Post on 23-Aug-2020
EE 435
Lecture 10
Laboratory Support
OTA circuits
Current Mirror Op Amps
-- Alternative perspective
-- Loop phase-shift concerns
Lab Support:
VDD
M1M2
VB2
M3 M4
VINVIN
CL
M5
VOUT
IT
VSS
4
Design space for single-stage op amp
EB131
0
V
2
λλ
1A
331 EBTTDDiC VVVVV
DD L EB1
P 1GB
V C V
Performance Parameters in Practical
Parameter Domain { VEB1 VEB3 VEB5 P}:
EB3DDOUT VVV
T2icOUT VVV
SSEB5EB1T1ic VVVVV
Simple Expressions (7) in Practical Parameter Domain
DD SS L
PSR
V V C
VDD
M1M2
VB2
M3 M4
VINVIN
CL
M5
VOUT
IT
VSS
Recall from Lecture 6
5
Design example for single-stage op amp
EB131
0
V
2
λλ
1A
331 EBTTDDiC VVVVV
DD L EB1
P 1GB
V C V
Performance Parameters in Practical
Parameter Domain { VEB1 VEB3 VEB5 P}:
EB3DDOUT VVV
T2icOUT VVV
SSEB5EB1T1ic VVVVV
DD SS L
PSR
V V C
VDD
M1M2
VB2
M3 M4
VINVIN
CL
M5
VOUT
IT
VSS
1. Select Parameter Domain (will use practical parameter domain)
{ VEB1 VEB3 VEB5 P}
2. Pick VEB1 to meet gain requirement ) { VEB1 VEB3 VEB5 P}
3. Pick P to meet GB requirement
4. Pick VEB3 and VEB5 to meet signal swing requirements
5. Map back from the Practical Parameter Domain to the
Natural Parameter domain (next page)
EB1
1 3 0
1 2V
λ λ A
{ VEB1 VEB3 VEB5 P}
Assume design to meet A0, GB and signal swing specs.
Recall from Lecture 6
6
Design example for single-stage op ampPerformance Parameters in Practical
Parameter Domain { VEB1 VEB3 VEB5 P}:
VDD
M1M2
VB2
M3 M4
VINVIN
CL
M5
VOUT
IT
VSS
From expression it follows that 2k ox kDk EBk
k
C WI V
2L
1
2
1 n OX EB1 DD SS
3
2
3 p OX EB3 DD SS
5
2
5 n OX EB5 DD SS
T B2 EB5 ss THn
DD SS
W 1 P
L C V V V
W 1 P
L C V V V
W 2 P
L C V V V
PI or V V V V
V V
Mapping from Practical Parameter Domain { VEB1 VEB3 VEB5 P} to Natural
Parameter Domain { W1/L1 W3/L3 W5/L5 IT}
Recall from Lecture 6
Design Space Exploration
Consider the 5T Op Amp with CM Biasing
5T Op Amp Design
Process Paramaters Fixed Contraints
μnCOX 350 uAV^2 VDD 2 V Input Quantities in
μpCOX 75 uAV^2 VSS -2 V
VTHn 0.4 V CL 10 pf
VTHp -0.4 VL1=L2=…=Lk 0.5 um
λ 0.01 V^-1 VB2 0.6 V
Op Amp Design Variables Performance Characteristics Practical Design Values in um Small Signal Parameters (if desired)
No VEB1 VEB3 VEB9 P (mw) A0 BW (MHz) GB (MHz) SR (V/uS) Vomax Vomin VCM IT (mA) W1 W2 W3 W4 W9 gm1 gm3 gm9 go1 go3 go9
1 0.1 -0.1 0.1 5 1000 0.20 199.04 0.125 1.9 0 0.1 1.25 357.1 357.1 1666.7 1666.7 714.3 0.0125 0.0125 0.025 6.25E-06 6.25E-06 1.3E-05
2 0.2 -0.2 0.1 5 500 0.20 99.52 0.125 1.8 -0.1 0.1 1.25 89.3 89.3 416.7 416.7 714.3 0.00625 0.00625 0.025 6.25E-06 6.25E-06 1.3E-05
3 0.4 -0.1 0.1 5 250 0.20 49.76 0.125 1.9 -0.3 0.1 1.25 22.3 22.3 1666.7 1666.7 714.3 0.003125 0.0125 0.025 6.25E-06 6.25E-06 1.3E-05
