Compensating caps and filters

jhaible jhaible at debitel.net
Sun Jul 2 02:01:47 CEST 2000


>To make the filter oscillate you need positive feedback!!
>IMO this doesn't count as the purpose isn't to improve a gain stage.
>And quite tricky to compensate. *grin*


You're partly right - the overall negative feedback becomes positive
with the additional phase shift from the caps. What I forgot to mention is
that each stage has a NFB loop to make the integrator a RC-type filter.

The interesting thing is that a typical OTA filter unit, complete with
gm cell, integration cap and voltage follower, plus NFB around that all,
is quite similar to the ordinary opamp circuit. In fact, you could use
an ordinary opamp, with its internal compensation cap, as a OTA-type
VCF, if you could control the Iabc current of the internal differential
amplifier. For fixed cutoff frequencies, this is widely known as
"capacitorless filter". But it's going to be even better. In some old
Farfisa schemos, I found a capacitorless VCF. They used a quad opamp
with programmable current sources, the LM346. They actually built a
one-chip capacitorless 4-pole LPF. The *topology* is remarkable similar
to the SSM2040 and similar OTA filters. The difference is that instead of
external 1nF caps it works with the internal (5pF ?) compensation caps,
and a much lower Iabc to match. What performance would one expect
from something like that ??
I was curious, and so I breadboarded it. It works remarkably well for
cutoff frequencies of a couple of hundred Hz or higher, but below that
(nA range ?) stability becomes a problem. No wonder, as the external
current not only sets the internal Iabc current, but also *every* other
bias current of the opamp.
Nevertheless, the results were astounding. I guess in the Farfisa
Synthorchestra
it's only used in the upper range (?) to control the brilliance of some
sounds.
But it's complete with resonance and all. When I'm doing the next batch of
scans for my homepage, I'll probably include this circuit. Not recommended
to build, but interesting for being the "missing link" between opamps
and OTA filters. In a way, most opamps with NFB are OTA filters @ fixed
cutoff frequency.

JH.





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