4.4 MOSFETS in IC CSA CGA.pdf

Department of Electrical and
Computer Engineering
ECSE-330B Electronic Circuits I
MOSFETs 1
Biasing in MOSFET Amplifiers
• Biasing: Creating the circuit to establish the desired
DC voltages and currents for the operation of the
amplifier
• Four common ways:
1. Biasing by fixing VGS
2. Biasing by fixing VG and connecting a resistance in the
Source
3. Biasing using a Drain-to-Gate Feedback Resistor
4. Biasing Using a Constant-Current Source

MOSFETs 2
• Biasing by fixing VGS
When the MOSFET device is
changed (even using the same
supplier), this method can result in
a large variability in the value of
ID. Devices 1 and 2 represent
extremes among units of the same
type.
( )2
2
1
t
GS
n
D V
V
L
W
k
I −
′
=
ox
n
ox
ox
n
n C
t
k μ
ε
μ =
=
′
VDD
-VEE
VGG
RG
RD
RS
M1
VG
VD
VS

MOSFETs 3
• Biasing by fixing VG and connecting a resistance in the Source
Degeneration Resistance

MOSFETs 4
• Biasing using a Drain-to-Gate Feedback Resistor
Large Resistor

MOSFETs 5
• Biasing Using a Constant-Current Source
Figure 4.33 (a) Biasing the MOSFET using a constant-current source I. (b) Implementation of the constant-current source I
using a current mirror.
Current Mirror
Used in Integrated Circuits

MOSFETs 6
Current Mirror DC Analysis
• The width and length (the W/L aspect
ratio) and the parameters of the two
transistors can be different
• We can choose W/L freely
• In this circuit, consider W/L of both
MOSFETs are the same and transistors
are identical. The Gate-Source voltages
are also the same, then
( )2
1
1
2
1
t
GS
n
REF V
V
L
W
k
I −
′
=
( )2
1
1
2
1
t
GS
n V
V
L
W
k
I −
′
=
REF
REF
I
I
W
L
L
W
I
I
=
=
⋅
= 1
1
1
1
1
W1
L1
VGS VGS
+
-
-
+
W1
L1
I

MOSFETs 7
• Designing IREF
R
V
V
V
I SS
GS
DD
REF
+
−
=
( )2
1
1
2
1
t
GS
n
REF V
V
L
W
k
I −
′
=
• It is often needed to find the value of R in order to
achieve a desired IREF

MOSFETs 8
Biasing of MOSFET Amplifier
• 1- Intro to MOS Field Effect Transistor (MOSFET)
• 2- NMOS FET
• 3- PMOS FET
• 4- DC Analysis of MOSFET Circuits
• 5- MOSFET Amplifier
• 6- MOSFET Small Signal Model
• 7- MOSFET Integrated Circuits
• 8- CSA, CGA, CDA
• 9- CMOS Inverter & MOS Digital Logic

MOSFETs 9
MOSFET Design Space
• Modern integrated circuits use MOSFETs
extensively
– Very high densities of transistors – up to 109
transistors/cm2 in some ULSI memory arrays.
– Off-chip discrete resistors and capacitors are NOT
commonly used
– On-chip resistors and capacitors generally small
– Multistage amplifiers are usually DC-coupled
• Transistors used wherever possible to implement
current sources, resistors, capacitors,

MOSFETs 10
Using MOSFETs to implement R’s and C’s
• Resistors:
Active Loads (large R’s)
Diode-connected loads (small R’s)
MOSFET Triode-Region (moderate R’s)
• Capacitors
Most obvious is the gate-body capacitor
Can be used to have variable-capacitors as well
• Current Mirrors

MOSFETs 11
MOSFET Active Loads
• MOSFETs used as an active load for high resistances:
– MOSFET is held in saturation with the source and gate
held at a constant DC voltage
– Drain connected to circuit
vgs = 0
gmvgs = 0
D
o
I
r
⋅
=
λ
1
– ro is inversely proportional to ID
Rin= ?
Rin=ro

