# MOSFET Circuits at DC

Electronics - Devices

Oct 7, 2013 (4 years and 9 months ago)

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MOSFET Circuits at DC

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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.

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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.

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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

.

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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
.

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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).

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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?

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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 .

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Large Signal Operation

Sedra

4.4.1

Common Source (CS) = Grounded Source

Voltage Amplifier

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Amplifier Voltage Transfer Characteristic (VTC)

Sedra 4.4.4

A:

B:

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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.

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MOSFET Operation as a Switch

Sedra

4.4.3

A:

C:

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Biasing for Linear Amplifier

Sedra

4.4.4

Voltage Gain:

= Slope at Q.

Slope is

ve

→ Inverting Amplifier (180
°

phase shift)

legroom

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MOSFET Small Signal Operation

Sedra 4.6.2

v
gs

<<
V
GS

g
m
:
transconductance

of MOSFET

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Separating DC Analysis and Signal Analysis

Sedra 4.6.3

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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

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

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

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

R
L

by

a

large

capacitor
.

Thus

the

capacitors

act

as

short

circuits

at

signal

frequencies
.

Small Signal Analysis

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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!

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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

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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

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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

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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.

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Biasing MOSFET Amplifier Circuits
(Q point)

Sedra

4.5.3

Biasing by Drain
-
to
-
Gate Feedback Resistor

R
G

in M
W

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Biasing MOSFET Amplifier Circuits
(Q point)

Sedra

4.5.4

Biasing by Constant
-
Current Source

R
G

in M
W

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Current Mirror Circuit

Sedra

4.5.4

V
D

=
V
G

Q
1

in sat.

V
GS Q2

=
V
GS Q1

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Biasing MOSFET Amplifier Circuits
(Q point)

Sedra

4.5.4

Constant
-
Current Source

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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.