[sdiy] Matched transistors - critical parameters?

[David G Dixon]

Well, in the interests of full disclosure, I typically put current mirrors
on my filter OTAs.  Of course, I haven't used an OTA in years, ever since I
discovered the 2164 quad VCA, and I've never looked back.  No expo
converter, no currents or current mirrors -- just a single buffered control
voltage sent to all VC pins, and four VCAs gain-matched to within a few
hundredths of a dB.

But, looking at the schematic of the LM13700, I see no reason why the Iabc
pin current draw of different OTAs shouldn't be very similar.

> And maybe this is why multiple OTA filters sound so different 
> from each other.. my workshop is in storage, but maybe 
> someone can take a few measurements. I think that 
> transconductance is not guaranteed to be better than 50% in agreement.
> 
> ----- Original Message -----
> From: "David G Dixon"
> > In my experience, with multistage filters, if you are 
> sending the same 
> > current to the Iabc inputs of OTAs, you don't even need to 
> mirror the 
> > current -- just connect all of Iabc's directly to the 
> current source.
> > Each
> > one should suck the same amount of current.