[sdiy] Matched transistors and EFM circuits?

[dj hohum]

Hi Harry,

Thanks for responding.

On 6/8/07, Harry Bissell & Nora <harrybissell at copper.net> wrote:
> In my experience with the EFM boards (in particular the ladder
> circuit) unmatched transistors result in severe DC offsets as you sweep
> the filter center frequency.  So bad in my case thatI had to add
> a shunt regulator to the bias capacitor (in a Moog its about 220uF
> and the EFM should be that big as well...).
>
> Using matched transistor pairs eliminated that problem totally.

Ok, good. I presume that you mean simply matched pairs? Or all of them
matched closely?

> In the MS-20 clone, the matched transistors might make the current
> sources a little better with temperature. If you are not using
> tempcos, why bother ???

Well, good question. I don't really understand how ladder filters work
at all, they are just a bit of magic to me. Further, although I'm
aware that matching transistors in an expo converter is important for
temperature stability, I'm not completely sure what's going on in the
discrete OTAs in the MS-20 circuit. Let's just say I don' t really
understand how an OTA works in general. Hence, I really don't know
what it would affect to use unmatched transistors.

As far as temperature compensation goes in the MS-20 filter, again, I
wouldn't really know where to put the tempcos.

Thanks for your help
Daryl
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