4 0.05 -0.1 0.1 5 2000 0.20 398.09 0.125 1.9 0.05 0.1 1.25 1428.6 1428.6 1666.7 1666.7 714.3 0.025 0.0125 0.025 6.25E-06 6.25E-06 1.3E-05
5 0.1 -0.1 0.1 1 1000 0.04 39.81 0.025 1.9 0 0.1 0.25 71.4 71.4 333.3 333.3 142.9 0.0025 0.0025 0.005 1.25E-06 1.25E-06 2.5E-06
6 0.1 -0.1 0.1 10 1000 0.40 398.09 0.25 1.9 0 0.1 2.5 714.3 714.3 3333.3 3333.3 1428.6 0.025 0.025 0.05 1.25E-05 1.25E-05 2.5E-05
7 0.1 -0.1 0.1 0.1 1000 0.00 3.98 0.0025 1.9 0 0.1 0.025 7.1 7.1 33.3 33.3 14.3 0.00025 0.00025 0.0005 1.25E-07 1.25E-07 2.5E-07
8 0.1 -0.1 0.1 20 1000 0.80 796.18 0.5 1.9 0 0.1 5 1428.6 1428.6 6666.7 6666.7 2857.1 0.05 0.05 0.1 0.000025 0.000025 0.00005
9 0.1 -0.1 0.2 1 1000 0.04 39.81 0.025 1.9 0 0.1 0.25 71.4 71.4 333.3 333.3 35.7 0.0025 0.0025 0.0025 1.25E-06 1.25E-06 2.5E-06
10 0.1 -0.2 0.1 0.1 1000 0.00 3.98 0.0025 1.8 0 0.1 0.025 7.1 7.1 8.3 8.3 14.3 0.00025 0.000125 0.0005 1.25E-07 1.25E-07 2.5E-07
Design Space Exploration
5T Op Amp Design
Process Paramaters Fixed Contraints
μnCOX 350 uAV^2 VDD 2 V Input Quantities in
μpCOX 75 uAV^2 VSS -2 V
VTHn 0.4 V CL 10 pf
VTHp -0.4 V L1=L2=…=Lk 0.5 um
λ 0.01 V^-1 VB2 0.6 V
Op Amp
No VEB1 VEB3 VEB9 P (mw) A0 BW (MHz) GB (MHz) SR (V/uS) Vomax Vomin VCM IT (mA) W1 W2 W3 W4 W9
1 0.1 -0.1 0.1 5 1000 0.20 199.04 0.125 1.9 0 0.1 1.25 357.1 357.1 1666.7 1666.7 714.3
2 0.2 -0.2 0.1 5 500 0.20 99.52 0.125 1.8 -0.1 0.1 1.25 89.3 89.3 416.7 416.7 714.3
3 0.4 -0.1 0.1 5 250 0.20 49.76 0.125 1.9 -0.3 0.1 1.25 22.3 22.3 1666.7 1666.7 714.3
4 0.05 -0.1 0.1 5 2000 0.20 398.09 0.125 1.9 0.05 0.1 1.25 1428.6 1428.6 1666.7 1666.7 714.3
5 0.1 -0.1 0.1 1 1000 0.04 39.81 0.025 1.9 0 0.1 0.25 71.4 71.4 333.3 333.3 142.9
6 0.1 -0.1 0.1 10 1000 0.40 398.09 0.25 1.9 0 0.1 2.5 714.3 714.3 3333.3 3333.3 1428.6
7 0.1 -0.1 0.1 0.1 1000 0.00 3.98 0.0025 1.9 0 0.1 0.025 7.1 7.1 33.3 33.3 14.3
8 0.1 -0.1 0.1 20 1000 0.80 796.18 0.5 1.9 0 0.1 5 1428.6 1428.6 6666.7 6666.7 2857.1
9 0.1 -0.1 0.2 1 1000 0.04 39.81 0.025 1.9 0 0.1 0.25 71.4 71.4 333.3 333.3 35.7
10 0.1 -0.2 0.1 0.1 1000 0.00 3.98 0.0025 1.8 0 0.1 0.025 7.1 7.1 8.3 8.3 14.3
Design Variables Performance Characteristics Practical Design Values in um
Embedded Spreadsheet
9
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
Review from last lecture:
10
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
BBIG is the output conductance of IBB
Review from last lecture:
11
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
Review from last lecture:
12
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)
Review from last lecture:
13
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
Review from last lecture:
14
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
Review from last lecture:
15
• 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?
