[sdiy] moog transistor matching circuit question

[matt holland]

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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