MOSFETs 12
Diode-Connected MOSFETs
• A Diode connected MOSFET can be used to achieve
small resistances:
– The Drain is directly connected to Gate, and therefore it
can only be operated in saturation (or cutoff)
Source Absorption Theorem

MOSFETs 13
MOSFET Current Mirrors
• Used extensively in
MOSFET IC applications
• Often ro,is neglected. Since
there is no gate current, the
drain currents of M1 and M2
are identical
• In practice, IREF ≠ I due to
finite ro. (Not included in EC1)
VGS VGS
+
-
-
+
0 0
( ) )
1
(
2
1 2
1
1
X
t
X
n
REF V
V
V
L
W
k
I λ
+
−
′
=
1
1 DS
GS
X V
V
V =
=
VX
The current I will also depend on VDS2
I

MOSFETs 14
• The width and length (the W/L
aspect ratio) of MOSFETs can
be designed almost freely
• Since the W/L of M1 and M2
need not be the same, the size
ratios can affect current ratios
( )2
1
1
2
1
t
GS
n
REF V
V
L
W
k
I −
′
=
( )2
2
2
2
1
t
GS
n V
V
L
W
k
I −
′
=
1
1
2
2
W
L
L
W
I
I
REF
⋅
=
W1
L1
W2
L2
VGS VGS
+
-
-
+
I

MOSFETs 15
Current Scaling (Steering)
• Ratio of
aspect ratios
can be
selected to
achieve
nearly any
scale factor
I/IREF
W
L
W
L
W
L
W
L
Note: All gates are connected

MOSFETs 16
Current Mirroring – Pushing and Pulling

MOSFETs 17
Small Signal
• Transistor M1 is diode
connected and acts like a
resistor to s.-s. ground.
ro2

MOSFETs 18
Outline of Chapter 5
• 1- Intro to MOS Field Effect Transistor (MOSFET)
• 2- NMOS FET
• 3- PMOS FET
• 4- DC Analysis of MOSFET Circuits
• 5- MOSFET Amplifier
• 6- MOSFET Small Signal Model
• 7- MOSFET Integrated Circuits
• 8- CSA, CGA, CDA
• 9- CMOS Inverter & MOS Digital Logic

MOSFETs 19
DC and AC - Body-Effect / CLM
Three types of analysis:
Neglect AC Body-Effect & AC CLM
/ Neglect AC CLM
Use AC Body-Effect
Use AC Body-Effect / Use CLM
Three types of analysis:
Neglect DC Body-Effect & DC CLM
/ Neglect DC CLM
Use DC Body-Effect
Use DC Body-Effect / Use DC CLM
DC Analysis
AC Analysis
(small-signal)
Use whatever
DC values for
V and I in the
small-signal
analysis

MOSFETs 20
Common Source Amplifier (CSA)
• Current source I implemented
with current mirror.
• Current mirror provides
active load at drain
• Source terminal grounded –
no DC or AC Body effect

MOSFETs 21
CSA with Current Mirror
CSA
ro2

MOSFETs 22
CSA Small Signal Analysis
• From MOSFET
Current-Mirror:
only ro2 appears in
analysis
i
gs v
v =
1
( ) 1
2
1
1 gs
o
o
m
out v
r
r
g
v −
=
( )
2
1
1 o
o
m
i
out
V r
r
g
v
v
A −
=
=

MOSFETs 23
CSA Input/Output Resistance
• Input Resistance
• Output resistance
2
1 o
o
OUT r
r
R =
vgs1 = 0
gm1vgs1 = 0
∞
⇒
IN
R

MOSFETs 24
CSA Calculations
• In practice, difficult to keep all
transistors operating in saturation
VOUT is hard to control, and
sensitive to: W/L, VG, and
CLM
Ω
=
−
=
=
=
=
=
′
=
′
=
=
=
= −
k
R
V
V
V
V
L
W
L
W
k
k
V
V
V
V
REF
SS
DD
V
A
n
V
A
p
t
t
1
2
,
5
40
,
50
125
50
1
01
.
0
1
1
2
2
2
1
1
2
1
μ
μ
λ
λ
V
V
V
o
V
A
m
OUT
G
A
k
r
m
g
V
V
mA
I
V
V
100
52
.
38
192
.
5
855
.
3
596
.
2
567
.
2
1
−
=
Ω
=
=
=
=
=
Hand: SPICE:
V
V
V
OUT
G
A
V
V
mA
I
V
V
4
.
102
895
.
2
57
.
2
58
.
2
−
=
=
=
=

