and Innovation: https://edupocalypsenow.wordpress.com
> My blog on Electronics and Programming: https://lantertronics.blogspot.com
> My YouTube channel on Electronics and Programming:
> https://www.youtube.com/c/lantertronics
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