EKT104 ANALOG
ELECTRONIC CIRCUITS
[LITAR ELEKTRONIK ANALOG]
INTRODUCTION TO FET
AMPLIFIER:
FET REVIEW
DR NIK ADILAH HANIN BINTI ZAHRI
adilahhanin@unimap.edu.my
1
FIELD EFFECT TRASISTOR (FET)
•
Advantages Of FET
•
Types Of FET & Its Operation
2
FET Advantages
• Voltage-controlled amplifier
 input impedance very high
• Low noise output
 useful as preamplifiers when noise must be very low because
of high gain in following stages
• Better linearity
 distortion minimized
• Low inter-electrode capacitance
at high frequency, inter-electrode capacitance can make
amplifier work poorly. FET desirable in Radio Frequency stages
(high frequency)
3
TYPES OF FET
FET
JFET
MOSFET
 n channel
Enhancement mode
 p channel
 n channel
MESFET
 p channel
Depletion mode
 n channel
 p channel
4
JUNCTION FIELD EFFECT
TRANSISTOR (JFET)
D
ohmic
contact
n-channel
G
p n p
D
Structure
G
n p n
S
S
D
D
G
Symbol
S
p-channel
G
S
5
METAL-OXIDE-SEMICONDUCTOR FIELD
EFFECT TRANSISTOR (MOSFET)
DEPLETION
p
n
p
dielectric
ENHANCEMENT
metal
p
n
p
6
n-channel
p-channel
JFET OPERATION
depletion region
n
n
VDD
p
VGG
p
n
VDD 
p
p
n
• Gate-source is reversed-biased
•  zero current at gate
• IDS flow through the channel and the value is determined by the width of
depletion region and the width of the channel
7
MOSFET OPERATION
electron
inversion layer
G
S
D
n+
n+
G
S
n+
D
-------
p-type
p-type
SS
SS
n+
• No voltage applied to gate
• +ve voltage applied to gate
• Current is zero
• Electron inversion layer is created
• Current is generated between
source and drain
8
FET BIASING


JFET BIAS CIRCUITS
•
•
Self-bias
Voltage-divider bias
MOSFET BIAS CIRCUITS
•
•
Voltage-divider bias
Drain-feedback bias
9
EQUIVALENCE BIASING OF
JFET & BJT
JFET
 VGS
I D  I DSS 1 
 VP
ID  IS
IG  0 A
BJT



2
<==>
I C  I B
<==>
IC  I E
<==>
VBE  0.7V
10
JFET BIAS CIRCUITS
- SELF-BIAS+VDD
RD
VGS  VG  VS   I D RS
IG = 0
RG
RS
VDS  VDD  I D RD  RS 
11
JFET BIAS CIRCUITS
- VOLTAGE-DIVIDER BIAS+VDD
R1
RD
ID
 R2
VG  
 R1  R2
VG
R2
RS
ID

 VDD

VG  VGS

RS
12
MOSFET BIAS CIRCUITS
- VOLTAGE-DIVIDER BIAS+VDD
R1
RD
VGS

R2
 
 R1  R2

 VDD

VDS  VDD  I D RD
R2
where I D  K VGS  VTN 
2
K in formula can be calculated by substituting data
sheet values ID(on) for ID and VGS at which ID(on) is
specified for VGS
13
MOSFET BIAS CIRCUITS
- DRAIN-FEEDBACK BIAS+VDD
RG
RD
VGS  VDS  VDD  I D RD
IG = 0
14
LOAD LINE
•
•
self-biased JFET
voltage-divider bias JFET
15
LOAD LINE
- SELF-BIASED JFET+VDD
9V
Example:
RD
Determine the Q-point for the
JFET circuit. The transfer
characteristic curve is given in
the figure.
2.2K
RG
RS
10M
680
16
SOLUTION
For ID=0,
VGS=-IDRS=(0)(680)=0V
From the curve,
ID (mA)
IDSS=4mA; so ID=IDSS=4mA
VGS=-IDRS=-(4m)(680)=-2.72V
Q point is the
intersection between
the transfer
characteristic
curve and the
load line
-VGS (V)
4 IDSS
ID=2.25mA
VGS=-1.5V
Q
2.25
-6
VGS(off)
-2.72 -1.5
17
LOAD LINE
- VOLTAGE-DIVIDER BIAS JFET+VDD
8V
Example:
Determine the Q-point for
the JFET circuit. The transfer
characteristic curve is given
in the figure.
R1
RD
2.2M
680
R2
2.2M
RS
3.3K
18
SOLUTION
For ID=0,
 R2 
 2.2 
VDD  
VGS  VG  
 8  4V
 4.4 
 R1  R2 
For VGS=0,
ID 
VG  VGS VG
4


 1.2mA
RS
RS 3.3K
ID (mA)
12 IDSS
ID=1.8mA
VGS=-1.8V
Q point is the
intersection between
the transfer
characteristic
curve and the
load line
Q
1.8
-VGS (V)
-3 -1.8
VGS(off)
1.2
4
VGS (V)
19
EXERCISES
-Load Line JFET1. Determine the Q-point for the JFET circuit.
The transfer characteristic curve is given in
the figure.
+VDD
6V
RD
820
ID (mA)
IDSS = 5mA
-VGS (V)
VGS(off)=-3.5
RG
RS
10M
330
20
EXERCISES
-Load Line JFET2. Determine the Q-point for the JFET circuit.
The transfer characteristic curve is given in
the figure.
ID (mA)
IDSS = 5mA
RD
3.3M
1.8K
2.2M
VGS(off)=-4V
12V
R1
R2
-VGS (V)
+VDD
RS
3.3K
21
FET CHARACTERISTICS
•
•
JFET
MOSFET
22
JFET CHARACTERISTICS
• Drain Characteristic
ID
VGS
IDSS
VGS=0
ohmic region
breakdown region
Saturation
region
Constant
current
area
VGS(off)
VP
(Pinch –off voltage)
VDS
23
JFET CHARACTERISTICS
• Transfer Characteristic
ID

I D  I DSS 1 

VGS 2
VP
IDSS
N-CHANNEL
-VGS
VP
24
JFET DATA SHEET
For MMBF5459 VGS (off) = -8.0V (max)
IDSS = 9.0 mA (typ.)
25
MOSFET CHARACTERISTICS
• Transfer Characteristic (Depletion MOSFET)
ID

I D  I DSS 1 

VGS 2
VP
-VGS
N-CHANNEL
VGS(off)=VP
IDSS
26
MOSFET CHARACTERISTICS
• Transfer Characteristic (Enhancement MOSFET)
ID
I D  K VGS  VTN 
2
N-CHANNEL
K in formula can be calculated by
substituting data sheet values ID(on)
for ID and VGS at which ID(on) is
specified for VGS
VTN
+VGS
27
E-MOSFET DATA SHEET
ID(on) = 75 mA (minimum) at
VTN = 0.8 V and VGS = 4.5V
K
I D ( on)
VGS  VTH 2

75mA
2

5
.
48
mA
/
V
4.5  0.82
28