ENGI 242/ELEC 222 January 2004

FET Biasing 1

JFET Biasing

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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

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FET Biasing 2

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DC Analysis for JFET Fixed Bias Network

⎛ ⎞⎜ ⎟⎝ ⎠

2GS

D DSSP

VI = I 1 - V

GS GGV = - V

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JFET Fixed Bias Plotting Shockley’s Equation

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FET Biasing 3

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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

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JFET Fixed Bias

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FET Biasing 4

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Graphical Solution for JFET Fixed Bias

⎛ ⎞⎜ ⎟⎝ ⎠

⎛ ⎞⎜ ⎟⎝ ⎠

2GS

D DSSP

2GG

D DSSP

V I = I 1 - V

V I = I 1 - V

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JFET Self Bias Configuration

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FET Biasing 5

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JFET Self Bias DC Equivalent Circuit

VRG = 0

IG = 0

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Transconductance Curve

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FET Biasing 6

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JFET Self Bias Load Line

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JFET Self Bias

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FET Biasing 7

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JFET Self Bias Transconductance Curve

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Graphical Solution of JFET Self Bias

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FET Biasing 8

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JFET Self Bias Effect on Variation of RS

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Self Bias

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FET Biasing 9

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JFET Voltage Divider Bias

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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

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FET Biasing 10

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JFET Voltage Divider Bias Load Line

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The Effect of RS on Q-point

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FET Biasing 11

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JFET Voltage Divider Bias

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JFET Voltage Divider Bias Q-point

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FET Biasing 12

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Voltage Divider Bias

MOSFET Biasing

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FET Biasing 13

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D-MOSFET Self Bias

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Determining the Q-point for D-MOSFET Self Bias

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FET Biasing 14

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N Channel D-MOSFET Voltage Divider Bias

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Q Point of D-MOSFET Voltage Divider Bias

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FET Biasing 15

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Effect on Change in Q Point with Variation of RS

With an N ChannelD-MOSFET,VGS may be positive

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D-MOSFET Fixed Bias

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FET Biasing 16

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E-MOSFET Feedback Bias

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DC Equivalent For the E-MOSFET Feedback Bias

IG = 0

VGS = VDS

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FET Biasing 17

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Transfer Curve for the E-MOSFET Feedback Bias

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Determining the Q-point for E-MOSFET Feedback Bias

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FET Biasing 18

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Q-point for the E-MOSFET Fixed Bias

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EMOS Feedback Bias Example

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

Don

2GSon GSTH

2D GS GSTH

I k = (V - V )

I = k(V - V )

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FET Biasing 19

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EMOS Feedback Example

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Voltage Divider Bias for an N E-MOSFET

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FET Biasing 20

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Q-point for E-MOSFET Voltage Divider Bias