O Alasali
MOSFET Circuits at DC
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 1: Design the circuit so that the transistor operates at
I
D
= 0.4mA and
V
D
=
0.5V. The transistor has
V
t
= 0.7V,
m
n
C
ox
= 100
m
A/V
2
,
L
= 1
m
m, and
W
= 32
m
m.
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 2: Design the circuit to obtain a current
I
D
of 80
m
A. Find the value
required for
R
, and find the dc voltage
V
D
. Let the transistor have
V
t
= 0.6V,
m
n
C
ox
=200
m
A/V
2
,
L
= 0.8
m
m, and
W
= 4
m
m.
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 3: Design the circuit to establish a drain voltage of 0.1V. What is the
effective resistance between drain and source at this operating point? Let
V
t
=1V
and
k
n
’
(
W/L
=1 mA/V
2
.
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 4: Analyze the circuit to determine the voltages at all nodes and the currents
through all branches. Let the transistor have
V
t
=1V and
k
n
’
(
W/L
) = 1 mA/V
2
.
O Alasali
MOSFET Circuit at DC
Sedra Exercise 4.8
The PMOS transistor has
V
T
=
1V,
k
p
’
= 60
m
A, and
W/L
=
10
.
a)
Find
the range for
V
G
for which the transistor conducts
.
b)
In
terms of
V
G
find the range of
V
D
for which
the
transistor operates in the triode region
.
c)
In
terms of
V
G
find the range of
V
D
for which
the
transistor operates in the saturation region
.
d)
Neglecting
channel length modulation find the values of

V
OV
,
V
G
and
V
D
to operate the transistor in saturation
mode with
I
D
=75
m
A
.
e)
If
l
=
0.02/V find the value of
r
o
corresponding to the
overdrive voltage found in (d
).
f)
For
l
=
0.02/V
and
V
OV
found in (e
) find
I
D
at
V
D
= 3V
and 0V, also calculate the o/p
resistance in saturation
and. compare
with (e).
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 5: Design the circuit so that the transistor operates in saturation with
I
D
=0.5mA and
V
D
=3V. Let the transistor have
V
t
=
1V and
k
p
’
(
W/L
) = 1
mA
/V
2
.
What is the largest value
R
D
can have while maintaining saturation?
O Alasali
MOSFET Circuit at DC
Sedra
4.3
Example 6: The transistors in the circuit are matched with
k
n
’(
W
n
/L
n
)
=
k
p
’(
W
p
/
L
p
)
=
1 mA/V
2
and
V
tn
=
V
tp
=1V. Find the drain currents
I
DN
and
I
DP
, as well as voltages
v
O
, for
v
I
= 0V, +2.5, and
2.5 V .
O Alasali
Large Signal Operation
Sedra
4.4.1
Common Source (CS) = Grounded Source
Voltage Amplifier
Load Line:
O Alasali
Amplifier Voltage Transfer Characteristic (VTC)
Sedra 4.4.4
A:
B:
O Alasali
Amplifier VTC Example
Sedra
4.5.1
Given a MOSFET with
k
n
’
W
/L
= 1mA/V
2
,
V
t
=1V used in the circuit with
R
D
=18kW,
V
DD
= 10V, and the transistor
is
biased
at
V
OQ
= 4V. Find the gain of the circuit.
Student exercise: Find the gain of the circuit if
V
OQ
was 5V.
O Alasali
MOSFET Operation as a Switch
Sedra
4.4.3
A:
C:
O Alasali
Biasing for Linear Amplifier
Sedra
4.4.4
Voltage Gain:
= Slope at Q.
Slope is
–
ve
→ Inverting Amplifier (180
°
phase shift)
headroom
legroom
O Alasali
MOSFET Small Signal Operation
Sedra 4.6.2
v
gs
<<
V
GS
g
m
:
transconductance
of MOSFET
O Alasali
Separating DC Analysis and Signal Analysis
Sedra 4.6.3
O Alasali
MOSFET Small Signal hybrid
Equivalent Circuit
Sedra
4.6.5
Replace all constant DC voltage sources by short circuits.
Replace all constant DC current sources by open circuits.
g
m
and
r
o
values depend on the bias (
Q
) point chosen.
MOSFET Amplifier Example
Sedra 4.6.5
For
the
amplifier
circuit
determine
the
small

