[sdiy] matching discreete transistors can be so easy ...

[jhaible at debitel.net]

... with one of these solderless breadboards.

Last time I selected transistor pairs for low offset voltage, it was a nightmare.
When I touched a transistor with my hands for a few seconds, it would take
2 minutes ofterwards to get a valid Vbe reading. I remember an evening with
a tiny hourglass and styrofoam, last time I did it. Gloves didn't help much, and
My hands are too clumsy for tweezers. (Especially when the transistor leads
don't fit into the test socket easily.)

Now I don't have to select pairs - I have to select "11-tuples". The PS-3200 clone
needs groups of 11 transitors that are tightly matched. Time to try the breadboard
idea.

I've built the well-known Moog test circuit (basically a 100uA current source), but
instead of using a transitor test socket, I connected three flexible wires and built
a little probe with 3 contacts that can be plugged into the breadboard parallel
to a transistor.

Now matching is much more fun: Plug 30 transistors into a small breadboard,
then wait 2 minutes for the _whole batch_, and probe them one by one by
inserting the 3 contact-probe next to a transitor. No chnge in temperature
from this probing, and the whole procedure is really _fast_. I didn't even
have to draw a big table and sort the transistors. Of the 30 in the board,
8 were close enough, so these stay in the board, and the others are replaced
from the drawer. Anther 2 minutes of stabilizing, and then it was fine. 

I didn't know selecting transistors could be so much fun. I have a *lot*
of such groups of 11 yet to select, and I don't know how I could do it otherwise.

JH.