[sdiy] moog high pass flter - Transistor Matching

[Tim Ressel]

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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