signal
gain,
input
resistance
and
largest
input
signal
possible
.
The
MOSFET
k
n
’
W
/L
=
0
.
25
mA/V
2
,
V
t
=
1
.
5
V
and
V
A
=
50
V
.
The
input
signal
v
i
is
coupled
to
the
gate
by
a
large
capacitor,
also
the
output
signal
is
coupled
to
the
load
R
L
by
a
large
capacitor
.
Thus
the
capacitors
act
as
short
circuits
at
signal
frequencies
.
MOSFET Amplifier Example Contd.
Sedra
4.6.5
For
the
amplifier
circuit
determine
the
small

signal
gain,
input
resistance
and
largest
input
signal
possible
.
The
MOSFET
k
n
’
W
/L
=
0
.
25
mA/V
2
,
V
t
=
1
.
5
V
and
V
A
=
50
V
.
The
input
signal
v
i
is
coupled
to
the
gate
by
a
large
capacitor,
also
the
output
signal
is
coupled
to
the
load
R
L
by
a
large
capacitor
.
Thus
the
capacitors
act
as
short
circuits
at
signal
frequencies
.
DC Bias Circuit
MOSFET Amplifier Example Contd.
Sedra 4.6.5
For
the
amplifier
circuit
determine
the
small

signal
gain,
input
resistance
and
largest
input
signal
possible
.
The
MOSFET
k
n
’
W
/L
=
0
.
25
mA/V
2
,
V
t
=
1
.
5
V
and
V
A
=
50
V
.
The
input
signal
v
i
is
coupled
to
the
gate
by
a
large
capacitor,
also
the
output
signal
is
coupled
to
the
load
R
L
by
a
large
capacitor
.
Thus
the
capacitors
act
as
short
circuits
at
signal
frequencies
.
Q point evaluation
DC Bias Circuit
MOSFET Amplifier Example Contd.
Sedra 4.6.5
For
the
amplifier
circuit
determine
the
small

signal
gain,
input
resistance
and
largest
input
signal
possible
.
The
MOSFET
k
n
’
W
/L
=
0
.
25
mA/V
2
,
V
t
=
1
.
5
V
and
V
A
=
50
V
.
The
input
signal
v
i
is
coupled
to
the
gate
by
a
large
capacitor,
also
the
output
signal
is
coupled
to
the
load
R
L
by
a
large
capacitor
.
Thus
the
capacitors
act
as
short
circuits
at
signal
frequencies
.
Small Signal Analysis
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.1
Biasing by Fixing V
GS
Fixed
V
GS
→
Fixed
I
D
→
Desired Gain
Problem:

C
ox
,
V
t
, and
W/L
vary during manufacturing

V
t
and
m
n
vary with temperature
Not a good
method!
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.2
Biasing by Fixing V
G
& Connecting a Resistance at Source
Let
I
D
↑
→
V
GS
must
↓
→
I
D
↓
(
V
G
fixed):
R
S
provides
Negative Feedback
R
S
: degeneration resistance
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.2
Biasing by Fixing V
G
& Connecting a Resistance at Source
Dc bias circuit
Coupling ac input signal
R
G1
&
R
G2
in M
W
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.2
Biasing by Fixing V
G
& Connecting a Resistance at Source
Dc bias circuit
Coupling ac input signal
dual power supply
O Alasali
Fixing
V
G
& Connecting a Resistance at Source Example
Sedra
4.5.2
Design the circuit to have
I
D
=0.5mA. Given
V
t
=1V,
k
n
’
W
/L
=1mA/V
2
,
V
DD
=15V.
Also find
I
D
% for a transistor whose
V
t
= 1.5V.
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.3
Biasing by Drain

to

Gate Feedback Resistor
R
G
in M
W
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.4
Biasing by Constant

Current Source
R
G
in M
W
O Alasali
Current Mirror Circuit
Sedra
4.5.4
V
D
=
V
G
→
Q
1
in sat.
V
GS Q2
=
V
GS Q1
O Alasali
Biasing MOSFET Amplifier Circuits
(Q point)
Sedra
4.5.4
Constant

Current Source
O Alasali
Constant Current Biasing Using Current Mirror
Example
Determine the bias current
I
Q1
, the
gate

to

source
voltages of the transistors, and the drain

to

source
voltage of
M
1
. Assume
I
REF
= 200
m
A,
V
t
=
0.4
V,
k
‘
n1
(
W/L
)=
0
. 5
mA/V
2
,
k
‘
n2
(
W/L
)=
k
‘
n3
(
W/L
)=
0.3
mA/V
2
, and all transistors are in saturation.
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