lecture 10: folded-cascode amplifiers current mirror op …class.ece.iastate.edu/ee435/lectures/ee...
TRANSCRIPT
1
EE 435
Lecture 10:
Folded-Cascode Amplifiers
Current Mirror Op Amps
2
Basic Op Amp Design
• Fundamental Amplifier Design Issues
• Single-Stage Low Gain Op Amps
• Single-Stage High Gain Op Amps
• Other Basic Gain Enhancement Approaches
• Two-Stage Op Amp
Where we are at:
3
M1
M3
-A
VIN
VOUT
High output impedance quarter-circuits
Regulated Cascode Amplifier
or “Gain Boosted Cascode”
Quarter Circuit
• A is usually a simple amplifier, often the reference op amp with + terminal
connected to the desired quiescent voltage
• Assume biased with a dc current source (not shown) at drain of M3
Review from last lecture:
4
Gain-Boosted Telescopic Cascode Op Amp
VDD
V OU
T
CL
VB2
VB3
V S
S
VB5 M 1
1
A1A2
A3A4
IT
VINVIN
M1M2
M3 M4
M5
M7
M6
M8
Advantages:
Significant increase in dc gain
Limitations:
:• Signal swing (4VDSAT+VT between VDD and VSS)
• Reduction in GB power efficiency
- some current required to bias “A” amplifiers
• -additional pole in “A” amplifier -may add requirements for some compensation
• Area Overhead for 4 transistors and 4 amplifiers-actually minor concern since performance will usually
justify these resources
(with or w/o current mirror counterpart circuits)
Review from last lecture:
5
Are there other useful high output impedance
circuits that can be used for the quarter circuit?
V1G M1V1 G 1
V1G M1 G 1
G2
CL
2
Vd
Vo++
1
0
1 2
1 2
1
2
2
MV
L
M
L
GA
G G
G GBW
C
GGB
C
Review from last lecture:
6
Implementation of Biased Folded
Cascode Amplifier?
VDD
VIN
M1
M3 M
5
VB1 V
B3
Biased Folded CascodeImplementation of Biased
Folded Cascode
VDD
VIN
M1
M3V
B1IB1
IB2 V
OUT
VSS
Review from last lecture:
7
VDD
VIN
M1
M3M5
VB1 VB3
Analysis of Biased Folded Cascode
go1gM1V1V1
go3gM3V3V3go5 gM5V5 V5
CL
VOUT
VIN
VX
0
OUT m
OUT o3 L m3 3 X o3
X o1 o3 o5 m1 1 m3 3 OUT o3
3 X
1 IN
OUT o3 L m3 o3 3
m1 IN 3 m3 o1 o3 o5 OUT o
1
o3INL o1 o
m3
3
5
V g sC g V V g
V g g g g V g V V g
V V
V V
V g sC g g V
g V V g +g g g +V
V g
gVsC g g
g
g
GM
GO
How can this be seen by inspection?
• First observe if all go’s are 0, GM=gm1
• Then observe M3 “cascodes” the impedance g01+go5
8
Biased Folded Cascode Quarter Circuit
VDD
VIN
M1
M3 M
5
VB1 V
B2
CL
VOUT
OUT m1
o3INL o1 o5
m3
V g
gVsC g g
g
m3m1
V0
o1 o5 o3
ggA
g g g
m1
L
gGB
C
9
Basic Amplifier Structure Comparisons(ideal current source biasing)
Common Source
Cascode
Regulated
Cascode
Folded Cascode
Small Signal Parameter Domain
m3m1
V0
o1 o5 o3
ggA
g g g
mV0
o
gA
g
m3m1V0
o1 o3