MOSFETs 25
Common Gate Amplifier (CGA)
• A pMOS current mirror is used
as IREF including the output
resistance.
• Since source terminal not at
signal ground, the body effect is
present.
• The gate terminal held at a DC
voltage. (AC Ground)
Typically used as second stage of a
multi-stage amplifier circuit

MOSFETs 26
CGA – DC Analysis
• Current mirror is assumed to be ideal
during the DC analysis, thus IREF=I
• DC voltage at the source terminal (VS)
must be obtained from driving the
current IREF through the transistor.
• This assumes that the input voltage
source VI is set to zero
• RI is part of the source voltage
• Solve for VO, with VS and VG known,
and including CLM
( ) ( )
[ ]
S
O
t
S
G
n V
V
V
V
V
L
W
k
I −
⋅
+
−
−
′
= λ
1
2
2
1

MOSFETs 27
CGA
• Replace with a current source
including output resistance ro2
Choice of analysis:
Neglect AC Body-Effect & CLM
/ Neglect CLM
Use AC Body-Effect
Use AC Body-Effect / Use CLM
ro2

MOSFETs 28
CGA – No Body Effect or CLM
2
o
gs
m
o r
v
g
v ⋅
−
=
I
I
m
m
gs v
R
g
g
v
+
−
=
1
1
1
1
I
m
o
I
o
V
R
g
r
v
v
A
+
=
=
1
2
i=0
Non Inverting

MOSFETs 29
CGA – RIN & ROUT, No Body Effect or CLM
1
1
m
IN
g
R =
RIN
ROUT
2
o
OUT r
R =
vI = 0

MOSFETs 30
CGA – With Body Effect & no CLM
( ) 2
1
1 o
x
mb
x
m
o r
v
g
v
g
v +
−
=
( ) 2
1
1 o
mb
m
x
o
V r
g
g
v
v
A +
=
=
x
gs
bs v
v
v −
=
= 1
1
( ) 2
1
1
1
1 o
bs
mb
gs
m
o r
v
g
v
g
v +
−
=
( )
I
IN
IN
o
mb
m
i
x
x
o
i
o
total
V
R
R
R
r
g
g
v
v
v
v
v
v
A
+
⋅
+
=
⋅
=
=
− 2
Include RI and
solve for total
voltage gain in
term of RIN
RIN
Solve at vx first:
vX

MOSFETs 31
CGA – RIN With Body Effect & no CLM
x
mb
m
x v
g
g
i )
( 1
1 +
=
x
gs
bs v
v
v −
=
= 1
1
1
1
1
mb
m
IN
g
g
R
+
=
RIN
iIN
vX
Neglect Input
Voltage Source
x
mb
m
in v
g
g
i )
( 1
1 +
=
iX

MOSFETs 32
CGA - With Body Effect & CLM
Neglect Input
Voltage Source
gs
x v
v −
=
2
o
x
o r
i
v ⋅
=
( )
1
1
1
o
o
x
x
mb
m
x
r
v
v
v
g
g
i
−
+
+
=
vX
iX
iX
( ) ⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
+
⋅
=
= 1
1
1
2
1
1
mb
m
o
o
o
x
o
V g
g
r
r
r
v
v
A

MOSFETs 33
CGA – RIN With Body Effect & CLM
Neglect Input
Voltage Source
2
o
x
o r
i
v ⋅
=
( )
1
1
1
o
o
x
x
mb
m
x
r
v
v
v
g
g
i
−
+
+
=
1
1
1
1
2
1
1
mb
m
o
o
o
x
x
IN
g
g
r
r
r
i
v
R
+
+
+
=
=
RIN
iX
vX

4.4 MOSFETS in IC CSA CGA.pdf

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