possible tempco alternative link

[Rene Schmitz]

Hi Martin and DIYers!

>We could adapt the additonal series R so that it fits the tempco
>at eg. 30C.  This means 1321Ohm series resistor and gives the needed
>3361ppm @30C.  The original tempco is scaled with 0.4308 by this, so we
>get a new tempco table for KTY81 + series R 1321Ohm, and compare that
>to the theoretical ideal needed tempco:
>
>Tj C  tempco   Ohm (typ) needed tempco
>------------------------------------------------
>
>  0   3662    2136       3723.7
> 10   3576    2189       3594.3
> 20   3446    2282       3473.7
> 25   3403    2321       
> 30   3360    2361       3361.1
> 40   3231    2443       3255.7
> 50   3145    2530       3156.8
> 60   3059    2620       3063.9
> 70   2973    2713       2976.4
> 80   2886    2811       2893.8
> 90   2800    2912       2815.8
>100   2714    3017       2742.0
>
>We can see that for an ideal Transistor pair, the nonlinear tempco of
>the KTY series is not harmfull at all, it helps. It gives better
>compensation then a wirewound linear tempco.

I'm speechless! I didn't analyze this so deep. Many thanks Martin, for
doing the math for me. It matches to my experimental findings, my VCO3
which uses these, is quite stable. (Although I have to admit that the
scheme I use has an opamp output offset drift problem. But this has nothing
to do with the KTY in general.) 
Seems as if I have to change the description on my website.
Until now I assumed that the required tempco was linear, so I thouhgt that
the KTY would be inferior to a wirewound tempco. Your figures tell me that
I'm wrong.

The KTYs come in different resistor values the KTY 81-1xx is 1k, the KTY
81-2xx is 2k.
And they're available with different tolerances: -110 means 1k, +/-1%; -120
means 1k, +/-2%. One could try if a 2k KTY with a parallel resistor gives
(even) better results.

>Of course there are the typical problems: Ib is not zero, Ics variation,
>matching, self heating, thermal gradients. It is not clear to me in the
>moment if these effects are not stronger then anything else considered
>here.

Well here my argument is that it is the same regardless of the type of
tempco used.
There is not much that you can do against these errors, exept using good
pairs, and thermal coupling of the resistor to the transistors. The KTY
comes in a TO-92, and can hence be easily glued to something like a 2SC1583
or a minidip.
Btw, I've put up a page which compares Rbe for various transistor pairs,
I've measured these for a bunch of devices. The link is on the startpage of
my site at uni-bonn.

Bye
 René




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