[sdiy] pin 5 3080

[Martin Czech]

SNIP
:::thought the whole point of the OTA was to vary the
:::current/gain. Please help put me right!

What is an ota like this?  A two stage design, IMHO. A diff amp feeding
some current mirrors.  So the main indegredients of an op amp are
there. But you have no output stage as usual, but a current source
output. Thus the device takes the input voltage difference times the
Iabc "program current" times some constant to make an output CURRENT. In
the case of an op amp we would write: takes the input difference times
some constant to deliver some output VOLTAGE. That's all. The "program
current" Iabc on pin 5 is mirrored into the diff amp, thus modulating
transconductance.  Now, who has said that you should crank Iabc up and
down? You can just as well input a constant current. Now the ota is
described as:

the device takes the input voltage difference  times some constant to
make an output CURRENT.

We get closer to the op amp, didn't we?  Now we only have to convert
the output current into a voltage, that's easy: Ohms law, a resistor
to reference will do.  Or a op amp current to voltage converter, if
you want it more complicated, but also more linear (constant voltage @
ota output pin).

Voila:

the ota with fixed Iabc and current -> voltage converter @ output is a
differential voltage amplifier with reasonable gain, now you can use it
as an op amp.

... errr, almost. The 3080 of course has no such thing as a "compensation"
(stupid expression, you can't compensate for input->output lag, instead
gain is trashed down into the ground...). So, depending on the gain (Iabc),
parasitics (board, socket, pins) etc. etc. some external lead cap
must be provided in the feedback loop to ensure stability in
closed loop applications.

Open loop: comparator, reasonable fast (clear, no comp), low temp drift of
input offset voltage 

I don't know if the usage of an ota as op amp is still adequate today,
I would guess that: NO, we have better dedicated devices,
but clones are clones...

m.c.