cmfb
TRANSCRIPT
![Page 1: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/1.jpg)
Differential Amplifiers and common mode feedback
![Page 2: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/2.jpg)
Differential amplifiers
• Cancellation of common mode signals including clock feed-through
• Cancellation of even-order harmonics
• Increased signal swing
Symbol:
![Page 3: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/3.jpg)
Two-Stage, Miller, Differential-In, Differential-Out Op Amp
Output common mode range (OCMR) = VDD-VSS - VSDPsat - VDSNsat
peak-to-peak output voltage
≤ 2·OCMR
![Page 4: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/4.jpg)
Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Push-Pull Output
Able to actively source and sink output current
Output quiescent current poorly defined
![Page 5: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/5.jpg)
Cascode Op Amp with Differential-Outputs, push-pull output
![Page 6: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/6.jpg)
Differential-Output, Folded-Cascode
OCMR = VDD -VSS - 2VSDP(sat) -2VDSN(sat)Quite limited
![Page 7: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/7.jpg)
Two-Stage, Differential Output, Folded-Cascode
M11-M13 and M10-M12 provide level shift
![Page 8: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/8.jpg)
Common Mode Output Voltage Stabilization
Common mode drift at output
causes differential
signals move into triode
region
![Page 9: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/9.jpg)
Common Mode feedback
• All fully differential amplifier needs CMFB
• Common mode output, if uncontrolled, moves to either high or low end, causing triode operation
• Ways of common mode stabilization:– external CMFB– internal CMFB
![Page 10: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/10.jpg)
Vin
Vo1
Vbb
Vo2
I2
I1
Cause of common mode problem
Vo1
Vo1Q
actual Q point M2 is in triode
Vin=VinQVbb=VbbQ
Vbb=VbbQ+Δ
Vin=VinQ+ΔVin
Unmatched quiescent currents
![Page 11: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/11.jpg)
Vin
Vo
Vxx Ix
Iy
Vo
Vyy
Iy(Vo)
Ix(Vo)VOCM
yyyOCMo
yyyOCMo
VIVV
VIVV
i.e. If
i.e. If
yyo VV to from feedback neg. need
![Page 12: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/12.jpg)
CM
measurement
CMFB
+
-VoCM
Voc
Vo+
Vo-
Vo+ +Vo-
2
desired common mode voltage
Basic concept of CMFB:
evb
![Page 13: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/13.jpg)
CM
measurement
CMFB+
-VoCM
Voc
Vo+
Vo-
Vo+ +Vo-
2
Basic concept of CMFB:
evb e
Find transfer function from e to Voc, ACMF(s)Find transfer function from an error source to Voc Aerr(s)Voc error due to error source: err*Aerr(0)/ACMF(0)
![Page 14: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/14.jpg)
example
Vb2
Vi+ Vi-
VCMFB Vb1
CC CC
Vo+ Vo-
+-
Vo+
Vo-
VocVCMFB
![Page 15: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/15.jpg)
Voc
VoCM
Example
Need to make sure to have negative feedback
?
?
![Page 16: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/16.jpg)
Source follower
averager
BIAS4
1.5pF 1.5pF
M13A M13B
300/3 300/3
20K 20KOUT+ OUT-
M2A M2B150/3 150/3
BIAS3
300/2.25 300/2.25
IN-
M3A
M3B
IN+
300/2.25 300/2.25
M1A M1B
BIAS2
BIAS1
75/3
M7A
75/3
M7B
M6C 75/2.25
75/2.25
M6AB
M11
150/2.25
M8
150/2.25
200/2.25
200/2.25
M10
M5
CL=4pF 4pFM9A
50/2.25
M4A
50/2.25
M9B
50/2.25
M4B
50/2.25
M12A
1000/2.25
M12B
1000/2.25
VDD
VSS
Folded cascode amplifier
![Page 17: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/17.jpg)
R1 R2Vo-Vo+
node cascodedan at
is when especially-
achieve todifficult -
large very , use
effect. loading resistive :Prob
if,2
21
21
o
oo
V
RR
RRVV
Resistive C.M. detectors:
![Page 18: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/18.jpg)
Resistive C.M. detectors:
Vo.c.
R1 R1Vo-Vo+
Vi-Vi+
![Page 19: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/19.jpg)
• O.K. if op amp is used in a resistive feedback configuration
• & R1 is part of feedback network.
• Otherwise, R1 becomes part of g0 & hence reduces AD.C.(v)
![Page 20: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/20.jpg)
Buffer Vo+, Vo- before connecting to R1.
Vo+ Vo-
Voc
R1 R1
Simple implementation:
source follower
Vo-Vo+Vo.c.
* Gate capacitance is your load to Amp.
![Page 21: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/21.jpg)
Why not:
Vo-Vo+
Vo.c.
* Initial voltage on cap.
![Page 22: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/22.jpg)
C1 C2 short diff C A
freq. high at :Prob
if 21,2
CCVV oo
![Page 23: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/23.jpg)
Use buffer to isolate Vo node:
gate cap is load
or resistors
![Page 24: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/24.jpg)
Switched cap CMFB
Vo+
Vo- VoCM.
