Transcription of The FET Constant-Current Source/Limiter
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The FET Constant-Current Source/Limiter
AN103 Siliconix10-Mar-97 1 The FET Constant-Current Source/LimiterIntroductionThe combination of low associated operating voltage andhigh output impedance makes the FET attractive as aconstant- current source. An adjustable- current source (Fig-ure 1) may be built with a FET, a variable resistor, and asmall battery. For optimum thermal stability, the FET shouldbe biased near the zero temperature coefficient +Figure Transistor current SourceRSRLNO TAGW henever the FET is operated in the current saturated re-gion, its output conductance is very low. This occurswhenever the drain-source voltage VDS is at least 50%greater than the cut-off voltage VGS(off). The FET may bebiased to operate as a Constant-Current source at any cur-rent below its saturation current Source BiasingFor a given device where IDSS and VGS(off) are known, theapproximate VGS required for a given ID is(1)VGS VGS(off) 1 IDIDSS 1 k where k can vary from to , depending on device ge-ometry.
The best FETs for current sources are those having long gates and consequently very low goss. The Siliconix 2N4340, J202, and SST202 exhibit typical g oss = 2 S at V DS = 20 V. These devices in the circuit of Figure 4 will provide a current source adjustable from 5 A to 0.8 mA with internal impedance greater than 2 M at 0.2 mA.
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