[sdiy] Matched Trannies

[Mark Smith]

	I have been using IMX17 from Rohm for almost 2 years in ladder filters with good results.
	This is a dual BC846/BC547 type transistor in a SOT23-6 package.
	It makes for a very compact filter - a whole ladder filter in less than 2 sqr inches.
	I have also had success replacing 2SC1583 with them too - although not in expo convertors.

	They are certainly well worth looking at.

	mark

> 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
> ----  IEE Cambridge Branch: http://www.iee-cambridge.org.uk  ----