6.012 - Microelectronic Devices and Circuits - Fall 2005
Lecture 22-1
Lecture 22 - Multistage Amplifiers (II)
DC Voltage and Current Sources
November 29, 2005
Contents:
1. DC voltage sources
2. DC current sources and sinks
Reading assignment:
Howe and Sodini, Ch. 9, §§9.4
6.012 - Microelectronic Devices and Circuits - Fall 2005
Lecture 22-2
Key questions
• How does one synthesize voltage and current sources?
• How can this be done in an economic way?
Lecture 22-3
6.012 - Microelectronic Devices and Circuits - Fall 2005
1. DC voltage sources
2 Features of voltage source:
• A well controlled voltage
• voltage does not depend on current drawn from source
(low internal resistance).
I-V characteristics of voltage source:
I
ideal
real
0
0
VS
V
Equivalent circuit model of voltage source:
RS
+
Vs
-
Lecture 22-4
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Consider MOSFET in ”diode configuration”:
ID
ID
+
VGS
VDS
+
VGS −
−
VGS=VT
0
0
VT
VDS
I-V characteristics:
ID =
W
W
µCox (VDS − VT )2
µCox (VGS − VT )2 =
2L
2L
Beyond threshold, MOSFET looks like ”diode” with quadratic
I-V characteristics.
Lecture 22-5
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 How does one synthesize a voltage source with this?
Assume a current source is available.
iOUT
VDD
IREF
iOUT
iD
+
0
VT
vOUT
vOUT
−
-IREF
VGS = VDS takes value needed to sink current:
ID = IREF + iOU T =
W
µCox(vOU T − VT )2
2L
Then:
iOU T =
W
µCox (vOU T − VT )2 − IREF
2L
Solving for vOU T :
vOU T
IREF + iOU T
= VT +
W
µCox
2L
Lecture 22-6
6.012 - Microelectronic Devices and Circuits - Fall 2005
vOU T is function of IREF and W/L of MOSFET:
• IREF ↑ ⇒ vOU T ↑
• W/L ↑ ⇒ vOU T ↓
iOUT
iOUT
W/L
IREF
VT
VT
vOUT
−IREF1
vOUT
−IREF
−IREF2
2 Small-signal view of voltage source:
D
G
+
ro
it
+
gmvgs vgs
S
Rout =
Rout is small (good!).
1
1
//ro gm
gm
-
vt
6.012 - Microelectronic Devices and Circuits - Fall 2005
Lecture 22-7
2 PMOS voltage source:
VDD
iOUT
+
IREF
VOUT
−
Same operation and characteristics as NMOS voltage source.
PMOS needs to be bigger to attain same Rout .
Lecture 22-8
6.012 - Microelectronic Devices and Circuits - Fall 2005
2. DC current sources and sinks
2 Features of current source:
• A well controlled current,
• supplied current does not depend on voltage across
(high internal resistance)
I-V characteristics of current source:
I
real
Is
ideal
0
V
0
Equivalent circuit model of current source:
Is
RS
Lecture 22-9
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Connect voltage source to another MOSFET:
VDD
IREF
iOUT
+
M1
M2
+
vOUT
−
VREF
−
⎛
⎞
⎛
⎞
IOU T
1 ⎜W ⎟
⎝ ⎠ µCox (VREF − VT )2
2 L 2
IREF
1 W
⎜⎝ ⎟⎠ µCox(VREF − VT )2
2 L 1
Then:
IOU T =
W
L 2
IREF W
L 1
IOU T scales with IREF by W/L ratios of two MOSFETs
(current
(curr
ent mirror circuit).
Well ”matched” transistors important.
Lecture 22-10
6.012 - Microelectronic Devices and Circuits - Fall 2005
• Small-signal view of current source:
it
+
1
gm1
+
gm2vgs2
vgs2
ro2
-
-
Rout = ro2
I-V characteristics of NMOS current source:
iOUT
(W/L)2
IREF
(W/L)1
1/ro2
VDS
SAT2
vOUT
vt
Lecture 22-11
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 PMOS current source
• NMOS current source sinks current to ground.
• PMOS current source ssources
current from positive
ources
o
supply.
