[sdiy] Testing OTA performance

[jhaible at debitel.net]

> > Hey SDIYers,
> > 	I am testing some parameters for various OTA's.  The OTA's being  
> > tested are the 3080, LM13700, JRC13700, and JRC13600.  I have  
> > measured the control current versus gain, and I also want to measure  
> > the noise output of the OTA's.  I am thinking about hooking a large  
> > amplifier up to the output of the OTA, with no input into the OTA,  
> > and seeing how flat that output signal is.
> 
> "no input" as in grounded inputs or as in free-floating inputs?
> I hope you meant grounded.

And ground them with low impedance (short circuit, or something like
100 Ohm), in order not to measure the noise of the resistor, when you
have a really good OTA.


> > ideas out there?  Also, any ideas on how to measure the distortion of  
> > these OTA's?
> 
> There are three parameters that comes to mind:
> 1) Input amplitude
> 2) Iabc current
> 3) Frequency

I wouldn't bother too much with distortion measurements.
If you really have an OTA (and not some other VCA), the
input differential pair will give you the same distotion
in practice as the textbook will predict.
That's without the linearising diodes (if there are any).

> Don't forget the moon-phase and time to next equinoxe!

I would add another important test: CV feedthru, or variation
of offset voltage with the control current.

For this, set up a 1-pole VCF (OTA, cap, buffer opamp, feedback)
and change Iabc over a wide range. (like, 100nA to 1mA)
(Caution: There are some OTAs that die on 1mA, but these are the
exception.)
The advantage over testing it in a VCA configuration is that
you don't have to re-scale the makeup gain for every Iabc
value.
If you only want to use the OTA as a VCA, you can also do the 
following:
Add an offset trimpot. Feed pulses for Iabc (0A min, 1mA max,
1kHz), adjust trimpot for minimum audio at the output (with no
input signal, of course). Some OTAs can be trimmed to better
rejection than others.

JH.

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