[sdiy] Matched Transistor Specs

Steve Lenham steve at bendentech.co.uk
Tue Sep 1 11:28:11 CEST 2020


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.

Cheers,

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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