12 Fet Dc Biasing

JFET Biasing

ENGI 242/ELEC 222

January 2004 ENGI 242/ELEC 222 2

JFET Fixed Bias

RG is present to limit current in case VGG is connected with wrong polarityThis would forward bias the gate-source junction causing high currents, which would destroy the transistor

Since GS is reverse biased, IG = 0 and VRG = 0

GS

GG G G

GG

GS

G

V + I R + V = 0SinV = -

ce I = V

0

January 2004 ENGI 242/ELEC 222 3

DC Analysis for JFET Fixed Bias Network

2GS

D DSSP

VI = I 1 - V

GS GGV = - V

January 2004 ENGI 242/ELEC 222 4

JFET Fixed Bias Plotting Shockley’s Equation

January 2004 ENGI 242/ELEC 222 5

JFET Fixed Bias Graphical Solution

GS GG

2GS

D DSSP

2GG

D DSSP

Since V = - V

V I = I 1 - V

V I = I 1 - V

January 2004 ENGI 242/ELEC 222 6

JFET Fixed Bias

January 2004 ENGI 242/ELEC 222 7

Graphical Solution for JFET Fixed Bias

2GS

D DSSP

2GG

D DSSP

V I = I 1 - V

V I = I 1 - V

January 2004 ENGI 242/ELEC 222 8

JFET Self Bias Configuration

January 2004 ENGI 242/ELEC 222 9

JFET Self Bias DC Equivalent Circuit

VRG = 0

IG = 0

January 2004 ENGI 242/ELEC 222 10

Transconductance Curve

January 2004 ENGI 242/ELEC 222 11

JFET Self Bias Load Line

January 2004 ENGI 242/ELEC 222 12

JFET Self Bias

January 2004 ENGI 242/ELEC 222 13

JFET Self Bias Transconductance Curve

January 2004 ENGI 242/ELEC 222 14

Graphical Solution of JFET Self Bias

January 2004 ENGI 242/ELEC 222 15

JFET Self Bias Effect on Variation of RS

January 2004 ENGI 242/ELEC 222 16

Self Bias

January 2004 ENGI 242/ELEC 222 17

JFET Voltage Divider Bias

January 2004 ENGI 242/ELEC 222 18

JFET Voltage Divider Bias DC Analysis

Input Output

2G DD

1 2

R V = V R + R

DS DD D D SV = V - I (R + R )G GS D S

GS G D S

-V + V + I R = 0V = V - I R

January 2004 ENGI 242/ELEC 222 19

JFET Voltage Divider Bias Load Line

January 2004 ENGI 242/ELEC 222 20

The Effect of RS on Q-point

January 2004 ENGI 242/ELEC 222 21

JFET Voltage Divider Bias

January 2004 ENGI 242/ELEC 222 22

JFET Voltage Divider Bias Q-point

January 2004 ENGI 242/ELEC 222 23

Voltage Divider Bias

MOSFET Biasing

ENGI 242/ELEC 222

January 2004 ENGI 242/ELEC 222 25

D-MOSFET Self Bias

January 2004 ENGI 242/ELEC 222 26

Determining the Q-point for D-MOSFET Self Bias

January 2004 ENGI 242/ELEC 222 27

N Channel D-MOSFET Voltage Divider Bias

January 2004 ENGI 242/ELEC 222 28

Q Point of D-MOSFET Voltage Divider Bias

January 2004 ENGI 242/ELEC 222 29

Effect on Change in Q Point with Variation of RS

With an N ChannelD-MOSFET, VGS may be positive

January 2004 ENGI 242/ELEC 222 30

D-MOSFET Fixed Bias

January 2004 ENGI 242/ELEC 222 31

E-MOSFET Feedback Bias

January 2004 ENGI 242/ELEC 222 32

DC Equivalent For the E-MOSFET Feedback Bias

IG = 0

VGS = VDS

January 2004 ENGI 242/ELEC 222 33

Transfer Curve for the E-MOSFET Feedback Bias

January 2004 ENGI 242/ELEC 222 34

Determining the Q-point for E-MOSFET Feedback Bias

January 2004 ENGI 242/ELEC 222 35

Q-point for the E-MOSFET Fixed Bias

January 2004 ENGI 242/ELEC 222 36

EMOS Feedback Bias Example

VGSTH = 4VVGSon = 7.5VIDon = 5mAVDD = 22V

Don

2GSon GSTH

2D GS GSTH

I k = (V - V )

I = k(V - V )

January 2004 ENGI 242/ELEC 222 37

EMOS Feedback Example

January 2004 ENGI 242/ELEC 222 38

Voltage Divider Bias for an N E-MOSFET

January 2004 ENGI 242/ELEC 222 39

Q-point for E-MOSFET Voltage Divider Bias