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Two-Stage op-amp 2007 S. J. Ashtiani 33
Switched-Capacitor CMFB
CC: Generated common-mode signal CS: Sets the DC bias of CC Wide
swing (rail-to-rail) Used in switched-capacitor circuits Prone to
charge injection Capacitive loading
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Two-Stage op-amp 2007 S. J. Ashtiani 34
CMFB Compensation Issues
Q1 Q2
Q3 Q4
Q5 Q6
Q7 Q8
Q9
VBCN
VBCP
Vin+ Vin-
Vout-
VBP
Vout+
CMFB
Q2
Q4
Q6
Q8
VBCN
VBCP
Vin-
Vout-
VBP
Q9/2CMFB
VCM
VCNTRL
CMFB usually has lower fT: (Desired) Potential instability if
other non-dominant poles contribute Simpler CMFB circuit: More
stable!
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Two-Stage op-amp 2007 S. J. Ashtiani 35
Class-A/AB Stages: Rabii 97
VBN
Q1 Q1
Q3 Q4
CC
Q6
Q8 Q10
Q12
CC
Q5
Q7Q9
Q11
Vin+ Vin-Vo-Vo+
VCMFB1
Vo1- Vo1+
Two independent CMFB circuits are required Class A/AB: Output
bias currents changes with input voltage t= gm1/CC
P2=(gm11+Kgm5)/CL, P3=gm7/(Cgs7+Cgs9) SR: Not limited by the second
stage
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Two-Stage op-amp 2007 S. J. Ashtiani 36
Class-AB Stages: Hogervorst 94 Class AB
For symmetry: IQ3=IQ4(in quiescent mode)
For Maximum positive half: IQ3=0,
For Maximum negative half: IQ4=0,
Rail-to-Rail output swing
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Two-Stage op-amp 2007 S. J. Ashtiani 37
OTA with Nested Miller Compensation
Compensation of three-stage OTAs 3poles, 2 zeros:
If Cm>>C1,C2:
One RHP and One LHP Zero |RHP|
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Two-Stage op-amp 2007 S. J. Ashtiani 38
Ahuja Compensation (Ahuja 83)
Input stage not loaded by CC Eliminates the RHP Zero Larger
P2
t=-gm1/CC P2=-gm2/(CC+CL)CC/C1
Additional power consumption Better high-Frequency PSRR
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Two-Stage op-amp 2007 S. J. Ashtiani 39
Cascode Compensation (Ribner 84) Faster SR
Larger BW for same CC
No additional current path
Very Difficult to compensate Third-order system One real and
two
complex poles
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Two-Stage op-amp 2007 S. J. Ashtiani 40
Cascode Compensation :Small-Signal Model
( )( )
22
3212
2
3222
123
22322
2
1
)(
T
mmm
T
Cmm
T
CmCLm
CmmT
m
CCggfgs
CCCggs
CCfgCCgs
sCCggCC
g
sH++
++
=
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Two-Stage op-amp 2007 S. J. Ashtiani 41
Cascode Compensation: Closed-Loop Settling
