[sdiy] [OT] PTAT with PN junction

[ASSI]

On Thursday 13 March 2003 07:44, Tim Ressel wrote:
> There seems to be a hole in my knowledge. I an using a
> transistor PN junction as a temperature sensor (PTAT)
[...]

What you really want is two junctions at the same temperature forward 
biased with different current densities, a single junction is never 
proportional to anything. In integrated circuits this is simply done by 
scaling the area of one versus the other so you can use a current 
mirror to drive both at the same current. With discretes, it is usually 
simpler to just use two different currents (the current ratio needs to 
be controlled carefully), otherwise you need an array and gang 7 vs. 1 
diode or some such.

Check out the design manual at http://www.arraydesign.com for instance 
for the gory details and lots of useful ideas on bipolar circuits 
(especially the section on deltaVbe).


Achim.
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