Differential Amplifiers and common mode feedback

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Differential Amplifiers and common mode feedback
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Differential amplifiers

• Cancellation of common mode signals including
  clock feed-through
• Cancellation of even-order harmonics
• Increased signal swing
• Symbol

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Two-Stage, Miller, Differential-In,
Differential-Out Op Amp
peak-to-peak output voltage 2OCMR
Output common mode range (OCMR) VDD-VSS -
VSDPsat - VDSNsat
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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
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Cascode Op Amp with Differential-Outputs,
push-pull output
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Differential-Output, Folded-Cascode
OCMR VDD -VSS - 2VSDP(sat) -2VDSN(sat) Quite
limited
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Two-Stage, Differential Output, Folded-Cascode
M11-M13 and M10-M12 provide level shift
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Common Mode Output Voltage Stabilization
Common mode drift at output causes differential
signals move into triode region
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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

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Cause of common mode problem
Unmatched quiescent currents
VbbVbbQ?
Vbb
I2
VinVinQ
VbbVbbQ
Vo1
Vo2
Vin
I1
Vo1Q
Vo1
actual Q point M2 is in triode
VinVinQ?Vin
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Vxx
Ix
Vo
Ix(Vo)
VOCM
Vin
Iy(Vo)
Vyy
Iy
Vo
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Basic concept of CMFB
CM measurement
Vo Vo- 2
Vo
Vo-
Voc
-
CMFB
Dvb
e
VoCM

desired common mode voltage
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Basic concept of CMFB
CM measurement
Vo Vo- 2
Vo
Vo-
Voc
-
CMFB
Dvb
e
e
VoCM

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 errAerr(0)/ACMF(0)
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example
Vb2
CC
CC
Vi
Vi-
Vo
Vo-
VCMFB
Vb1
Vo
Voc
VCMFB


-
Vo-

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Example
Voc
?
?
VoCM
Need to make sure to have negative feedback
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VDD
150/3
150/3
M7A
300/3
300/3
M2A
M2B
75/3
M13A
M13B
BIAS4
averager
1.5pF
1.5pF
M7B
75/3
M3B
OUT
OUT-
BIAS3
20K
20K
M3A
300/2.25
300/2.25
300/2.25
300/2.25
M6C
75/2.25
Source follower
IN-
IN
M1A
M1B
M12B
M6AB
M12A
1000/2.25
75/2.25
1000/2.25
200/2.25
BIAS2
M11
M10
M9A
M9B
CL4pF
4pF
150/2.25
50/2.25
50/2.25
BIAS1
M8
M5
200/2.25
M4A
M4B
150/2.25
50/2.25
50/2.25
VSS
Folded cascode amplifier
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Resistive C.M. detectors
R1
R2
Vo-
Vo
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Resistive C.M. detectors
Vo.c.
R1
R1
Vo-
Vo
Vi-
Vi
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• 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)

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Buffer Vo, Vo- before connecting to R1.
Voc
Vo
Vo-
R1
R1
Simple implementation source follower
Vo.c.
Vo-
Vo
Gate capacitance is your load to Amp.
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Why not
Vo.c.
Vo-
Vo
Initial voltage on cap.
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C1
C2
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Use buffer to isolate Vo node
gate cap is load
or resistors
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Switched cap CMFB
Vo
F2
F1
F1
VoCM.
Vo-
VoCM.
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To increase or decrease the C.M. loop gain e.g.
Vo.c.d.
Vo.c.
Vo.c.d.
Vo.c.
VC.M.F.B.
VC.M.F.B.
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Another implementation

• Use triode transistors to provide isolation
  z(s) simultaneously.

M1, M2 in deep triode. VGS1, VGS2gtgtVT
Voc
Vo-
Vo
M1
M2
In that case, circuit above M1, M2 needs to
ensure that M1, M2 are in triode.
can be a c.s.
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deep triode oper
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Example
Input state
Vo
Vo-
Vb
M1
M2
e.g. Vo, Vo-2V at Q Vb 1V , Then M12 will
be in deep triode.
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Vo
Vo-
Vb1
Vb2
VX
M1
M2
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Two-Stage, Miller, Differential-In,
Differential-Out Op Amp
M10 and M11 are in deep triode
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Vo Vo- 2

VoCM.
VCMFB
Vo
Note the difference from the book accommodates
much larger VoCM range
Vo-
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Small signal analysis of CMFB
Example
IB
IB
VCM
M4
M3
Vo
Vo-
M1
M2
-?i ?i
?i ?i
M5
?i -?i
-?i -?i
VCMFB
?i0
2?i
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• Differential Vo Vo? by ?Vo, Vo-? by ?Vo
• Common mode Vo Vo? by ?Vo, Vo-? by ?Vo

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IB
IB
VCM
M4
M3
Vo
Vo-
M1
M2
?i
?i
M5
-?i
-?i
VCMFB
?i0
2?i
M7
?i7
-
1 gm6
-2?i
M6
-2?i
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CMFB loop gain example
Vb2
CC
CC
Vi
Vi-
Vo
Vo-
VCMFB
Vb1
Vo
Voc
VCMFB


-
Vo-

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-gm5vro2
-gm5vro2gm6
Vo
gm5v
v
Poles p1 p2 z1 same as
before
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VDD
150/3
150/3
M7A
300/3
300/3
M2A
M2B
75/3
M13A
M13B
BIAS4
averager
1.5pF
1.5pF
M7B
75/3
M3B
OUT
OUT-
BIAS3
20K
20K
M3A
300/2.25
300/2.25
300/2.25
300/2.25
M6C
75/2.25
Source follower
IN-
IN
M1A
M1B
M12B
M6AB
M12A
1000/2.25
75/2.25
1000/2.25
200/2.25
BIAS2
M11
M10
M9A
M9B
CL4pF
4pF
150/2.25
50/2.25
50/2.25
BIAS1
M8
M5
200/2.25
M4A
M4B
150/2.25
50/2.25
50/2.25
VSS
Folded cascode amplifier
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Removing the CM measurement
Vo
VoCM
Vo-
VCMFB
Directly connect Vo, Vo- to the gates of CMFB
diff amp.
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VDD1.65V
M11
M12
M3
M4
M26
M27
Vo1
Vo2
M21
M22
M23
M24
M1C
M2C
IDC100?A
VCM
M13
M2
Vi1
Vi2
M1
M14
M51
M52
M25
-VSS-1.65V
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CMFB with current feedback
M3
M4
IB
Vo
Voc
VoCM
CM detect
Vo-
M5
M6
M7
M1
M2