KT81 in feedback loop

[Martin Czech]

What can we theoretically expect from expo KT81 temperature compensation,
if the tempco is in the opamp feedback loop?

The following assumptions were made:

1. no other errors contribute
2. a typical KT81 curve was fitted
3. a six octave range was observed
4. the expo converter runs from -3V to 3V in these 6 oct.
5. it is tuned to 1V/oct.
   

The relative error (compared with an ideal tempco) is approx. given,
depending on temperature and input voltage. The point of taylor
development is 273+35K, ie.  the error is 0 for this temperature.



Temperature [K]  rel error  voltage [V]
---------------------------------------
273+35 K         0          -3 , +3
273+40 K         -0.03 %    -3 
273+40 K          0.03 %    +3 
273+30 K         -0.03 %    -3 
273+30 K          0.03 %    +3 
273+45 K         -0.15 %    -3
273+45 K          0.15 %    +3
273+25 K         -0.15 %    -3
273+25 K          0.15 %    +3


No matter how far we go from the development temperature, the error is
approx. the same in both directions. The absolute value of error is also
approx. the same for positive and negative voltages.  Ie. the error that
is introduced by using a KT81 instead a "real" tempco is a positive tilt
of the frequency/voltage plot, the osc is runnig too sharp for voltages
>0 and too flat for voltages < 0. The error seems to be acceptable. Note:
this is the upper limit of quality one can get, practical results will
of course be worse than this.

For those who are interested I've included all formulas in gnuplot format
in the attachment.

m.c.
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