FET Constant Current Source / Limiter Datasheet

FET Constant-Current Source / Limiter

The combination of low associated operating voltage and high output impedance makes the FET attractive as a constant-current source. An adjustable-current source (Figure 1) may be built with a FET, a variable resistor, and a small battery. For optimum thermal stability, the FET should be biased near the zero temperature coefficient point.

A change in supply voltage or a change in load impedance, will change ID by only a small factor because of the low output conductance goss.

The value of goss is an important consideration in the accuracy of a constant-current source where the supply voltage may vary. As goss may range from less than 1 mS to more than 50 mS according to the FET type, the dynamic impedance can be greater than 1 MW to less than 20 kW. This corresponds to a current stability range of 1 mA to 50 mA per volt. The value of goss also depends on the operating point. Output conductance goss decrease approximately linearly with ID.

Whenever the FET is operated in the current saturated region, its output conductance is very low. This occurs whenever the drain-source voltage VDS is at least 50% greater than the cut-off voltage VGS(off). The FET may be biased to operate as a constant-current source at any current below its saturation current IDSS.

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