Review from last lecture:
16
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
Review from last lecture:
17
OTA Circuits
• OTA often used open loop
• Excellent High Frequency Performance
• Gain can be made programmable with dc current
• Large or very large adjustment ranges possible
gm
IOUT
IABC
circuitsMOSforIK
circuitsBJTforIKg
ABC
ABC
m
2 to 3 decades of adjustment for MOS
5 to 6 decades of adjustment for BJT
Review from last lecture:
18
OTA Applications
gm
VIN
VOUT
R
Voltage Controlled Amplifier
Note: Technically current-controlled, control variable
not shown here and on following slides
INmOUTVRgV
gm is controllable with IABC
Review from last lecture:
19
OTA Applications
INmOUTVRgV
Voltage Controlled Inverting Amplifier
gm
VIN
VOUT
RL
Review from last lecture:
20
OTA Applications
RIN
gm
RIN
gm
m
IN
gR
1
Voltage Controlled Resistances
m
IN
gR
1
Review from last lecture:
21
OTA Applications
gm1
VIN
VOUT
gm2
gm1
VIN
VOUT
gm2
in
m
m
OUTV
g
gV
2
1
Voltage Controlled Resistorless Amplifiers
in
m
m
OUTV
g
gV
2
1
Noninverting Voltage Controlled Amplifier Inverting Voltage Controlled Amplifier
Extremely large gain adjustment is possible
22
OTA Applications
gm
VIN
VOUT
C
in
m
OUTV
sC
gV
in
m
OUTV
sC
gV
Voltage Controlled Integrators
Noninverting Voltage Controlled Integrator Inverting Voltage Controlled Integrator
gm
VIN
VOUT
C
23
Comparison with Op Amp Based Integrators
VIN
VOUT
CR
INOUTV
sRCV
1
VIN
VOUT
CR
R1
R1
INOUTV
sRCV
1
OTA-based integrators require less components and significantly
less for realizing the noninverting integration function !
24
Properties of OTA-Based Circuits
• Can realize arbitrarily complex functions
• Circuits are often simpler than what can be obtained with Op Amp counterparts
• Inherently offer excellent high frequency performance
• Can be controlled with a dc voltage or current
• Often used open-loop rather than in a feedback configuration (circuit properties depend directly on gm)
• Other high output impedance op amps can also serve as OTA
• Linearity is limited
• Signal swing may be limited but can be good too
• Circuit properties process and temperature dependent
25
• Current-Mirror Op Amp offers strategy for
gm enhancement
• Very Simple Structure
• Has applications as an OTA
• But – how good are the properties of the
CMOA?Is this a real clever solution?
26
Current Mirror Op Amp W/O CMFB
VDD
VSS
VIN
VIN
M : 1 1 : M
IT
VSS
1 : 1
VOUT
1mmEQMgg
86 OOOEQggg
86
1
OO
m
VO
gg
gMA
And can use higher output impedance
current mirrors to decrease gOEQ
VDD
VSS
VINVIN
IT
VB1
VSSVSS
M1 M2
M3 M4M5 M6
M7M8M9
VOUT
CL
L
T
C
MISR
27
SR of Current Mirror Op Amp VDD
VSS
VINVIN
IT
VB1
VSSVSS
M1 M2
M3 M4M5 M6
M7M8M9
VOUT
CL
L
T
C
MISR
VDD
VSS
VINVIN
VOUT VOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
CLCL
T
L
MISR
2C
28
Fully Differential Current Mirror Op
Amp with Improved Slew Rate
1 : M MM M : 1
VSS
VIN VIN
IT
1 : 1 1 : 1
VOUT VOUT
VDD
VSS
CLCL
Need CMFB circuit and requires modest circuit
modification to provide CMFB insertion point
29
VDD
VSS
VINVIN
IT
VB1
VSSVSS
M1 M2
M3 M4
M5AM6A
M7M8AM9A
M6B
VOUT
M5B
M9B
M8B
VSSVSS
VOUT
Fully Differential Current Mirror Op Amp with
Improved Slew Rate
Need CMFB circuit and requires modest circuit
modification to provide CMFB insertion point
This circuit was published because of the claim for improved SR (Fig 6.15 MJ)
30
VDD
VSS
VINVIN
IT
VB1
VSSVSS
M1 M2
M3 M4
M5AM6A
M7M8AM9A
M6B
VOUT
M5B
M9B
M8B
VSSVSS
VOUT
TIMP
L
MISR
C
Fully Differential Current Mirror Op Amp with
Improved Slew Rate
Need CMFB circuit and requires modest circuit
modification to provide CMFB insertion point
L
T
AmpOpCM
C
IMSR
2
Improved a factor of 2 !
