BC847BS (was Re: Components with potential)

[Tim Godfrey]

At 09:01 AM 9/25/98 +0100, you wrote:
>Unfortunately the datasheets don't say much. They say "closely matched
current 
>gain" but nothing about Ube or any figures. I suppose somebody have to buy a 


I wanted to point out that there are parts available that contain two
discrete, independent transistors in one package. I ran across some like
this from ROHM a few years ago when I was looking for a real matched dual
PNP to make a current mirror. ROHMs data sheet was similarly nebulous, so I
ended up calling the factory and they confirmed that the package contained
two separate transistor chips. (their parts are at
http://www.rohm.co.jp/products/shortform/21trstr/trstr5.html#1)

After looking at the Phillips data sheet for the BC857BS, I have a strong
suspicion that this may be the same situation. The first clue is that they
call it a "double transistor". Whenever I've seen matched transistor pairs
on the same substrate they are called "dual". The other clue is the
statement "No mutual interference between the transistors". I think this
means they are not on the same substrate. The final indications is that
there is no mention of Vbe matching. 

For the kind of applications we are interested in (expo converters and
current mirrors) it is very important to have two transistors on a single
substrate. If they are not, there is limited thermal coupling through the
plastic case. Differences in power dissipation between the two transistors
will result in differences in junction temperature, and thus prevent
accurate current mirror or exponential operation.

Tim.


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