<div dir="ltr">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 =)<div><br></div><div>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. </div><div><br></div><div>I then found this page <a href="http://www.cgs.synth.net/modules/simple_transistor_matching.html">http://www.cgs.synth.net/modules/simple_transistor_matching.html</a> 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.</div><div><br></div><div>Matt</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jan 9, 2016 at 11:03 PM, Simon Brouwer <span dir="ltr"><<a href="mailto:simon.o@brousant.nl" target="_blank">simon.o@brousant.nl</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
<div><div>Hi Matt, </div>
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<div>You're correct in your understanding of the circuit.</div>
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<div>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.</div>
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<div>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.</div>
<div>Use a largish voltage (say more than 10V) and choose the resistor to get about 100uA. Use a DMM to measure the Vbe voltage.</div>
<div>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.</div>
<div>You could make the test socket out of an IC socket (the type with machined pins).</div>
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<div>Best regards</div>
<div>Simon</div>
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<blockquote type="cite" style="margin-left:0px;padding-left:10px;border-left:solid 1px blue">Op 9 januari 2016 om 17:29 schreef matt holland <<a href="mailto:matt@mattholland.org" target="_blank">matt@mattholland.org</a>>:<div><div class="h5"><br><br>
<div dir="ltr">Hi All,
<div> </div>
<div>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. </div>
<div> </div>
<div>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.</div>
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<div>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.</div>
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<div>Apologies if this has been covered before and appreciate any help.</div>
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<div>mh</div>
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<div>[1] <a href="http://www.musicfromouterspace.com/analogsynth_new/TRANSISTORMATCHER/images/moogmatchersmall.gif" target="_blank">http://www.musicfromouterspace.com/analogsynth_new/TRANSISTORMATCHER/images/moogmatchersmall.gif</a> </div>
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