AW: VCO hacks, tweaks

[terry michaels]

Message text written by Haible Juergen
>
Ian and Terry,

what do you think about Rene's differential amp 
temperature compensation ?
It's at <http://titan.cs.uni-bonn.de/~schmitz/thcomp.gif>,
and it was discussed on the list some time ago, but
I think you joined us later. Now with all this recent talk
about tempco resistor accuracy, how does Rene's
circuit compare to the resistor method ?

Personally, I think it's very elegant, using 4 transistors 
from the same array for both, expo conversion and
temerature compensation. But I have never tried it -
it's been some time since I've designed my last VCO.

So what do you think about it ?

JH.
<

Hi Juergen:

Took a look at Rene's circuit, the concept is sound.  The T1, T2
differential amplifier along with the second op-amp has a gain that varies
directly with absolute temperature:  Vo = q/2KT times E in, times a
constant dependent on resistor values and supply voltage.  The first op-amp
forces whatever input voltage is needed to get a constant output from the
diff. pair, that same voltage is applied to the expo convertor.  Works on
paper, I'll have to try it someday.

The beauty of this circuit is it could be easily integrated onto a complete
VCO chip.

BTW, the data sheet for the Curtis CEM 3340 VCO talks about a precision
multiplier fed by a temperature dependent constant to provide temperature
compensation of the expo convertor.  Since they only show a block diagram,
there is no way to know exactly how it was done, it might have been done in
a similar way to Rene's design.

I hope I didn't duplicate whatever previous discussions there were on this
subject, if so I apologize.

Terry