HFT

[Ian Fritz]

Hi Folks --

Just to make sure this is clear, the diode plus resistor scheme has been
around for many years. I picked it up from Hal Chamberlin's book, but it has
appeared several other places alse. I can't find the original reference, so
I'm not sure who did it originally. Franco? Hemseth? Maybe someone has a
reference? Its purpose is to compensate for the emitter bulk resistance of
the converter transistor, and the detailed analysis shows that it does so
exactly, provided that the temperature dependence of the diode and the
transistor base-emitter junction are the same. I recommend not leaving the
diode out.

At one time I experimented with different tracking schemes for the SAW VCO
and got the best results combining both the Rbulk correction and the reset
time correction (small resistor in series with the timing cap).  You can see
that the design on my site has both!

  Ian

>
> The 1M and diode arrangement that I "adapted" from Ian's, takes a sniff
> of the current as well - the voltage at the output of the servo opamp is
> nearly
> proportional to the expo current. So this does basically the same thing as
> Oberheims
> solution. One could argue that one can neglect the voltage drop across the
> diode,
> and thus leaving it out. There is a description (including the maths) in
> the archives.
> Also Rick Jansen had a drawing on his page of such a compensation using
the
> third transistor as the diode. It included some maths, and a reference to
> S. Franco "Design with operational amplifiers and analog integrated
> circuits". I never tried to find that book. You can find the Rbulk values
> of some duals on my homepage.
>