ggA
g g
m3m1V0
o1 o3
ggA A
g g
m
L
gGB
C
m
L
gGB
C
m
L
gGB
C
m
L
gGB
C
10
Basic Amplifier Structure Comparisons
EB3EB131
VOVV
1
λλ
4A
EB3EB1351
VOVVλλθλ
4θA
Common Source
Cascode
Regulated
Cascode
Folded Cascode
EB
VOV
1
λ
2A
EBLDD V
1
CV
2PGB
Practical Parameter Domain
EB1LDD V
1
CV
2PGB
EB3EB131
VOVV
A
λλ
4A
EB1LDD V
θ-1
CV
2PGB
Θ=pct power in A
EB1LDD V
θ
CV
2PGB
Θ=fraction of current
of M5 that is in M1
11
Biased Folded-Cascode Amplifier
VDD
VIN
M1
M3 M
5
VB1 V
B3
Quarter Circuit
M2
M4
M6
VB4
VB2
VSS
Counterpart Circuit
12
QUARTER CIRCUIT
Folded-Cascode Operational Amplifier
Op Amp
VDD
VDD
VB1
VB2
VB5
VSS
VSSVSS
VDD
VOUTVOUT
VB1
VINVIN
VB2
VB5
IT
VB3
VB4
VDD
VIN
VB1 VB3
13
Folded-Cascode Operational Amplifier (redrawn)
These transistors pair-wise form a current source
and one in each pair can be removed
VB5
VIN VIN
VOUT VOUT
VDD
VSSVSS
IT
VB4
VB3
VB2
VB1
M1 M2
M7
M9
M8
M10
M5 M6
M3 M4
M11M12
VDD
VB5
IT2
14
Folded Cascode Op Amp
Needs CMFB Circuit for VB4
Either single-ended or differential outputs
Can connect counterpart as current mirror to eliminate CMFB
Folding caused modest deterioration of AV0 and GB energy efficiency
Modest improvement in output swing
VIN
VIN
VOUT V
OUT
VDD
VSS
VSS
IT
VB4
VB3
VB2
VB1
M1
M2
M7
M9
M8
M10
M5 M
6
M3 M
4
IT2
15
Folded Cascode Op Amp
(Single-ended Output)
VIN
VIN
VDD
VSS
VSS
IT
VB3
VB2
M1
M2
M7
M9
M8
M10
M5 M
6
M3 M
4
VOUT
CL
IT2
VB1
1mmEQgg
OEQL
mEQ
V
gsC
gsA
m9
O9
O7
m3
O3
O5O1OEQ
g
gg
g
gggg
L
m
C
gGB 1
L
mEQ
C
gGB
OEQ
mEQ
V
g
gA
0
m9
O9
O7
m3
O3
O5O1
m
V0
g
gg
g
ggg
gA
1
16
Operational Amplifier Structure Comparison
Reference
Op Amp
Telescopic
Cascode
Regulated
Cascode
Folded
Cascode
31
1
2
1
OO
m
VO
gg
gA
L
m
C
gGB
2
1
Small Signal Parameter Domain
L
T
C
ISR
2
L
T
C
ISR
2
m5
o5o7
m3
o3o1
m1
o
g
gg
g
gg
2
g
A
L
m
C
gGB
2
1
3m9
o9o7
1m3
o3o1
m1
o
Ag
gg
Ag
gg
2
g
A
L
m
C
gGB
2
1L
T
C
ISR
2
m9
o9o7
m3
o3o5o1
m1
o
g
gg
g
ggg
2
g
A
L
m
C
gGB
2
1L
T
C
ISR
2
17
Operational Amplifier Structure Comparison
3
EB775
1
EB331EB1
V0
A
Vλλ
A
VλλV
2A
EB1LDD V
1
C2V
θ-1PGB
LDDC2V
θ-1PSR
Reference
Op Amp
Telescopic
Cascode
Regulated
Cascode
Folded
Cascode
Practical Parameter Domain
EB131
V0
V
1
λλ
1A
EB1LDDV
1
C2V
PGB
LDDC2V
PSR
LDDC2V
PSR
EB575EB331EB1
V0VλλVλλV
2A
EB1LDDV
1
C2V
PGB
Θ=pct power in A
EB979EB3351EB1
V0Vλλθ1VλλθλV
2θA
EB1LDD V
θ
C2V
PGB
LDDC2V
PθSR
Θ=fraction of
current of M5
that is in M1
18
Folded Cascode Op Amp
(Single-ended Output)
L
m
C
gGB 1
m9
O9
O7
m3
O3
O5O1
m
V0
g
gg
g
ggg
gA
1
What is a practical design parameter set?
How many degrees of freedom are there?
DOF ?
