A Precision Silicon Transistor Thermometer

This article describes a mass-producible electronic thermometer employing an inexpensive transistor as a temperature sensor. The instrument features ±0.1°C accuracy from -50 to + 125°C; ±0.02°C stability throughout a 1000-day 125°C temperature cycle test; and probes that are freely interchangeable with no calibration by user. Probes need be factory-calibrated at only one temperature, and are based on a novel low-thermal-mass hermetic transistor package. Also described are the theoretical analysis and experiments carried during development. It is shown that sensor transistor Vbe will vary almost linearly with temperature if collector current is an appropriate quadratic function of absolute temperature. Effects of various current functions on Vbe linearity are theoretically analyzed and experimental results given for comparison. The technique used to find the optimum current function is explained, and the circuit which generates the function is described. It is shown that using a more expensive function generator and a 3-point sensor calibration will yield ±0.01°C accuracy. Also discussed is the degree to which different commercially available transistors conform to the theoretical predictions. Criteria are given for selecting an appropriate transistor type.