Linear and Switching Mode applications of JFET-s in On-board Electronic Circuits (Current references and Low Current Switches)

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Dr. Szabó József
Department of Broadband Infocommunications and Electromagnetic Theory

The bipolar and field effect transistors (BJT and JFET) are regurarly used in satellite onboard system electronics, even in these days. The output characteristics of the two device has a lot in common, but there is a great difference in their transfer characteristics, because one of them is controlled by charge carriers and the other one is controlled by an electric field. Both devices are susceptible for designing circuits, for instance JFET- or BJT-input operational amplifier, of course their parameters may greatly differ. Simple circuit functions can be sufficied with both of these devices, but because of the difference between their controlling methods it is recommended to use JFET device by certain applications, because it is easier to control and in their stacioner off or on state they do not consume energy. Commercially available current diodes are JFET based devices, where the gate and source outputs are short wired. The use of these devices did not become popular, because the transfer characteristic at zero controlling voltage has a strong temperature dependence. Some types are produced in multiple current categories, but only a small fracture of them can be viewed as temperature indepent devices at some level. If we complement a JFET device with an appropiately chosen resistance, than we can produce a cheap current generator with the exclusion of temperature dependence, whereas devices realized with BJT are much more complex and expensive. The other common application field of JFET devices are low leakage current switches, because their static control practically do not require energy. A JFET can be a device, which can switch extremely low current and in its off state ensures perfect wire break. In this case the leakage current is the limitation of the application. The leakage of a vacuum relay can be significantly lower, but they may also be more costly, their controlling more complex, and also their mass and sizes from a constructial point of view more disadvantegaous. In those applications, where the requirement is not so strict, like 10-100 pA, than some types of JFET devices are applicable. The manufacturers in most cases do not give information about the parameters of the device in extreme conditions. The aim of the final thesis is the elaboration of qualification procedure, which with we can produce special circuits with the selection of commercially available devices.


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