[sdiy] Matched Transistor Specs

David G Dixon dixon at mail.ubc.ca
Tue Sep 1 20:30:11 CEST 2020

As a professor who teaches heat transfer to engineering students, I will
contradict Steve's statement, and say that two transistors in a tiny-ass SMD
package will be at the same temperature within the limits of ones ability to
measure it.  If one transistor suddenly becomes the source of significant
heat, and the other does not, then the hot transistor will heat up the
entire package very quickly, and within, probably, milliseconds, the two
transistors will be at virtually the same temperature.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Steve
Sent: Tuesday, September 01, 2020 2:28 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Matched Transistor Specs

I seem to remember that many of the general-purpose SMD pairs are not
monolithic but are in fact two separate transistor die in a single package.
As they come from the same wafer they are probably pretty well matched, but
won't track with temperature like a monolithic pair.

The really good pairs are not just monolithic but interleave the two
transistor structures so as to equally share out any minute variations and
temperature gradients across the die and make matching even tighter.

For really good log/expo conformance you also want low base resistance
(Rbb) but it is rarely specified except for expensive parts specifically
targetted at that sort of application.


Steve L.
Benden Sound Technology

On 01/09/2020 09:24, Miles Stevens wrote:
> My experience with dual transistors is mainly the NXP BCM series, they 
> are incredibly handy for tight layouts in general, but I don't find 
> them up to snuff for an expo pair.
> On Tue, 1 Sep 2020 at 14:50, Aa Bb <alfa.rpar.as at gmail.com 
> <mailto:alfa.rpar.as at gmail.com>> wrote:
>     Dear Aaron
>     For exponential circuit must be used transistors for wich some
>     parameters (H21,...)  are defined in conditions Ucb=0 V.  Number of
>     available transistors for such purposes is small.
>     On Tue, 1 Sep 2020, 01:23 Aaron B., <aaron at zadzmo.org
>     <mailto:aaron at zadzmo.org>> wrote:
>         I've been looking at dual transistors for exponential circuits.
>         If I'm
>         okay with surface mount, there seems to be a bewilderingly large
>         number
>         of parts available.
>         Besides the maximum voltage/current ratings, what should someone
>         look
>         for in a good exponential converter pair? Do any of the typically
>         published specs like Vceo, Is, beta, etc matter much in this use
>         case?
>         -- 
>         Aaron B. <aaron at zadzmo.org <mailto:aaron at zadzmo.org>>

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