[sdiy] moog transistor matching circuit question

[Simon Brouwer]

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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