[sdiy] moog transistor matching circuit question

[Tom Bugs]

Someone on the MW forum did recently do a very short run PCB - before 
Christmas, I think - maybe there'll be more?
Around here: 
https://www.muffwiggler.com/forum/viewtopic.php?t=81990&start=250

I've never really looked much into transistor matching, but wondered if 
the Peak Atlas testers may be any good?
I recently got one of their Capacitor ESR testers and it is very quick 
and convenient.
The company had some reconditioned units (of various different tester 
models) available via Ebay at considerably less than the standard prices..
[Had a quick check now, but couldn't find their particular listing..]


On 09/01/2016 20:52, Mattias Rickardsson wrote:
>
> I have a spare set of matched transistors for the TTSH supplied by 
> Thonk, if somebody needs them. Contact me privately if interested.
>
> Speaking of transistor matchers, are there any DIY PCBs for them? I'd 
> love to not have to build a temporary one on a prototype board next 
> time. :-)
>
> /mr
>
> Den 9 jan. 2016 5:30 em skrev "matt holland" <matt at mattholland.org 
> <mailto:matt at mattholland.org>>:
>
>     Hi All,
>
>     I'm in the process of helping a buddy build a TTSH and as a result
>     am being thrust into the world of transistor matching. I could
>     just get on with it and breadboard one of the handful of circuits
>     out there but instead I've been staring at the Moog circuit linked
>     to here [1] and trying to understand the theory behind it. The non
>     DUT transistor is a current soruce, with the op amp giving the
>     ability to set the current without having to bother biasing it
>     with a precise Vbe. The two resistors on the non-inverting input
>     bias the inverting input to -5V, which enforce a 98uA (~100uA)
>     emitter current through the 51k resistor through the magic of
>     negative feedback.
>
>     It doesn't seem like the current value is even important at all so
>     long as it stays the same from one DUT bjt to the next (i.e. temp
>     is same from one measurement to the next for the tester as well as
>     the DUT). Then we measure the Vbe across the DUT which is sort of
>     a proxy for measuring Is (Vb = Vt*ln(Ic/Is) since it Is that
>     models all the physical parameters that vary from one bjt to the
>     next and is the temp dependent parameter that gets cancelled out
>     in the expo converter to the extent that they're identical.
>
>     The bit that I'm still having trouble with is the 10k resistor.
>     It's there in the NPN tester but not in the PNP tester. I've seen
>     this described as a "flaw" in the PNP tester but I'm not sure what
>     it's doing int the NPN tester. I threw together a spice sim that
>     steps the value of it from 0 to 10k with absolutely no meaningful
>     change in the measured Vbe.
>
>     Apologies if this has been covered before and appreciate any help.
>
>     mh
>
>     [1]
>     http://www.musicfromouterspace.com/analogsynth_new/TRANSISTORMATCHER/images/moogmatchersmall.gif
>
>
>
>
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