[sdiy] What do people use for matched transistors these days?

Neil Johnson neil.johnson71 at gmail.com
Fri Sep 15 20:51:32 CEST 2023


Pete Hartman wrote:
>
> Hm.
> - can these still apply the same logic of "two transistors on the same strip are probably close enough" ?
> - I'm curious what the use case is for a dual transistor where they're not matched.... I mean these cases are a *little* smaller than two individual SOT-23's but there are also smaller packages that could be used.

Yes indeed, that's one of their selling points - they'll be from
similar areas on the same wafer so they're going to be quite well
matched.  BUT BUT BUT they are not on the same piece of silicon
(poorer thermal tracking) and they are not laid out on the silicon for
matching, so they'll never match (pun intended) the matching you'll
get with proper matched pair devices (e.g., LM394, etc).

For example, Infineon's datasheet for the BC856S says on the front page:
• For AF input stages and driver applications
• High current gain
• Low collector-emitter saturation voltage
• Two (galvanic) internal isolated transistor with good matching in one package

The galvanic isolation gives the game away - there is literally an
air-gap between the two transistors.

If you need two transistors that are reasonably matched and reasonably
thermally-linked then these devices will do the job.  If you need two
transistors, then putting down one part is cheaper than putting down
two parts.

Neil



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