Low Frequency Characteristics of JFETs

Electronic Devices Laboratory [email protected] CE/EE 3110

Low Frequency Characteristics of Junction Field Effect Transistors

Low Frequency Characteristics of JFETs


• Junction FET (JFET) one form of an FET transistor Voltage-variable depletion width used to control transistor operation Causes pinch-off and device saturation Exhibits both a linear and saturated I-V characteristic Small signal operation modeled by transconductance and shunt resistance

in pinched-off mode of operation



Variation of depletion width across JFET channel (a) and current-voltage characteristics versus gate voltage (b).



2N5485 n-channel JFET IDS-VDS characteristics (upper left), input-output characteristic (iD(sat.) vs vGS) of a JFET (upper right), and small

signal model of a JFET in pinched off mode of operation (bottom).



Measurements:

1) ID(Sat.) = IDSS(1+VGS/VP)2

2) VP = -VGS + VDS(Sat.)

3) gm = ΔiD/ΔνGS (νDS = const.)

4) rd = ΔνDS/ΔiD (νGS = const.)

Experimental values for Vp, ID(Sat.), gm, and rd and theoretical values for I D(Sat.).

Plot the IDS versus VDS characteristic and show pinch-off locus in the plotPlot both the experimental and theoretical curves for ID(Sat.) versus νGS

Determine gm and rd at νDS = 8 V




JFET IDS – VDS Curves



JFET IDS – VDS Curves and Pinch off



Plots of gm and rd Versus VGS at VDS = 8.0 V

Low Frequency Characteristics of JFETs

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