PMOS current mirror:
VDD
M2
M1
iOUT
IREF
Lecture 22-12
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Multiple current sources
Since there is no DC gate current in MOSFET, can tie
up multiple current mirrors to single current source:
VDD
M1
MR
M2
M3
iOUT2
iOUT1
iOUT3
IREF
IOU T n =
W
IREF WL n
L R
Similar idea with NMOS current sinks:
VDD
IREF
MR
+
VREF
−
iOUT1
iOUT2
iOUT3
M1
M2
M3
Lecture 22-13
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Multiple current sources and sinks
Often, in a given circuit, we need current sources and
sinks. Can build them all out of a single current source:
VDD
M1
MR
M2
iOUT1
iOUT2
iOUT4
IREF
M4
M3
IOU T 1 =
IOU T 2 =
IOU T 4 =
W
IREF WL 1
L R
W
IREF WL 2
L R
W
IOU T 1 WL 4
L 3
=
W
W
L 4 L 1
IREF W W L 3 L R
Lecture 22-14
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Generating IREF :
Simple circuit:
VDD
IREF
IREF =
R
+
VOUT
−
VDD −VOU T
R
VOU T = VT +
IREF
W µC
ox
2L
For large W/L, VOU T → VT :
IREF VDD − VT
R
• Advantages:
– IREF set by value of resistor.
• Disadvantages:
– VDD also affects IREF .
– VT and R are function of temperature ⇒ IREF (T ).
In real world, more sophisticated circuits used to generate
IREF that are VDD and T independent.
Lecture 22-15
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Can now understand more complex circuits.
Examples:
+2.5 V
RS
IREF
vs
+
−
VBIAS
+
−
M3
M1
+
M2
vOUT
−
−2.5 V
Amp stages:
What does it do?
Lecture 22-15
6.012 - Microelectronic Devices and Circuits - Fall 2005
2 Can now understand more complex circuits.
Examples:
+2.5 V
RS
IREF
vs
+
−
VBIAS
+
−
M3
M1
+
M2
vOUT
−
−2.5 V
Amp stages:
What does it do?
Lecture 22-16
6.012 - Microelectronic Devices and Circuits - Fall 2005
2.5 V
RS
IREF
Vs
+
−
VBIAS
+
−
M3
What does it do?
+
RL
M2
−2.5 V
Amp stages:
M1
vOUT
−
Lecture 22-16
6.012 - Microelectronic Devices and Circuits - Fall 2005
2.5 V
RS
IREF
Vs
+
−
VBIAS
+
−
M3
What does it do?
+
RL
M2
−2.5 V
Amp stages:
M1
vOUT
−
Lecture 22-17
6.012 - Microelectronic Devices and Circuits - Fall 2005
+2.5 V
IREF
RS
vs
+
−
VBIAS
+
−
Q1
iOUT
RL
M3
M2
−2.5 V
Amp stages:
What does it do?
Lecture 22-17
6.012 - Microelectronic Devices and Circuits - Fall 2005
+2.5 V
IREF
RS
vs
+
−
VBIAS
+
−
Q1
iOUT
RL
M3
M2
−2.5 V
Amp stages:
What does it do?
Lecture 22-18
6.012 - Microelectronic Devices and Circuits - Fall 2005
+3 V
M2
M2B
RS
6 kΩ
vs
+
−
VBIAS
+
−
−3 V
Amp stages:
What does it do?
M1
M4
iOUT
+
M3
RL
vOUT
−
Lecture 22-18
6.012 - Microelectronic Devices and Circuits - Fall 2005
+3 V
M2
M2B
RS
6 kΩ
vs
+
−
VBIAS
+
−
−3 V
Amp stages:
What does it do?
M1
M4
iOUT
+
M3
RL
vOUT
−
Lecture 22-19
6.012 - Microelectronic Devices and Circuits - Fall 2005
+2.5 V
M4
M6
IREF
RS
vs
M3
Q1
M5
VBIAS
+
−
+
−
−2.5 V
Amp stages:
What does it do?
Q2
+
vOUT
M2
−
Lecture 22-19
6.012 - Microelectronic Devices and Circuits - Fall 2005
+2.5 V
M4
M6
IREF
RS
vs
M3
Q1
M5
VBIAS
+
−
+
−
−2.5 V
Amp stages:
What does it do?
Q2
+
vOUT
M2
−
6.012 - Microelectronic Devices and Circuits - Fall 2005
Lecture 22-20
Key conclusions
• Voltage source easily synthesized from current source
using MOSFET in diode configuration.
• Current source easily synthesized from current source
using current mirror circuit.
• Multiple current sources and sinks with different magnitudes of current can be synthesized from a single
current source.
• Voltage and current sources rely on availability of well
”matched” transistors in IC technology.