possible tempco alternative link

[Martin Czech]

I should have added the link:

http://www-us.semiconductors.philips.com/similar/KTY81


Note: They are reported to work, and I believe this.
      But I have not tryed myself so far.

But even if they cause (yet unknown) problems for vco precision stuff,
they might be useable for other compensation, e.g. extra log,exp
circuits, vca,vcf, lfo, wave shaper etc etc.

KTY81-1 series look like a SOD70 plastic transistor, bipolar type
Tempco @10C 8300ppm/K
       @70C 6900ppm/K

KTY82-1 series look like a SMD SOT23 plastic transistor, bipolar type
Tempco @10C 8300ppm/K
       @70C 6900ppm/K

KTY83-1 series look like a diode, these are the unipolar types!
Tempco @10C 8000ppm/K
       @70C 6700ppm/K

KTY84-1 series look also like a diode, these are the unipolar types!
They seem to be optimised for linearity.
Tempco @10C 7700ppm/K
       @70C 6600ppm/K


all are about 1000Ohm @ 25C,positive tempco, max current is 10mA-2mA
(hot), but the lower the current, the less self heating of course.
For our applications 1mA should suffice.

The series have up to 7-9 members, these seem to be resistance sorting
bins.

The tempco is too large, but this is not harmfull, because we can always
lower it with series resistors (but of course not get higher).  Further
the tempco is not linear, which seems to be a bad thing on first sight, a
series resistor will linearize, so that the tempco doesn't vary that much.

But:

the ideal tempco needed varies also with temperature.
Some examples of an IDEAL transistor pair:

Tj C  needed temco for 1V/oct
----------------------------
  0   3723.7
 10   3594.3
 20   3473.7
 30   3361.1
 40   3255.7
 50   3156.8
 60   3063.9
 70   2976.4
 80   2893.8
 90   2815.8
100   2742.0


The KTY81 data sheet tells

Tj C  tempco   Ohm (typ)
------------------------

  0   8500     815
 10   8300     868
 20   8000     961
 25   7900    1000
 30   7800    1040
 40   7500    1122
 50   7300    1209
 60   7100    1299
 70   6900    1392
 80   6700    1490
 90   6500    1591
100   6300    1696


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.

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.

Anyway: thanks to Rene for digging out these parts.

A typo: The precision linear temperature elements are of course Pt,
Platinum.  They are called Pt100 etc. Farnell, Conrad seems to carry them.
Very precise, available, but 4-5 Euro. Ok, we don't need hundreds of them.


m.c.