VoCM.
Φ2 Φ1Φ1
![Page 25: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/25.jpg)
To increase or decrease the C.M. loop gain:e.g.
Vo.c.d.Vo.c.
VC.M.F.B.
Vo.c.d.Vo.c.
VC.M.F.B.
![Page 26: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/26.jpg)
Another implementation• Use triode transistors to provide isolation
& z(s) simultaneously.
Vo-Vo+
Voc
M1 M2
can be a c.s.
M1, M2 in deep triode.
VGS1, VGS2>>VT
In that case, circuit above M1, M2 needs to ensure that M1, M2 are in triode.
![Page 27: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/27.jpg)
2 oo VV
deep triode oper
![Page 28: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/28.jpg)
Example:
M1 M2
Vb
Vo+Vo-
Input state
e.g. Vo+, Vo-≈2V at Q & Vb ≈1V ,
Then M1&2 will be in deep triode.
![Page 29: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/29.jpg)
Vo+Vo-
Vb1
Vb2
M1 M2
VX
oo
ooX
X
MM
GG
oc
oo
VV
VVV
IV
RR
VV
V
VV
,
,
)(
,
,
,
21
21
C.G. cascoded is
to but
const
If
![Page 30: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/30.jpg)
Two-Stage, Miller, Differential-In, Differential-Out Op Amp
M10 and M11 are in deep triode
![Page 31: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/31.jpg)
Vo++ Vo-
2 VoCM.
VCMFB
Vo+
Vo-
large be can gain
CMFBoo VVV
2
Note the difference from the book
accommodates much larger VoCM range
![Page 32: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/32.jpg)
M1
M3
VCM M4
M2
IB IB
Vo+ Vo-
+Δi
-Δi
-Δi
+Δi+Δi
+Δi -Δi
-ΔiVCMFB
M5
Δi=0 2Δi
Small signal analysis of CMFB
Example:
![Page 33: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/33.jpg)
• Differential Vo: Vo+↓ by ΔVo, Vo-↑ by ΔVo
• Common mode Vo: Vo+↑ by ΔVo, Vo-↑ by ΔVo
o
m
m
mCMFB
mmmmo
m
Vk
g
gki
gV
ggggV
gi
5
1
5
43211
21
2
![Page 34: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/34.jpg)
M1
M3
VCM M4
M2
IB IB
Vo+ Vo-
-Δi
+Δi+Δi
-ΔiVCMFB
M5
Δi=0 2Δi
M6
M7Δi7
-2Δi+
--2Δi
1
gm6
![Page 35: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/35.jpg)
accurate made be canratiosgeometric by gain*
:gain increase To
om
m
m
mCMFB
m
m
m
mmG
mG
Vg
g
g
gV
g
gii
g
gigVi
giV
6
7
5
1
6
75
6
7777
67
1
12
2
12
![Page 36: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/36.jpg)
CMFB loop gain: example
Vb2
Vi+ Vi-
VCMFB Vb1
CC CC
Vo+ Vo-
+-
Vo+
Vo-
VocVCMFB
![Page 37: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/37.jpg)
Vo
v
2
ooocCMFB
VVVV from gain
gm5v
-gm5vro2
-gm5vro2gm6
762
65
dsdsds
mmocmc ggg
ggA
Poles: p1
p2
z1 same as before
![Page 38: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/38.jpg)
Source follower
averager
BIAS4
1.5pF 1.5pF
M13A M13B
300/3 300/3
20K 20KOUT+ OUT-
M2A M2B150/3 150/3
BIAS3
300/2.25 300/2.25
IN-
M3A
M3B
IN+
300/2.25 300/2.25
M1A M1B
BIAS2
BIAS1
75/3
M7A
75/3
M7B
M6C 75/2.25
75/2.25
M6AB
M11
150/2.25
M8
150/2.25
200/2.25
200/2.25
M10
M5
CL=4pF 4pFM9A
50/2.25
M4A
50/2.25
M9B
50/2.25
M4B
50/2.25
M12A
1000/2.25
M12B
1000/2.25
VDD
VSS
Folded cascode amplifier
![Page 39: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/39.jpg)
Directly connect Vo+, Vo- to the gates of CMFB diff amp.
Vo-
Vo+ VoCM
VCMFB
Removing the CM measurement
![Page 40: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/40.jpg)
VDD=+1.65V
-VSS=-1.65V
M11 M12 M3 M4 M26 M27
Vo1 Vo2
M14
IDC=100υAM1C M2C
M13
M1
M2Vi1 Vi2
M51 M52
M21 M22
M25
VCM
M23 M24
![Page 41: CMFB](https://reader033.vdocuments.us/reader033/viewer/2022061121/546ea3acb4af9fd7158b45cd/html5/thumbnails/41.jpg)
CMFB with current feedback
VoCMCM
detect
VocVo+
Vo-
M3 M4
M1 M2
M6 M7
M5
IB
)by , t to(equivalen
by ,
by ,by If
2,
4 :Q desired
21
621
13
oGG
mo
oocooo
SSDDoCM
B
VVV
gVii
VVVVV
VVVI
II