but …
31
VDD
VSS
VINVIN
IT
VB1
VSSVSS
M1 M2
M3 M4
M5AM6A
M7M8AM9A
M6B
VOUT
M5B
M9B
M8B
VSSVSS
VOUT
TIMP
L
MISR
C
L
T
AmpOpCM
C
IMSR
2
Fully Differential Current Mirror Op Amp with
Improved Slew Rate
Improved a factor of 2 !
but …
M1IVPTDD AmpOp CM
IMP DD TP V I 1 2M
M
M
CV
PSR
LDD
AmpOpCM
12
IMP
DD L
P MSR
V C 1 2M
SR actually about the same for “improved SR circuit”
and basic OTA
32
Comparison of Current-Mirror Op Amps
with Previous Structures
86
1
2
OO
m
VO
gg
gM
A
64
46
LW
LWM
Does the simple mirror gain
really provide an “almost free”
gain enhancement ?
VDD
VSS
VINVIN
VOUTVOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
Ask the apple comparison question !
33
Comparison of Current-Mirror Op Amps
with Previous Structures
86
1
2
OO
m
VO
gg
gM
A
64
46
LW
LWM
Does the simple mirror gain really provide an “almost
free” really large gain enhancement ?VDD
VSS
VINVIN
VOUTVOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
Are we comparing Apples with Apples?• In the small-signal parameter domain?
• In the practical parameter domain?
• Does it matter if we are making a comparison?
34
Reference Op Amp
VDD
VB1
M1M2
VB2
M3 M4
VINVIN
CL
M9
CL
VOUTVOUT
IT
L
T
C
ISR
2
31
1
2
OOL
m
ggsC
g
)s(A
31
1
2
1
OO
m
VO
gg
gA
L
m
C
gGB
2
1
EB131
V0
V
1
λλ
1A
EB1LDDV
1
C2V
PGB
LDDCV
PSR
2
Consider single-ended output performance :
35
Comparison of Current-Mirror Op Amps
with Previous Structures
86
1
2
OO
m
VO
gg
gM
A
4
6
m
m
g
gM
Does the simple mirror gain really provide an
“almost free” gain enhancement ?
86
1
4
6
2
OO
m
m
m
VO
gg
g
g
g
A
64
46
LW
LWMI
OUT
IIN
M6
M4
Gain Enhancement Potential Less Apparent but still
Improved by gm6/gm4 ratio
36
Comparison of Current-Mirror Op Amps
with Previous Structures
86
1
2
OO
m
VO
gg
gM
A
EB1MMEB1T
M8M6EB1
T
QDM8M6EB
T
V
V2λ
1
λλV
1
2
IMλλV
M2
I
IλλV
MI
A
8681
0
22
2
1
Does the simple mirror gain really provide an
“almost free” gain enhancement ?
VDD
VSS
VINVIN
VOUTVOUT
IT
VB1
VSSVSS
VB2
M1 M2
M3 M4M5 M6
M7M8M9
Consider how the gain appears in the practical parameter domain
This is exactly the same as was obtained for the simple differential amplifier!
For a given VEB1, there is NO gain enhancement !
37
Comparison of Current-Mirror Op Amps
with Previous Structures
LEB
T
L
m
L
mEQ
CV
MI
C
gM
C
gGB
1
1
22
M1IVPTDD
VDD
VSS
VINVIN
VOUT VOUT
M : 1 1 : M
IT
VSS VSS
M1M2
How does the GB power efficiency compare with
previous amplifiers ?
GB efficiency decreased for small M !!
M1
M
CV2V
P
C2V
MIGB
LDDEB1LEB1
T
GB for Telescopic Cascode and Ref Op Amp !
38
Comparison of Current-Mirror Op Amps
with Previous Structures
M1IVPTDD
VDD
VSS
VINVIN
VOUTVOUT
M : 1 1 : M
IT
VSS VSS
M1M2
M1
M
C2V
PSR
LDD
How does the SR compare with previous
amplifiers ?
L
T
C
IMSR
2
L
T
AmpOpfRe
C
ISR
2
SR Improved by factor of M !but …
LDD
OpAmpfRe
CV
PSR
2
SR Really Less than for Ref Op Amp !!
39
Comparison of Current-Mirror Op Amps
with Previous Structures
VDD
VSS
VINVIN
VOUT VOUT
M : 1 1 : M
IT
VSS VSS
M1M2
How does the Current Mirror Op Amp really
compare with previous amplifiers or with
reference amplifier?
Perceived improvements may
appear to be very significant
Actual performance is not as good in
almost every respect !
But performance is comparable to
other circuits and the circuit structure
is really simple
Widely used architecture as well but
maybe more for OTA applications
End of Lecture 10