Electronic Devices Laboratory [email protected] CE/EE 3110
Low Frequency Characteristics of Junction Field Effect Transistors
Low Frequency Characteristics of JFETs
Electronic Devices Laboratory [email protected] CE/EE 3110
• 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
Low Frequency Characteristics of JFETs
Electronic Devices Laboratory [email protected] CE/EE 3110
Variation of depletion width across JFET channel (a) and current-voltage characteristics versus gate voltage (b).
Low Frequency Characteristics of JFETs
Electronic Devices Laboratory [email protected] CE/EE 3110
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).
Low Frequency Characteristics of JFETs
Electronic Devices Laboratory [email protected] CE/EE 3110
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
Low Frequency Characteristics of JFETs
Electronic Devices Laboratory [email protected] CE/EE 3110
Low Frequency Characteristics of JFETs
JFET IDS – VDS Curves
Electronic Devices Laboratory [email protected] CE/EE 3110
Low Frequency Characteristics of JFETs
JFET IDS – VDS Curves and Pinch off
Electronic Devices Laboratory [email protected] CE/EE 3110
Low Frequency Characteristics of JFETs
Plots of gm and rd Versus VGS at VDS = 8.0 V