[sdiy] moog high pass flter - Transistor Matching

[Scott Bernardi]

Although what you describe would work to compare two Vbe's, I'm not sure
how useful it would be to know just the mismatch between two
transistors.  If you had n transistors to work from, you'd have to
potentially make 2^(n-1) comparisons. Probably not bad if you were only
looking for one matched pair.
If you record the values of the Vbe's, then you make one measurement for
each transistor, and you compare values to see which pairs can go
together.

Tim Ressel wrote:

> Hi Y'all,
>
> If I recall rightly, the Moog matching circuit is a
> constant current source that attaches to the
> transistor to be measured. Then the Vbe is taken. Why
> not to this: Redo the circuit to produce two equal
> currents and run two transistors side-by-sde. Clip the
> trannies together with a wooden clothes pin for
> temperature tracking. Measure the difference in Vbe
> with a center-zero voltmeter.
>
> Would that turn it into a two-brewski effort??
>
> --tr
>
> --- Seb Francis <seb at is-uk.com> wrote:
> > Isn't this where the sixpack comes in?  If you keep
> > the beer at the same temperature (and I know
> > Australians are very particular about the
> > temperature of their beer) this helps stabilise the
> > temperature of your fingertips, and hence the
> > transistors ;-)
> >
> >
> > jhaible at debitel.net wrote:
> >
> > > > i find matching tranies not so hard, but time
> > consuming. the type of thing
> > > > done drinking a sixpack and watching a movie
> > (well thats what i was doing
> > > > when i built the lowpass)
> > >
> > > ... as long as you don't stir the air too much
> > when you're reaching for
> > > your sixpack. (;->)
> > >
> > > Seriously: the temperature is crucial if you match
> > transistors.
> > > If you read the same Vbe on the meter, but your
> > transistors have
> > > a temperature difference of a few degrees C, you
> > have no matched
> > > transistors.
> > >
> > > (Not saying that you don't know how to do it. Just
> > in general,
> > > I wonder how many "hand matched transitors" are
> > really matched.)
> > >
> > > JH.
> > >
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>
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