[sdiy] Matched Trannies

[Henri Kovalainen]

Hello Yves,

On Wed, 3 Jul 2002, Yves USSON wrote:
> 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 don't know if Jörgen is reading the list at the moment, but I thought I
should ease your mind by suggesting that you take a look at his Bergfotron
VCO, which uses the BC847BS as a transistor pair. He has also done some
throughout tests and the results can be seen on the page.

http://www.idg.se/personal/bergfors/bergfotron/vco.htm

It would appear that the device performs sufficiently as far as the
matching is concerned. I have also been told that this transistor family
is known to have very low bulk resistance, which means that they should
track better at higher currents.

I'm interested in the matter because I'm going to build a new bank of 8
VCOs for my modular and the BC847 seems like a good candidate.
Furthermore, because the device is very small, it's easier to design the
transistor-tempco pair so that they'll have better thermal contact as
Jörgen exemplary demonstrates.

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