[sdiy] moog transistor matching circuit question

[matt holland]

Thanks Simon. Consensus of everyone I asked is that it's the resistor isn't
needed, so then the lack of one in the PNP tester isn't a "flaw". I like
your idea of it being for current measurement, that seems very plausible.
I'm gonna go with that explanation =)

For the record I breadboarded up Ian's circuit last night and my whole
strip of 3904s appeared to be perfectly matched with my Fluke 87v set to
its mV range.

I then found this page
http://www.cgs.synth.net/modules/simple_transistor_matching.html which
seems like an even easier method for me. The 87v accuracy spec for the
diode test is 0.2%+1 so if my math is right two with the same reading would
be at most 3mV apart in reality.

Matt

On Sat, Jan 9, 2016 at 11:03 PM, Simon Brouwer <simon.o at brousant.nl> wrote:

> Hi Matt,
>
> You're correct in your understanding of the circuit.
>
> I think the 10k resistor is just for checking the current, it should be no
> coincidence that the "100uA" annotation is next to it. There is no harm in
> either putting it in or leaving it out.
>
> If you are not planning to match transistors on a regular basis: a dead
> simple matching jig consists of just a socket for the DUT, in which the DUT
> is connected as a diode (connect base and collector together), a resistor
> and a power supply.
> Use a largish voltage (say more than 10V) and choose the resistor to get
> about 100uA. Use a DMM to measure the Vbe voltage.
> Since the voltage across the resistor is very large compared to the
> variations in Vbe, the variation in current will be only small, and the
> actual difference in Vbe voltage between transistors will be only very
> slightly larger than what you measure using this circuit.
> You could make the test socket out of an IC socket (the type with machined
> pins).
>
> Best regards
> Simon
>
>
>
>
>
>
>
> Op 9 januari 2016 om 17:29 schreef 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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