Re: Curve Tracer

john larkin <JL@gct.com> wrote:

[...]

There are lots of dual transistors around these days, but they are
most always two die in one package, off the same wafer but otherwise
unmatched.

I've had quite good thermal-matching results with T0-220 power
transistors used in low-current circuits and mounted on an aluminium
bar.  The change of junction temperature with ambient temperature is
slow due to the thermal mass of the bar and its small surface area.  The
temperature difference between the junctions is small, even when they
are carrying different currents, because of the large chip area and the
low thermal resistnace between the junctions and the bar.

I've also bound two 'D'-packaged transistors together, flat sides
touching,  with thermal compound between them.  The ZTX series are good
in this respect because they have large junction areas but very small
'D' packages.

With diodes in a logarithming circuit, I use two pairs of 1N4148s (to
average-out any discrepancies) mounted vertically on a board in
'diagonal' formation .  (Viewed from above the 'North' and 'South' form
one pair and the 'East' and 'West' form the second pair)  The pairs are
joined at their top ends; their bottom ends go through holes in the
board in a square pattern. 

A heatshrink sleeve full of heatsink compound is then shrunk over them
so that the ccompoud penetrates and fills the voids between them.  This
results in a solid 4-wired 'component' with closely-thermally-matched
diodes.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk