ANALOGUE ELECTRONICS CIRCUITS 1 EKT 204 Introduction to FET Amplifier: FET (Review) 1
Jan 21, 2016
ANALOGUE ELECTRONICS
CIRCUITS 1
EKT 204
Introduction to FET Amplifier:
FET (Review)
1
FIELD EFFECT TRASISTOR (FET)
ADVANTAGES OF FET TYPES OF FET & ITS
OPERATION
2
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 RF stages (high frequency)
FET Advantages
3
Types of FETFET
JFET MOSFET MESFET
n channel Enhancement mode
p channel n channel
p channel
Depletion mode
n channel
p channel 4
Junction FET (JFET)
n-channel p-channel
G np p
S
D
G pn n
S
D
D
S
G
D
S
G
Structure
Symbol
ohmic contact
5
Metal-Oxide-Semiconductor MOS (MOSFET)
DE
PL
ET
ION
EN
HA
NC
EM
EN
T
n-channel p-channel
p
p
p
p
n
n
dielectric
metal
6
JFET Operation
n
n
p p
n
n
p p
VGG
VDDVDD
depletion region
• 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
p-type
n+n+
p-type
n+n+ - - - - - - -
G GD DS S
SSSS
• No voltage applied to gate
• Current is zero
• +ve voltage applied to gate
• Electron inversion layer is created
• Current is generated between source and drain
electron inversion layer
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
AI
II
V
VII
G
SD
P
GSDSSD
0
12
VV
II
II
BE
EC
BC
7.0
<==>
<==>
<==>
JFET BJT
10
JFET Bias Circuits - Self-Bias
+VDD
RD
RSRG
SDSGGS RIVVV
SDDDDDS RRIVV
IG = 0
11
JFET Bias Circuits - Voltage-divider Bias
+VDD
RD
RS
R1
R2
DDG VRR
RV
21
2
S
GSGD R
VVI
VG
ID
12
MOSFET Bias Circuits - Voltage-divider Bias
+VDD
R1 RD
R2
2TNGSD
DDDDDS
VVKIwhere
RIVV
DDGS VRR
RV
21
2
13
MOSFET Bias Circuits - Drain-Feedback Bias
+VDD
RDRG
DDDDDSGS RIVVV
IG = 0
14
LOAD LINE
SELF-BIASED JFET VOLTAGE-DIVIDER
BIAS JFET
15
LOAD LINE- SELF-BIASED JFET
+VDD 9V
RD
2.2K
RS
680
RG
10M
Example
Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
16
ID (mA)
-VGS (V)
4 IDSS
-6
VGS(off)
2.25
-1.5-2.72
Q
For ID=0,
VGS=-IDRS=(0)(680)=0V
From the curve,
IDSS=4mA; so ID=IDSS=4mA
VGS=-IDRS=-(4m)(680)=-2.72V
ID=2.25mAVGS=-1.5V
Q point is the intersection between
the transfer characteristiccurve and the
load line
17
LOAD LINE - VOLTAGE-DIVIDER BIAS JFET
+VDD
8V
RD
680
R1
2.2M
R2
2.2MRS
3.3K
Example
Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
18
ID (mA)
-VGS (V)
12 IDSS
-3
VGS(off)
1.8
1.2-1.8
Q
4VGS (V)
For ID=0,
VVRR
RVV DDGGS 48
4.4
2.2
21
2
mAKR
V
R
VVI
S
G
S
GSGD 2.1
3.3
4
For VGS=0,
ID=1.8mAVGS=-1.8V
Q point is the intersection between
the transfer characteristiccurve and the
load line
19
EXERCISES (Load Line JFET)
+VDD 6V
RD
820
RS
330
RG
10M
1. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
ID (mA)
-VGS (V)
IDSS = 5mA
VGS(off)=-3.5
20
2. Determine the Q-point for the JFET circuit. The transfer characteristic curve is given in the figure.
+VDD
12V
RD
1.8K
R1
3.3M
R2
2.2MRS
3.3K
ID (mA)
-VGS (V)
IDSS = 5mA
VGS(off)=-4V
EXERCISES (Cont.)
21
FET CHARACTERISTI
CS
JFET MOSFE
T
22
JFET CHARACTERISTICS
DRAIN CHARACTERISTIC
ohmic region
Saturation region
breakdown region
VDS
ID
VP
IDSS
VGS
VGS=0
VP=|VGS (off)|23
TRANSFER CHARACTERISTIC
N-CHANNEL
ID
VP
IDSS
-VGS
21P
GS
VV
DSSD II
JFET CHARACTERISTICS
24
JFET DATA SHEET
For MMBF5459 VGS (off) = -8.0V (max)
IDSS = 9.0 mA (typ.) 25
MOSFET CHARACTERISTICS TRANSFER CHARACTERISTIC
(Depletion MOSFET)ID
VGS(off)=VP
IDSS
-VGS
N-CHANNEL 21P
GS
VV
DSSD II
26
TRANSFER CHARACTERISTIC (Enhancement MOSFET)
ID
VTN +VGS
N-CHANNEL 2TNGSD VVKI
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
MOSFET CHARACTERISTICS
27
E-MOSFET DATA SHEET
2
22
)( /48.58.05.4
75VmA
mA
VV
IK
TNGS
onD
ID(on) = 75 mA (minimum) at
VTN = 0.8 V and VGS = 4.5V
28