Termistor and Transistors as Temperature Sensors
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Temperature measuring chips that use external sensors
generally are designed to use either thermistors or
transistors.

Exceptions are the Winbond W83782D and W83783S which claim
to be able to use both, however it isn't clear how
to configure these to use transistors.



Thermistors
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(summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm  -
 most thermistor companies don't have much on their web sites but
 Thermometrics has a comprehensive guide)

NTC (negative temperature coefficient) thermistors
have a parameter B (beta), units are K (kelvin).
B is the "material constant" of the thermistor and
is a measure of the change of resistance with respect to temperature.
It is the slope of the line graphing (ln R) vs. (1/T).

B values for three common termistor materials are
approximately 3400K, 3900K, and 4300K.

The other measure of thermistors is the temperature coefficient 'a' (alpha).
This is the percentage change in resistance for a given change
in temperature. This varies with temperature.

T = temp in Kelvin (C + 273)

A = (dR/dT) / R

A = - B / T**2

So by the last equation, a thermistor with a Beta of 3900 will change
resistance about 4.4% for 1 degree change at 25C (298K).
For a beta of 3423 as recommended in the 782D data sheet the change is 3.8%.

For thermistors, resistance (= voltage for a constant current)
is exponentially related to temperature.



Transistors
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To use a transistor as a sensor, hook the base of an NPN transistor
(such as the 3904) to the collector.

This is from Richard Dorf, "Electrical Engineering Handbook".

For a diode, the voltage Vd = Vt ln(Id/Is).
Vt = kT/q where k = Boltzman's constant 1.38 e-23;
                q = elementary charge 1.6 e-19;
                T = temp in kelvin.
		Is is reverse saturation current.

(more math and greek letters I don't understand completely)

dVd/dT works out to, at room temperature, about 2.1 mV/degree
or a temperature coefficient of about 0.32%/degree.

For diodes, voltage is linear with temperature.