[sdiy] OTA's, Iabc, and the magic of V to I

Magnus Danielson cfmd at swipnet.se
Mon Dec 24 14:52:54 CET 2001


From: "Nils Pipenbrinck" <np at inverse-entertainment.de>
Subject: [sdiy] OTA's, Iabc, and the magic of V to I
Date: Sun, 23 Dec 2001 21:16:55 +0100

Dear Nils,

> Maybe some of you remembers, I've started a thread rambling over OTA
> currents and curves a week ago or so.

Oh yes...

> I've done some more measurements, and I have to admit, that Magnus is right.
> The atanh curve I plottet was not caused by the shitty current source I've
> used (a resistor to V+) but the overdriven differential amplifier.

Phew! ;O)

> For some reason I assumed, that I can drive the OTA input with 300mV without
> going into the non-linear region. Well, I plotted some more curves with a
> working current source, and now I won't go over 45mV Vdiff. Don't ask me
> where I got these 300mV from. I also cross-checked with other circuits and
> found out that the ASM-1 VCA is just driving it with ca. 1mV. Maybe I'm even
> to optimistic with my 45mV. At least my next design will not distort that
> badly, and if it does I know how to fix it.

Goodie! You have started to learn some. However, there is a backside,
the more you dampen the input signal, the more noise you run into. I
think this is where it would be interesting to linearize the inputs
and see what levels could be usefull. It migth not be much of a gain
thougth.

Also, here we have a curious character, namely that as you dampen
further, the VCA will atanh clipp the signal and question is if this
is audioble.

> I still don't understand how the guts of the OTA's work, but that's not that
> important for me. I know how to practially use and design around them. Just
> wondering what will be the next thing I stumble into.. Gee, maybe I should
> start and write a website around these OTA ramblings. They are so easy to
> use once you've understood how to use them.

Indeed. I could take the effort of write up an explanaition of what
actually happeneds. It is not that hard to figure really, one has to
grasp the ideas of a diffrential pair and current mirrors, that's
about it actually ;O)

> A friendly and peacefull Xmas for all,

Indeed, Happy Christmas and may your OTAs live long and prosper!

God Jul och Gott Nytt År!
Magnus



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