Transcript
Differential Amplifiers and common mode feedback
Differential amplifiers
• Cancellation of common mode signals including clock feed-through
• Cancellation of even-order harmonics
• Increased signal swing
Symbol:
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
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
Cascode Op Amp with Differential-Outputs, push-pull output
Differential-Output, Folded-Cascode
OCMR = VDD -VSS - 2VSDP(sat) -2VDSN(sat)Quite limited
Two-Stage, Differential Output, Folded-Cascode
M11-M13 and M10-M12 provide level shift
Common Mode Output Voltage Stabilization
Common mode drift at output
causes differential
signals move into triode
region
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
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
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
CM
measurement
CMFB
+
-VoCM
Voc
Vo+
Vo-
Vo+ +Vo-
2
desired common mode voltage
Basic concept of CMFB:
evb
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)
example
Vb2
Vi+ Vi-
VCMFB Vb1
CC CC
Vo+ Vo-
+-
Vo+
Vo-
VocVCMFB
Voc
VoCM
Example
Need to make sure to have negative feedback
?
?
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
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:
Resistive C.M. detectors:
Vo.c.
R1 R1Vo-Vo+
Vi-Vi+
• 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)
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.
Why not:
Vo-Vo+
Vo.c.
* Initial voltage on cap.
C1 C2 short diff C A
freq. high at :Prob
if 21,2
CCVV oo
Use buffer to isolate Vo node:
gate cap is load
or resistors
Switched cap CMFB
Vo+
Vo- VoCM.
VoCM.
Φ2 Φ1Φ1
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.
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.
2 oo VV
deep triode oper
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.
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
Two-Stage, Miller, Differential-In, Differential-Out Op Amp
M10 and M11 are in deep triode
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
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:
• 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
M1
M3
VCM M4
M2
IB IB
Vo+ Vo-
-Δi
+Δi+Δi
-ΔiVCMFB
M5
Δi=0 2Δi
M6
M7Δi7
-2Δi+
--2Δi
1
gm6
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
CMFB loop gain: example
Vb2
Vi+ Vi-
VCMFB Vb1
CC CC
Vo+ Vo-
+-
Vo+
Vo-
VocVCMFB
Vo
v
2
ooocCMFB
VVVV from gain
gm5v
-gm5vro2
-gm5vro2gm6
762
65
dsdsds
mmocmc ggg
ggA
Poles: p1
p2
z1 same as before
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
Directly connect Vo+, Vo- to the gates of CMFB diff amp.
Vo-
Vo+ VoCM
VCMFB
Removing the CM measurement
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
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
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