[sdiy] Transistor Matching

[patchell]

    The two most common parameters to match are the DC current gain ( Hfe)
or Base Emmiter voltage (Vbe).  Generally speaking, around here, we are
concerned with Vbe.

    The reason for doing this is so that the transistor collector current,
which is dependent on Vbe, will track in two transistors.  This is
important for exponential converters and differential amplifiers, both of
which abound in our circuits.

    The circuit for doing this can be very simple.  I haven't been able to
look at Tim's circuit, because his website doesn't seem to be Netscape 4.7
compatable :-(

    Generally, I try to match the transistors to about 0.1 mV.  However,
this is not at all easy.  Just touching the transistor with your finger
will be enough to warm it up to make the reading inacurate.  Vbe is very
dependent on temperature.  Many electronic thermometers take advantage of
this fact.  Also, if the temperature is changing in the room fast enough,
this can also Through the readings off.  Vbe will change about 2 mV for
every degree C (or 9/5 degree F).  So the 0.1mV I attempt to match to is
probably just fooling myself.  Vbe is also dependent on current, you
generally, you want to use the same current source to do the tests with so
that you know that this isn't a variable.

David Reichert wrote:

> While the topic is up, I would like to pose a beginner style question
> which has most likely been brought up before, but I have yet to see it
> myself.
>
> Anyway, what exactly is involved in matching transistors? Which
> parameters are matched and to what tolerance? How does one go about it?
>
> If someone could answer this or point me to an appropriate source I
> would much appreciate it.
>
> > From: Tim Ressel <madhun2001 at yahoo.com>
> > Date: Mon, 9 Dec 2002 17:02:54 -0800 (PST)
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] Transistor Matching (was moog high pass flter)
> >
> > Yo,
> >
> > Question: How close does the trannies have to be
> > matched for say a ladder filter? is 6mV close enough?
> >
> > By the by, I breadboarded, for lack of a better name,
> > a differential transistor matcher. I used a CA3096 to
> > make both an NPN and PNP matcher. The idea is this:
> > you can run two transistors at the same time, and tie
> > them together to keep them at the same temperature.
> > Also the voltmeter now reads the difference directly.
> > Seems to work pretty well.
> >
> > --tr
> >
> >
> > __________________________________________________
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--
 -Jim
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