AN103

www.DataSheet4U.com AN103 The FET Constant-Current Source/Limiter Introduction 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. D S RL RS – + 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. DID = (DVDS)(goss) (3) 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. The relationship is ID g oss + IDSS gȀ oss (4) (5) (6) Figure 1. Field-Effect Transistor Current Source NO TAG where goss = gȀoss when VGS = 0 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 cur...