{IT,W1/L1,W5/L5,W3/L3,W9/L9,W7/L7,VB1,VB2,VB3}
9 DOF
Practical Design Parameters
{P,θ,VEB1,VEB3,VEB5,VEB7,VEB9,VB2,VB3}
where θ=IT/(IT+IT2)
VIN
VIN
VDD
VSS
VSS
IT
VB3
VB2
M1
M2
M7
M9
M8
M10
M5 M
6
M3 M
4
VOUT
CL
IT2
VB1
19
Textbook reference:
Some of the material we have been discussing appears in
Chapter 3, some in Chapter 5, and some in Chapter 6 of the
Martin and Johns text
In particular, the telescopic and folded cascode structures are
referred to as advanced op amps and appear in later chapters of
the text
20
Folded Gain-boosted
Cascode Amplifier
with ideal current source bias
modest improvement in output swing
L
m1
2C
gGB
m3
o3o1
m1o
gA
gg
gA
IB1
M1
M3
VOUT
IB2
VIN
-A
21
Folded Gain-boosted
Cascode Amplifier
-A
VDD
VIN
M1
M3 M
5
VB2
CL
VOUT
IBIAS
Ag
gggsC
g
V
V
m3
o3o5o1L
m1
IN
OUT
modest improvement in output swingL
m1
C
gGB
o3o5o1
m3m10
ggg
AggA
22
Basic Amplifier Structure Comparisons
L
m1
C
gGB
o3
m3
O1
m1VO
g
g
g
gA
Common Source
Cascode
Regulated
Cascode
Folded Cascode
Folded Regulated
Cascode
O
mVO
g
gA
L
m
C
gGB
Small Signal Parameter Domain
Ag
g
g
gA
o3
m3
O1
m1VO
L
m1
C
gGB
o3
m3
O5O1
m1VO
g
g
gg
gA
L
m1
C
gGB
A
g
g
gg
gA
o3
m3
O5O1
m1VO
L
m1
C
gGB
23
Basic Amplifier Structure Comparisons
EB3EB131
VOVV
1
λλ
4A
EB3EB1351
VOVVλλθλ
4θA
EB1LDD V
θ
CV
2PGB
Common Source
Cascode
Regulated
Cascode
Folded Cascode
Folded Regulated
Cascode
EB
VOV
1
λ
2A
EBLDD V
1
CV
2PGB
Practical Parameter Domain
EB1LDD V
1
CV
2PGB
EB3EB131
VOVV
A
λλ
4A
EB1LDD V
θ-1
CV
2PGB
Θ=pct power in A
Θ=fraction of current
of M5 that is in M1
EB3EB13512
2VO
VVλλλθ
A4θA
EB1
12
LDD V
θ-1θ
CV
2PGB
Θ1=pct of total power in A
Θ2=fraction of current of
M5 that is in M1
24
Folded Gain-boosted Telescopic
Cascode Op Amp
Needs CMFB Circuit for VB4
Either single-ended or differential outputs
Can connect counterpart as current mirror to eliminate CMFB
Folding caused modest deterioration in GB efficiency and gain
Modest improvement in output swing
L
m1
2C
gGB
m91
o9o7
m33
o3O5O1
m1
o
gA
gg
gA
ggg
2
g
A
V I
NV I
N
VDD
V S
S
IT
VB4
M1
M2
M7
M9 M 1
0
M5 M
6
M3 M
4
V OU
T
CL
VIN
VB2
VB3
V S
S
IT2
M8
VB1
A1A2
A3A4
25
Operational Amplifier Structure Comparison
Reference
Op Amp
Telescopic
Cascode
Regulated
Cascode
Folded
Cascode
Folded
Regulated
Cascode
31
1
2
1
OO
m
VO
gg
gA
L
m
C
gGB
2
1
Small Signal Parameter Domain
L
T
C
ISR
2
L
T
C
ISR
2
m5
o5o7
m3
o3o1
m1
o
g
gg
g
gg
2
g
A
L
m
C
gGB
2
1
3m9
o9o7
1m3
o3o1
m1
o
Ag
gg
Ag
gg
2
g
A
L
m
C
gGB
2
1L
T
C
ISR
2
m9
o9o7
m3
o3o5o1
m1
o
g
gg
g
ggg
2
g
A
L
m
C
gGB
2
1L
T
C
ISR
2
9m9
o9o7
3m3
o3o5o1
m1
o
Ag
gg
Ag
ggg
2
g
A
L
m
C
gGB
2
1L
T
C
ISR
2
26
Summary of Folded Amplifier Performance
• + Modest improvement in output signal
swing (from 5 VDS SAT to 4VDS SAT)
• + Can directly feed output back to input
to create buffer
• - Deterioration in AV0 (maybe 30% or
more)
• - Deterioration in GB power efficiency
(can be significant)
• - Minor increase in circuit size
27
Other Methods of Gain Enhancement
OCCOQC
QCM
V
gg
gA
0
L
mQC
C
gGB
Two Strategies:
1. Decrease denominator of AV0
2. Increase numerator of AV0
Previous approaches focused on decreasing denominator
Consider now increasing numerator
Recall:
VIN
VDD
VSS
VOUT
Quarter
Circuit
Counterpart
Circuit
CL
VBB
28
Determination of op amp characteristics
from quarter circuit characteristics
M1
OV
d L 1 2
GV 2AV sC G G
FVIN
IXX
VOUT
VSS
CL
Small signal differential amplifier
GsC
GsA
L
MVQC
)(
Small signal Quarter Circuit
F
P
VDD
VBB
OUTV
OUTV
d
2
V d
2
V
IBIAS
CLCL
or VSS
• Note that the counterpart circuit is simply serving as the biasing current source
• Could use counterpart circuits (or other circuits) from other quarter circuits for “P”
• Counterpart circuits connected as one-port
• Can think of making differential op amp directly from quarter circuit
29
Differential input op amp directly from
quarter circuit
M1
OV
d L 1 2
GV 2AV sC G G
F
VIN
IXX
VOUT
VSS
CL GsC
GsA
L
MVQC
)(
F
P
VDD
VBB
OUTV
OUTV
d
2
V d
2
V
IBIAS
CLCL
or VSS
BB
M1
OV
d L 1 I
GV 2AV sC G G
F
VDD
IBB
OUTV
OUTV
d
2
V d
2
V CLCL
VSS
IBB
F
VDD
IBB
OUTV
OUTV
d
2
V d
2
V CLCL
IBB
IBIAS
30
gmEQ Gain Enhancement Strategy
Consider using the quarter circuit itself to
form the op amp
gm is increased by the mirror gain !
