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

[Nils Pipenbrinck]

Hi Magnus (Scott, Scott and the lurkers),

> According to my datasheet shall the amplifier bias voltage vary from
> about 500 mV (at 100 nA and +25 degree C) to about 700 mV (at 1 mA and
> +25 degree C).

Wait wait wait..

Amplifier Bias Voltage?

I thought Pin5 of the 3080 is a current input pin. All references I found
are driving it with current. I know that I can also drive the bias input
with a small voltage (which in turn will let a current flow), but isn't that
the wrong way to do it?

> You may not realize it, but the pin is allready connected to a current
> mirror. In the schematic diagram of the chip D1 and Q3 forms a current
> mirror. Maybe another current mirror provides sufficient isolation.

I didn't realized that. Stupid me thought I just provide some current into
the base-emitter junction and the diode is just some kind of input
protection. That changes things quite a bit and makes my current mirror
approach pointless :)

> Also note that the atanh curves from signal-input to output is due to
> the diffrential pair (Q1 and Q2) and linearisation of that would
> require additional diodes to some positive voltage and then feed with
> current in order to logarithmize the input voltages.

Hm. I'm not sure about that. I didn't run the inputs hot. I just provided
some few millivolts into the diff-amp input. (the maximum it saw was about
150mV, but the output was almost saturated at 50mV). Afaik the datasheet
sais that anything below 300mV is fine for linear work.

But I am sure, that when I increased the input voltage, Iabc decreased, and
the output saturated. I don't know if the decrease in Iabc is caused by the
increasing output current or the input voltage, but I can find out by some
tests - this time with a better current source (one that doesn't suck when
the load changes).

> To use the input diffamp current to vary the gain is a gross
> oversimplification.

Not sure about that either. I don't have the schemantic of the 3080 here,
but I remember the current mirror was just connected to the common emitter
of the diff-amp. The current would directly translate into an amplified
difference voltage across the collectors and thus control the gain (at least
if I don't take the current flowing through the base-emitter junctions into
account, but this current should be quite low if not neglible).

> Just don't scratch your head too much on OTAs and current mirrors
> then...

I'll have a lot of time to spend in hotel-rooms the next days.. scratching
my head over current mirrors will be a lot of fun :)

Anyways - now I know, that a V to I converter just needs a tranny and an
opamp, so I can go and verify my ideas on my testboard.

Thanks and Cheers,

	Nils Pipenbrinck