[sdiy] Matched transistors - critical parameters?

Brock Russell brockr0 at shaw.ca
Wed Feb 8 06:37:01 CET 2012

Hi Matthew,

I've been working on some very small modules and synth voices too. 
I'm using some unmatched high gain dual transistors from On Semi, PNP

BC857CDW1T1G and NPN BC847CDW1T1G. These are smaller parts, SC-70-6 
(SOT-363). They are cheap, seem very well matched and fine for 
filters. Mouser has them in stock.


At 02:35 PM 07/02/2012, you wrote:
>Oren, Ian, All, thanks for the feedback on this.
>As I haven't looked into the theory of the circuits in question and 
>am just taking a "cookbook" approach at present, I wasn't sure if 
>the lack of gain in the new devices might be an issue.
>If this is all about heat, I don't see a problem.
>The DMMT5551 (and DMMT5401 for PNP) from Diodes Inc., are 
>"intrinsically matched," which is explained as being built on 
>adjacent dies from a single wafer. So, with the constant improvement 
>in fabrication technologies, I wouldn't be at all surprised if 
>electrical matching were not as good as, if not better, than what 
>might be considered dinosaurs, in silicon terms.
>As far as heat matching is concerned, and I realise I could well be 
>wrong on this, I can't see how the two dies, encapsulated in an area 
>of about 1.5 square millimetres or less, wouldn't be isothermal as 
>near as dammit. Not sure about the thermal conductivity of the lead 
>bonding but, even with emitters connected externally, there is still 
>a thermal path going that way.
>I'm trying to cram an entire synth, with modern components, into as 
>small a space as possible (all on one board, hard-wired.) No 
>accuracy is required, as this will not be played in the conventional 
>sense, but will receive control voltages from optical feedback in a 
>multi-sensory sculpture. This is why I was asking about getting 
>control voltages out of LDRs the other day, and why the LM358 VCOs 
>caught my interest.
>Hey, if I like the sound of it, I can always break it out into modular.
>Matthew Smith
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