[sdiy] ASM1 - MAT & caps

[Scott Bernardi]

There's actually two temperature dependencies in the Ic vs Vbe relationship:
Ic = Is*exp(-q/kT)
The Is term has a temperature dependence that approximately doubles every 10
degrees C. When you use two matched transistors in an exponentially converter,
they cancel each other out. What's left over is the -q/kT term that we use
tempco resistors to compensate for.
How well the Is terms cancel depends on the matching of the transistors and
the thermal coupling.  A delta Vbe is an indication of mismatch. 150uV is
plenty good, however.

Tom May wrote:

> Jim Patchell <patchell at silcom.com> writes:
>
> >     The smaller the Vos (delta Vbe), the better, but then again, I don't
> > think you will notice the difference.  Get the cheap one.
>
> I'm sounding like a broken record here, if anybody remembers what a
> broken record is.  What is better about a low Vos in the ASM-1
> exponential converter?
>
> Tom.
>
> >     -Jim
> >
> > sandrotraversi wrote:
> >
> > > ASM 1 again ...
> > >
> > > I found those MAT-02 but store has both EH (Vos 50uV) and FH (Vos
> > > 150uV)
> > > versions.
> > > Which is good ? EH is double expensive ...
> > >
> > > On VFC you may note a 30pf cap -
> > > I can't find this value, so I'm thinking to swap on a 33pf - it will
> > > affect
> > > in some way ?

--
Scott Bernardi
sbernardi at attbi.com