matching transistors & diodes

[Paul Schreiber]

Actually the best way to measure hfe is this "backwards" method
of using a constant current in the collector. This was a HP patent
in one of their board testers (expired), and refined by Doug Curtis & myself
(patent still good!) in the Tandy DVM ASIC we designed. It is part
of the automatic transistor indentification algorithm (the thing which
is actually patented).

Paul Schreiber
Synthesis Technology


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From: 	Martin Czech[SMTP:martin.czech at intermetall.de]
Sent: 	Friday, October 16, 1998 8:18 AM
To: 	synth-diy at mailhost.bpa.nl
Subject: 	matching transistors & diodes

There was a thread about matching transistors & diodes a couple
of weeks ago. We all know the "moog manual" method, constant Ie and
measure Ube. I've found somewhere a note that says : match at least at
two points.  That makes sense if the Ic is modulated over a wide range
(like in controlled gain stages). 

A switch for two reference currents is all that is needed to do this
with the above mentioned circuit. E.g. the test could be at 500uA and
1uA or so.

At 1uA you get something like the ideal diode characteristics and at
500uA the nonideal resitive component comes out. With only one measurement
point it could well be that two transistors seem to match, because ideal
diode characteristics and resistive components together are equal, this
could cause errors above or below the fixed Ic.

Now we've got two numbers, but we need a scalar to compare the transistors.
How can we come to such a scalar : arithmetic or geometric mean value
of the differences ?

Any ideas?

m.c.