Temperature Analysis—Constant Current Sources

To demonstrate temperature effects, we will look at two circuits that can be used as constant current sources. One circuit will be greatly affected by temperature and the other is designed to be relatively independent of temperature. The circuits use transistors and resistors. The temperature dependence of transistors has already been discussed in detail in Section 4.F. Before we look at the circuits, we will look at how PSpice handles temperature characteristics of resistors. 

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An ionization instrument for the analysis of gas has been developed in which the gas passes through a small chamber where it is irradiated by a small radioactive source. For a constant source of radiation, the ions produced in the gas d nd on the flow velocity of the gas and on its temperature, pressure and atomic composition. The dependence of the ionization on the atomic composition is a consequence of the different ionization potentials of the differrat types of atoms of the gas and the different probabilities for electron capture and collision. The ion current is collected on an electrode and measured. This current is a function of the gas pressure and velocity since the higher the pressure, the more ions form, while at higher velocity, the fewer ions are collected as more ions are removed by the gas prior to collection. Such ionization instruments are used in gas chromatographs and other instruments as well as in smoke detection systems (the normal radiation source is Am, usually 40 kBq), where secondary electrons condense on smoke particles, leading to lower mobility for the electrons and a decreased ion current. 

If we consider reactiorrs (10) and (11), assuming that the Ca, CaF, and SmF+ ion currents in reaction (10) have been measured rather accurately, we can draw the conclusion that the SmFj ion current measured in reaction (11) is in error by about two orders of magnitude. This can be a source of difference ( 50 kj/mol) between the enthalpies of atomization of SmF2 listed in the 6th and 10th columns of Table 76. It is difficult to detect which factor is responsible for such a large difference. However, some conclusions about the quality of these measurements can be drawn from analysis of temperature dependences of equilibrium constants. 

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