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Operational Amplifiers I (Ch. 24)p p
김 영 석김 영 석
충북대학교 전자정보대학
2011.3.1. .
Email: [email protected]
전자정보대학 김영석 24-1
Contents24.1 The Two-Stage Op-Amp
24.2 An Op-Amp with Output Buffer
24.3 OTA
24 4 G i E h t24.4 Gain Enhancement
전자정보대학 김영석 24-2
Block Diagram of OP-AMPOp-amp (Operational Amplifier) is a fundamental building block in Analog IC Design
Diff-amp + Gain Stage + Buffer
Design Spec:Design Spec:
Open-loop Gain
CMR
CMRR
PSRR
Output Voltage Range
Current Sourcing/Sinking
Power DissipationPower Dissipation
전자정보대학 김영석 24-3
24.1 Two-Stage Op-AmpLow-Freq Open-Loop Gain VVrgrrgA opmpoponmnOLDC /832)||( ≈⋅⋅⋅=Input Common-Mode Range
opmpoponmnOLDC
VVVVDDVmVVVV ssGSCMMIN
93028035014501.035.022,1
++
=+=Δ⋅+=
Power Dissipation: 30uW
Output Swing and Current Source/Sinking
mVVVVDDV THNSGCMMAX 93028.035.013 =+−=+−=
Output Swing and Current Source/Sinking
mVVVDDVmVVV
OUTMAX
OUTMIN
9001002
7
8
≈Δ−=≈Δ⋅=
Offset=Random+Systematic
AIAI SINKOUTSOURCEOUT μμ 10,10 ,, =>
Stability?
전자정보대학 김영석 24-4
Stability1) Rz=0, Cc=100fF
fFCCCC
MggVAgg
krRkrrR
mmpmmn
opopon
613
)7(/150
333,111||
21
21
=++=
====
Ω=≈Ω==
μ
kHzCRR
f
fFCfFCCC
fFCCCC
LgdL
gsgddg
287)(2
1
6.13
1
82
7241
==
==+=
=++=
MHzCCCCCC
Cgf
CRgR
cc
cm
cm
210)(2
)(2
2211
22
2211
=++
=π
π
MHzC
gfc
mz 240
22 ==
π
전자정보대학 김영석 24-5
Stability2) Rz=0, Cc=2.4pF
cc
mun
OL
pFCMHzC
gf
MHzfAPMgetTo
=⇒==
°−==∠°=
4.2102
100)10(,80
1
π c
ffffRHP zerotodueUnstablePMtionBut Simula
≈↑↓
≈=>
:!! ,0
21 unz ffffRHP zerotodue ≈↑↓ ,,: 21
전자정보대학 김영석 24-6
Stability3) Rz=6.5k, Cc=2.4pF
kRg
sistorNullingZeroofAddition
zmn
5.61Re
Ω==
−
i titdi ibllRB t
StablePM
1
! ,90°≈
iationprocesstodueimpossiblenearlyRg
But zmn
var , =
전자정보대학 김영석 24-7
Stability4) Rz=MOSFET, Cc=2.4pF
To Overcome Process Variations
Use MOSFET in Triode
271
1)16.9(1,
z
SGzSGPSGSGP
gWR
VVVV
==
==
Problem: Power Dissipation
1mzp
gVL
WKP Δ
전자정보대학 김영석 24-8
Stability5) SF, Cc
To remove RHP Zero, Use Source Follower + Cc
RHP Zero due to Signal Forwarding Through Cc
> R Si l F di Th h C=> Remove Signal Forwarding Through Cc
:Direction Reversenot But :back gFeedbackin
21
12
vSFCcvvCcSFvvout
=>=>=>=>=>=>=
SF ofRout low todue Impossible 21
전자정보대학 김영석 24-9
Stability6) CG, Cc
To remove RHP Zero, Use Common Gate Amp + Cc
RHP Zero due to Signal Forwarding Through Cc
> R Si l F di Th h C=> Remove Signal Forwarding Through Cc
:DirectionReversenotBut)(:back gFeedbackin 12
vCcMCGvvCGMCGCcvvout
=>=>=>=>=>=>=
MCG of Source Drain to frompath signal little todue Impossible :Direction Reversenot But 21 vCcMCGv =>=>=>
전자정보대학 김영석 24-10
Stability6) CG, Cc - Continued
ssgCsRgRg
vv mcg
cmm
s
out
(...)(...)(...)
