Matching OTAs?

[Mark Hammer]

If you bop over to this page: http://www.jhaible.de/matrix_fx/jh_matrix_fx.html  and scroll to the very bottom, you will see a short preamble and a link to a circuit ( http://www.jhaible.de/matrix_fx/jh_ota_selection.pdf ) for testing and matching LM13600s.

At this moment you are probably scratching your head like me, wondering why one would want to match them, and just exactly how "unmatched" they are normally.

Just figured I'd get the conversation rolling.

[R O Tiree]

I think the reason he wanted to match them might be due to the fact that he's using them as companders - one to compress and one to expand, so it made sense to have the expanders un-doing exactly what had been done by the compressors. I'm thinking he might have avoided a lot of trouble by simply including a small trim-pot into the Iabc inputs to calibrate them?

As to how "unmatched" they are, the datasheet for the CA3280 (which he appears to have used in the end) quotes forward transconductance as varying by 37.5% for small signals and 50% for large signals and he wanted them within 3%.

Very interesting concept, though - using an OTA between Out and Inv Input of another opamp to control its gain to compress the signal. I never saw anything quite like that before.