HFT

[terry michaels]

Message text written by "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.<

Hi Ian:
The Hemsath method of compensating for bulk resistance goes back a long
way.  The schematic for the Moog 901 Oscillator Controller, drawing #1100
dated 7/22/66 shows compensation for the series resistance of the diode
string used to generate the exponential function.  The 901 used 4 silicon
diodes in series as the non-linear element, in contrast to the later method
utilizing a bipolar transistor.  The service manual for the 901 describes
how the "high compensation" takes a portion of the current going through
the diode string and sums it with the control voltage inputs. This is not
the diode plus resistor method, it's the familiar technique of routing the
exponential current through a series pass transistor, with a resistive
divider feeding part of the current back to the control sum.

Terry Michaels