)1(
2
2211
++
+−=
FreqHigherCgCgf
ZeroRHPnotZeroLHPC
gf
cmcm
c
mcgz
:))6621((
) ,!!! (2
22 >=
=π
FreqHigherZeroLHPtodueOKNowfgf
FreqHigherCCCCCCCC
f
m
cc
:)(
Set
:))66.21()(2
()(2
1
2211212
≈
++>=
ππ
fFCMHzf
FreqHigherZeroLHPtodueOKNowfC
gf
cun
zc
mun
240@100
:) (2
1
==
≈=π
전자정보대학 김영석 24-11
Stability7) CG, Cc
Same as 6)
4Di iRB)(4:back gFeedbackin 12
CBMvCGBMCcvvout =>=>=>=
No Additional Power Dissipation
M4B of Source Drain to frompath signal little todue Impossible 4:Direction Reversenot But 21 vCcBMv =>=>=>
No Additional Power Dissipation
전자정보대학 김영석 24-12
Slew-Rate Limitations1) Miller Comp
nsmVpFA
CISRvofIncreaseLimitsiBut
vvONMvV.V.h: Low-to-Hig
SSLHoutC
outGp
c/3.8
4.220:
2,9050 7
≈==
↑↓=>=>↑=>=>
μ
AIvCurrentSinkMvCv
ONMMMvV.V.w: High-to-Lo
pFC
outoutcG
p
c
10 8 )2( , )1(
4,3,1,5090
4.2
7 ↓=>↑=>↑=>
=>↓=>=>
μ nsmVpFA
CCISR
Lc
DHL /3
4.310
)( 8 ≈=
+=
μ
2) Indirect Comp: Faster
nsmVpFA
CCISR
Lc
DHL /8
24.110
)(8 ≈=
+=
μpCC Lc .)(
전자정보대학 김영석 24-13
CMRRCMRR is determined by the diff stage
dBAA
AAAA
AAfACMRR
c
d
c
d
c
OL 50||log20||log20|)(|log202
2
2
≈===
전자정보대학 김영석 24-14
PSRR
77433 0,:
/)(
outSGDDG
out
OL
vvvvvvvfreqlowat
vvfAPSRR
≈↑=>↑===↑=>
=
+
++
77
/)(
)1(:
out
OL
outDGc
vvfAPSRR
gainvvvvshortCvfreqhighat
=
==≈==>↑=>
−−
++
78
78
1:
:
mBmoutc
oBmout
ggvshortCvfreqhighat
rgvvfreqlowat
+≈=>↑=>
+≈↑=>
−
−
전자정보대학 김영석 24-15
24.2 An Op-Amp with Output BufferWhen Driving Resistive Load, Output Buffer is required
)||)((),||(
84800,156.31500
2244
22211
21
oopoonmopmonomomm
OLDC
rrggArgrggA
dBAAA
+−=−=
==⋅==
전자정보대학 김영석 24-17
Effect of Device ShrinkL=100nm => 50 nm
Speed Enhancement
But, gain=84dB => 44dB due to CLM
전자정보대학 김영석 24-19
24.3 The Operational Transconductance Amp (OTA)OTA=Amp where all nodes are low impedance except input/output nodes
KVVrrgKvv
vA oomnp
outv 1@/65.16)||( 54 ==⋅⋅=
−=
MHzpF
VAC
gf
gKg
L
mnun
mmOTA
1
2412
/1502
=⋅
==
⋅=
πμ
π
MHzCrr
fLoo
dB 4.1)||(2
1
543 ==
π
전자정보대학 김영석 24-20
Increasing the OTA Output Resistance
54/500)||( dBVVRRgA ==⋅=
85
47)||(2
1
54/500)||(
3
PM
kHzCRR
f
dBVVRRgA
LocaspocasndB
ocaspocasnmnv
°=
==
===
π
)4.1,4.24: (85
3 MHzfdBAOTASimplePM
dBv ==
Important Note:
UnStableCAmpOp
StableMorefCOTA unL
↑=>
↓=>↑=>
:
:
UnStableCAmpOp L ↑=>− :
전자정보대학 김영석 24-21
OTA with an Output Buffer (An Op-Amp)2) Cascode OTA with Class-AB Output Buffer:
=> 100uA Lower Current than OTA with CS Output Buffer
전자정보대학 김영석 24-23
Comparison with CascodeCascode
M2-MOP: Fast
But, Limited CMR
OTA with Class AB
M1-M31-M3-M51-M5-MON: Slow
Better Input CMR/Speed
=> Folded-Cascode
전자정보대학 김영석 24-24
The Folded-Cascode OTA/Op-AmpBetter Input CMR and Speed
LdB
ocaspocasnmnv
CRRf
RRgA
π )||(21
)||(
3 =
⋅=
L
mnun
Locaspocasn
Cgf
CRR
π
π
2
)||(2
=
전자정보대학 김영석 24-25
½ ShrinkL=100nm => 50nm
fun=100MHz => 400MHz
AOL=70dB => 50dB (CLM)
td 10 > 5 @CL 1 Ftd=10ns => 5ns @CL=1pF
전자정보대학 김영석 24-27
24.4 Gain EnhancementShrink => Faster Speed, But Gain Reduction
=> Use Gain-Enhancement(GE) Technique
GEOLDCGEOLDC
rrggRRgAAAA
⋅+⋅⋅=
⋅=
)]||()[()]||([,
cGEocaspocasndB
oponmpmnocaspocasnmnOLDC
CARRf
rrggRRgA
⋅⋅=
⋅+⋅⋅=
)||(21
)]||()[()]||([
3 π
전자정보대학 김영석 24-29