Theory and application of self-heated thermistors

A thermistor is defined in conjunction with its application as a two-terminal circuit element. The three major thermistor characteristics, by which all applications are classified, are defined and discussed. Of the three characteristics, the static voltage-current characteristic is the subject of emphasis in the discussion, with mention made of both the corresponding applications and the theoretical and practical limitations on the performance of the incorporated thermistors. Small-and large-signal analysis techniques are described and a small-signal equivalent circuit for a thermistor is developed. Both the large- and small-signal analysis techniques are applied to typical thermistor sensor stage circuits wherein the operation of the thermistor is defined by its static voltage-current characteristic. It is shown that the thermistor small-signal analysis, as based on the application of a thermistor small-signal equivalent circuit, and the large-signal analysis, as accomplished by graphical techniques, are completely compatible with their respective counterparts in vacuum tube analysis. As an illustrative example, a thermal conductivity gas analyzer is designed and experimentally evaluated against its theoretical performance as predicted by large-signal analysis. Excellent agreement is shown. A portion of the total range of the gas analyzer is then chosen such as to be in keeping with the constraints imposed on the use of the small-signal equivalent circuit and, again, it is shown that the theoretical response is experimentally verified.