12 fet dc biasing
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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
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