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

[Nils Pipenbrinck]

Hiho list!

I'm on a personal quest to design and build my own sine-waveshaper (and get
some more expericences with OTA's).

Therefore I fooled around with a 3080 OTA last week. One thing I found out
was (to my surprise) that the resistance of the amplifier bias input pin
varies widely and nonlinear with the OTA output current.

I built a simple test-circuit to plot some curves. Just some resistors and
the OTA itself. Iabc was provided via a 100K resistor from V+. Vdiff was
setup with a voltage divider powered by a low impendance voltage source, and
the output was loaded with a 10K resistor (where I measured the voltage drop
across).

As soon as the output current went up (by changing the differential voltage)
Iabc went down (in my setup by factor 3 from 300µA to 100µA). This gave a
very nice atan shaped curves at the output. For my sine-waveshaper that's
brilliant, initially planed to overdrive the differential input, not to load
down the current source. I'm very satisfied with the resulting shape, but:

Sometimes you don't want distortion. How to go linear and provide a voltage
controlled current to this pin? Will a current mirror do the job for me?



I'm curious, because I see the following problem:

Say I have a mirror built around two trannies (T1 and T2), T1 is the
"master", collector current is generated via a resistor from CV. T2 is the
mirror which powers the Iabc of the OTA.

When the resistance of T2's collector path changes (because the OTA does so)
wouldn't this affect the base-emitter resistance of T1, which in turn
changes the "master" current? If so I would have gained nothing to make the
current to voltage converter more linear than the resistor I used in the
first place. (it's a mirror, no current copy machine :)

Gee, I'm on thin ice here, and I'm meditating over this for quite a time
now. Trannies make me scratch head all the time because any current and
resistance is somewhat related. The only possible way I can think of is a
sledgehammer method: Adding an opamp, measuring the voltage drop across a
resistor in the Iabc current path and do some kind of feedback compensation
to get rid of the drift.



There must be a simpler way - Could someone please enlighten me?


	Nils 'getting headaches' Pipenbrinck

Btw. I'm on a business trip for 3 days. I'm not sure if it's possible for me
to follow the list for this time, so if you answer me, and I stay quited
just wait a day or two.