[sdiy] Useful patents on OTAs and VCFs

[Aaron Lanterman]

Rossum - 3969682 - assigned to Oberheim (weird, I didn't know he had a 
relationship with them?) - goes through pretty much every conceivable way 
you could build a 1-pole filter form an OTA, a cap, and a voltage buffer.

The very last one, Fig 9, is what I mentioned in my previous e-mail - 
signal to + on ota, neg of ota to output, output through cap to ground; 
there's even a suggestion to feed this to the + terminal of another OTA 
stage, along with many caveats of the "make sure the input voltage swing 
is small enough" type.

Then there's ARP, 3805091 - looks basically like one of the Rossum 
circuits, except the cap runs across the feedback loop op an op amp, and 
in the patent they spell out a discrete OTA - well - that, and in Fig 7 
they give a differential version with two caps (still doing 1-pole)

- Aaron

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