Bipolar Transistor PTAT Cell and setup

Question

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: https://i.sstatic.net/5yQZH.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?

Thanks, Jon

Answer 1

If you keep the current levels reasonable so that self-heating is not excessive, the temperature should track ambient fairly well, with some offset. A heatsink would reduce the offset. You can predict the offset from the thermal resistance figures found in transistor datasheets. Hint: You probably want to be more like 10uA/100uA than 1A/10A.

You could use the Widlar current source, but it will have a temperature dependency. You could consider just using resistors in a 10:1 ratio connected to a relatively high voltage (think about first-order correction schemes for Vbe). Or just keep the current sources at a constant temperature.

Note also that you could use the same transistor for the two voltages, just time-multiplex between two currents and look at the difference in voltages.