Mgg m1MQC
M1
1 : M
IB
VIN
VOUT
CL
Consider this quarter circuit
Folding is required to establish the correct
bias current direction
Could have done this for other quarter
circuits as well but there is a particularly
important reason we are following this
approach with this quarter circuit – What
is it ?
OQCgOutput conductance of QC:
31
gmEQ Gain Enhancement Strategy
1 : M
VIN
IB
VOUT
M1
Redraw to absorb IB in the quarter circuit
BBOEQ OQC OIg g g
MQC M1g = g M
32
gmEQ Gain Enhancement Strategy
M : 1
IB
M1
1 : M
IB
M2V
IN
VOUT
VIN
VOUT
IT
1 : M
VIN
IB
VOUT
M1
33
Current Mirror Op Amps
OEQ
mEQV0
g
gA
2
1m
mEQ
gMg Premise: Transconductance gain increased by mirror gain M
Premise: If output conductance is small, gain can be very high
Premise: GB very good as well
L
mEQ
C
gGB
Very Simple Structure!
M : 1
IB
M1
1 : M
IB
M2VIN
VOUT
VIN
VOUT
IT
CLCL
Still need to generate the bias current IB
BBOEQ OQC OIg g g
-
OUTV0 + +
IN IN
VA =
V - V
(for VIN+=Vd/2)
34
Current Mirror Op Amps
VDD
VSS
VINVIN
VOUT VOUT
M : 1 1 : M
IT
VSS VSS
M1M2
VDD
VSS
VINVIN
VOUT VOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
Need CMFB to establish VB2
Can use higher output impedance current mirrors
Can use current mirror bias to eliminate CMFB but loose one output
Basic Current Mirror Op Amp
35
Is this a real clever solution?
36
Basic Current Mirror Op Amp
86 OOOEQggg
VDD
VSS
VINVIN
VOUT VOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
CL CL
2
1m
mEQ
gMg
O8O6
m1
VOgg2
gM
A
L
m
C
gMGB
2
1
CMFB not shown
L
T
C
IMSR
2
37
• Current-Mirror Op Amp offers strategy for
gm enhancement
• Very Simple Structure
• Has applications as an OTA
• Based upon small signal analysis,
performance appears to be very good !
• But – how good are the properties of the
CMOA?Is this a real clever solution?
38
Seminal Work on the OTA
N.E.C. PROCEEDINGS
From:
39
Seminal Work on the OTA
1969 N.E.C. PROCEEDINGS
December 1969
From:
40
VIN V
INQ1
Q2
I1
I2
IABC
Q3 Q
4
I3
I4
IOUT
12AB IIMII
IA I
B
34
4OUTB
3A
II
III
II
Original OTA
Current
Mirror
ABOUT IIMI
41
Original OTA
VIN V
INQ1
Q2
I1
I2
IABC
Q3 Q
4
I3
I4
IOUT
12AB IIMII
IA I
B
2B MII 1A MII
ABOUT IIMI
42
Original OTA
ABOUT IIMI
VIN V
INQ1
Q2
I1
I2
IABC
Q3 Q
4
I3
I4
IOUT
IA I
B
2B MII 1A MII
43
Original OTA
ABOUT IIMI
VIN V
INQ1
Q2
I1
I2
IABC
Q3 Q
4
I3
I4
IOUT
IA I
B
2B MII 1A MII
3-mirror OTA
44
Current Mirror Op Amp W/O CMFBV
DD
VSS
VIN
VIN
M : 1 1 : M
IT
VSS
1 : 1
VOUT
IOUT
VIN
gm
1mmEQMgg
Often termed an OTA
INmOUTVgI
Introduced by Wheatley and Whitlinger in 1969
45
End of Lecture 10