Of course, for critical applications, transistors should be matched in pairs for VBE at a nominal temperature. Specifically, pick up the transistors with tweezers. Insert them into a socket that's adjacent to a reference transistor running at equal, stable currents, such as 1.0 mA, with better than 0.1% matching.
To see the circuit as published in Electronic Design on June 14 of 1999, go to: http://www.elecdesign.com/Index.cfm?Action=Pease
and then go down to June 14 of 1999 to:
"What's All This Logarithmic Stuff, Anyhow?" at:
http://www.elecdesign.com/Articles/Index.cfm?ArticleID=4321
and find Figure 7. The resistors should be very well matched, and stable.
Any deviation between the Vbe of the DUT and the reference transistor's Vbe is used to grade the transistors into bins such as 1 or 2 mV wide, using the DVM as a difference meter. That may be generally good enough for matched pairs, for amplifiers, or logarithmic converters, but the Watt-meter circuit published in May 2002 really needs Vbe matched well within 0.5 mV, max, so it's a good idea to grade the transistors into 0.2 mV bins. Then, when you take the transistors out of that bin, or from adjacent bins, they are well-matched.
Oops! I almost forgot to say that you have to blow a big air blast--a steady flow of room-temperature air--over the DUTs. In fact, the DUTs waiting to go into the test should be kept in that same moving air.
After these transistors are matched, it's important to install them properly to keep them at the same temperature. Use some epoxy or glue, and some metal or junk as a thermal mass to keep the transistors at a fairly constant and equal temperature, and preferably along with the temperature-compensating resistor.
Another way to get matched transistors is to buy matched pairs, such as LM3046 or LM394. The '3046s are typically matched to within 1 mV, but the specs are about 3 mV max. You get a quad of transistors at this price. The LM394s are somewhat more expensive for a dual of WELL-matched transistors. But, you get a spec of 300 mV max for the LM394CH, as required for the watt-meter circuit. So that's a good standard technique for making matched pairs, such as for logarithmic circuits, or precision amplifiers -- or a watt-meter!
Best regards. / rap / Robert A. Pease
(A version of this was published in Electronic Design Magazine, June 14,1999.)
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