[sdiy] Matched Trannies

[Neil Johnson]

On Wed, 3 Jul 2002, Yves USSON wrote:
> When one needs a matched pair of transistors (PNPs or NPNs) for
> designing an expo converter ...

> Recently, I found another possible source of matched transistors these
> are SMD components labelled BC857S (siemens, supposed to be equivalent
> to two BC557 PNP) and BC847S (siemens, supposed to be equivalent to two
> BC547 NPN). These are real cheap : 0.18 euros (nearly 0.17$). My
> question is the following : Is there a good reason why I should not use
> these for designing expo converters or moog ladders ? As someone in the
> list gave them a try ?

I think last year, or about 18 months ago, someone looked at these too.
After some tech.questions I discovered that they are actually two
independent transistors in a single package, rather than two transistors
on the same silicon die -- I lost my synth-DIY archive, but I do remember
the answer from Philips was that they select two similar die and package
them up together.  You pay more for closer tolerances, but basically they
are separate slabs of sand.

While this no-doubt reduces their cost (damn good cost too!) because they
are not on the same silicon there will be issues with temperature tracking
and process variation, which might make them less suitable for
expo-converters, where the standard model assumes that both transistors
have the same characteristics and are at the exact same temperature.

I don't know about ladder filters though.  Anyone have real-world
experience??

Cheers,
Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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