[sdiy] How can I test an OTA?

[harrybissell]

Hi Bill...

somewhere on one of the diy websites is a copy of Gene Zumchak's
OTA Tutorial that you need to read (I think Rene Schmitz has a copy?)

The OTA looks like an opamp as far as the input pins are concerned...
(you must keep the current into these pins below 1mA however)..

and the bias input looks like a diode drop to the negative supply rail.

The output is a current, the product of the differential input voltage
and the bias current.

You can easily KILL the OTA at the bias current input... if you allow
an unlimited current (more than 1-2mA depending on the actual device)
to flow into it. For that reason, most folk put a 10K resistor in series

with the bias input. Even then, a slip of the scope probe can kill it.

You 'might' not see an output from an OTA if the next stage does not
convert the current to a voltage.  If there is a resistor to ground at
the
output, you should see something.

In most applications, you have to keep the differential input voltage
below 25mV or a LOT of distortion will result.  Overdriven OTA
distortion
is a rather pleasant sound imho so no problem....

H^) harry

bill berzinskas wrote:

> Hey everyone..
>
> I purchased some OTA's (CA3080) a few weeks ago..  I had an attempt at
> a few vca's and a filter.  I didn't really have all the parts for the
> filter, but figured i'd mess with it a bit anyway..   i got it working
> at one point, then something happened and my expo converter was
> sending like 10volts..  somewhere i was getting a small short that
> would take away about 1v per IC.. wierd.. i took it apart and tried a
> few more times w/ varying success..   i got all the parts for the
> ms-20 clone i'd like to build, and tried prototyping once, and it
> didn't work..  checking thru the circuit, i noticed i wasn't getting a
> signal out the other side of the OTA..
>
> I've got 2 questions really.  How can i test my OTA's to make sure
> they're still functioning properly? is there some simple circuit i can
> make on the breadboard to check them real quick?  Also, what kind of
> input are these looking for into the offset /bias input??
>
> Thanks
>
> --Billie
>
>
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