[sdiy] moog transistor matching circuit question

[Paul B]

heres a stripbaord layout for npn matching :)
http://electro-music.com/forum/topic-62167.html

On Sat, Jan 9, 2016 at 8:52 PM, Mattias Rickardsson <mr at analogue.org> 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>:
>
>> 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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