[sdiy] moog high pass flter - Transistor Matching

[Scott Bernardi]

I have a circuit where I can switch it between 5uA and 500uA.   I take
measurements and both currents (5uA first, then switch to 500uA).  With
a 3 1/2 digit DVM, I write down the two 4 digit Vbe (ex: .5643v,  .6370)

See http://home.attbi.com/~sbernardi/elec/og2/transistor_tester.html.
This one only measures one transistor at a time, which means you have to
measure all your transistors in one session because of temperature
differences.

I'm getting more convinced that differential measurements are the way to
go, so I'm working on another circuit that will bias up 12 transistors
at once, and then use a couple of 12 position rotary switches to choose
which two transistors to do a differential measurement on. It will
handle both npn's and pnp's, and the bias current is selectable between
5uA and 500uA.  I should have a preliminary schematic up within a couple
of days.

Tim Ressel wrote:

> Yo,
>
> Hmm... I'm getting more like 5mV. What current are you
> guys using for matching?
>
> --tr
>
> --- Ian Fritz <ijfritz at earthlink.net> wrote:
> > Hi Scott --
> >
> > At 07:54 PM 12/10/2002, Scott Bernardi wrote:
> >
> > >OK, I see your point, but I still just try to match
> > within half a millivolt or
> > >so. My 3 1/2 digit DVM works for that.
> >
> > I agree that half a mV is probably plenty good.  In
> > fact (as we've
> > discussed here before) if mismatch comes just from
> > geometric factors then
> > matching doesn't even matter -- the output is just
> > displaced by a fixed
> > ratio. If it comes from material flaws then it is
> > more likely to matter.
> >
> > >Probably the best argument for matching as a diff
> > pair is that they will
> > >both be
> > >at the same temperature, even with stray air
> > currents, because you're
> > >measuring
> > >them at the same time.
> >
> > I agree that you would want to make a final
> > differential check of the
> > selected pairs.  I think you could line up a bunch
> > of devices, shield them
> > against drafts and measure them quickly, though, and
> > avoid serious
> > temperature effects.  Especially if the offset from
> > the reference isn't too
> > great.
> >
> >    Ian
> >
> >
> >
> >
> > >Ian Fritz wrote:
> > >
> > > > Um, well you take one and measure the other n-1
> > against it.  Then pick the
> > > > good pairs by comparing the numbers, just as if
> > you had measured them
> > > singly.
> > > >
> > > > The advantage would be that you are using a
> > differential measurement and
> > > > therefore get better resolution.  You could in
> > principle use a 3 1/2 digit
> > > > DVM to determine offsets in the microvolt range.
> >  It would be quite
> > > > difficult to do this well with the .56 V
> > background you have in a single
> > > > measurement.
> > > >
> > > >    Ian
> > > >
> > > > At 08:49 PM 12/9/2002, Scott Bernardi wrote:
> > > > >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.
> >
>
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