I am in a measurement systems class at school, and we have a project to design a measurement system. I was reading my text book when a diagram caught my fancy, and I thought I would try to build it. I have a few questions regarding it.
Bipolar Transistor PTAT Cell: http://i.imgur.com/Slkftk4.png
The governing relationship is that the difference between the two emitter voltages is equal to the thermal voltage times the log of the proportion of the currents
Vptat = Vt*ln(I/Is)
The first question I have is how to do a constant current source to bias the BJT's. I was thinking along the lines of using a Widlar Current source like so. http://www.ami.ac.uk/courses/ami4409_amsicd/u01/images/amsicd_btccsd_img08.gif
Secondly, the temperature that this setup measures is the absolute temperature of the transistor because that is what effects the voltage to current relationship. So, if I wanted to make this cell measure ambient temperature, could I just stick a heat sink over the top of the transistors so the sink would help keep the transistor close to ambient temp?