Download Lecture 5b: Common-mode feedback

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Common-mode feedback
CMFB:
VCM:
common-modefeedbackamplifier(erroramplifier)
externalcommon-modevoltage
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Common-mode feedback
DifferenCalinputsignal:
Common-modeinputsignal:
Common-mode output
voltage Vocm midway
between the limits of the
signal swing (normally
power-supply voltages)
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Using a CMFB amplifier to set output voltages
One possible
implementaDon of
CMFB amplifier
•  CMFB amplifier is used to amplify the difference between the average of the differenDal
amplifiers outputs and Vbiasp.
•  If the gain of the CMFB is large, the average of the two outputs will be very close to Vbiasp.
•  Any variaDon in VCMFB affects each output by the same amount.
•  CMFB amplifier shouldn’t affect the differenDal amplificaDon in the differenDal amplifier.
•  When the differenDal amplifier outputs are equal, they should be Vbiasp.
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OperaDon of the CMFB circuit
a) No offset
Correct outputs,
i.e. Vbiasp
Needed to stabilize
the CMFB loop
b) With a 50mV offset:
Note how the CMFB isn’t doing anything.
Outputs not
balanced
⇒ We must use a CMFB that can
balance the output over the enDre
range of diff-amp output voltage.
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CompensaDng the CMFB loop
•  vC is the AC common-mode signal
•  Unity gain frequency of CMFB loop is
fun,cm =
A cm ⋅ gmn
2π ⋅ CL
•  If we want to compensate the CMFB loop with the same load capacitance used to compensate
the differenDal forward signal path, then we must ensure that the gain of the CMFB amplifier is
A cm ≤ 1
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CMFB design essenDals
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CMFB is only to adjust DC levels, not for signal quality
Should not limit the speed of the amplifier
Must be stable
Should not limit the signal swing
Common-mode range as large as possible
No differenDal to common-mode conversion
No common-mode to differenDal conversion
Must be funcDonal over all signal condiDons
As simple as possible (only DC level adjustment)
Low power
Not accurate (even 100mV error can be tolerable in many cases)
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Common-mode feedback loop
•  Gain of common-mode feedback loop:
•  If M1A and M1B are fully matched,
−3
−3
g g
10 10
only common-mode signal is fed back
ALOOP = −A CO m1 m2 ≈ −6 −6 = 106
gDS1gDS2
10 10
•  If M1A and M1B are fully matched,
•  Output resistance of common-mode circuit:
also differenDal signal will be fed back
⇒ AdM ↓
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R o,CM =
A COgm1
ΔA diff = −A CO
gm1 gm2
gDS1gDS2
⎡ ΔgDS1
Δg ⎤
+ 2 m1 ⎥
⎢2
gm1 ⎦
⎣ gDS1
Δgm ~ ΔL ⎫
⎪
1 ⎬ ⇒ ca. 1% error
ΔgDS1 ~
ΔL ⎪⎭
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Common-mode feedback loop
•  Pole of common-mode feedback loop:
pCMFB =
A COgm1
CL
•  Typically pCMFB limits common-mode feedback loop
⇒ ACO ↑ or gm1 ↑
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Common-mode detecDon with resistors
•  CMFB amplifier compares with an external CM-level
•  Gain of CMFB amplifier:
g
A CO = m1 > 1
gm4
Limited common-mode range
Min{VCM} = VGS1 + VDS,ISS
RCM >> ROUT > 1MΩ
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Common-mode detecDon with transistors only
•  Common-mode feedback with source follower ACM = 1
•  Limited common-mode range
min{CM} = VGS,FB1 + VGS6
max{CM} = VDD
•  Limited differenDal signal range
max{ΔVOUT} < VFB,SAT = √⁠2 VSAT,FB1
•  VX depends on ΔVOUT ⇒ Δvdiff ⟶ ΔVCM transformaDon!
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CMFB with double differenDal amplifier
•  Common-mode detecDon with
source-coupled pair MFB1A and MFB1B
•  Common-mode cpmparison and feedback with sourcecoupled pair formed by MFB1A+MFB1B and MFB1C
1⎛W⎞
⎛W⎞
⇒⎜ ⎟
= ⎜ ⎟
⎝ L ⎠MFB1A,MFB1B 2 ⎝ L ⎠MFB1C
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•  Gain and pole of CMFB:
A CO =
gm,MFB1C
gm,MFB2C
pole =
gm,MFB2
C GS,MFB2
Fast common-mode sehling
•  Limited common-mode range
•  Limited differenDal output swing, if |Vo1 – Vo2| > VFB,SAT
ACO → no common-mode feedback!
→
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Source degeneraDon common-mode feedback loop
•  Common-mode feedback is performed by M3 and M4
•  M3 and M4 work as source degeneraDon resistors
controlled by output common-mode voltage
•  M3 and M4 in linear region i.e. parallel conductance is
g CM = gDS3 + gDS4
W
(VDD − VOUT + − VT )
L
W
gDS4 = μC OX (VDD − VOUT - − VT )
L
ass. VOUT + = Vo + ΔV, VOUT - = Vo − ΔV
W
⇒ g CM = μC OX (VDD − Vo − ΔV − VT + VDD − Vo + ΔV − VT )
L
W
= 2μμ OX (VDD − Vo − VT )
L
gDS3 = μC OX
•  Gain of common-mode feedback:
A CO =
gm3
< 1 ; VDS3 < VDS,SAT
gDS3
⇒ slow!
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SC common-mode feedback
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Op-amp is used only during the φ2 phase
Cmbias is an input that determines common-mode output voltage
Phase φ1: both capacitors Ccm are charged to the desired value of output voltage Vocm
Phase φ2: both capacitors Ccm (charged to Vocm) are connected between the differenDal
output nodes and Cmbias
•  The average voltage applied to Cmbias node will be Vocm
•  The voltage across Ccm does not change when phase periods are small enough
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SC common-mode feedback
Start-up sehling to correct commonmode bias values:
VB4
VB1
CMF1
•  Fast sehling to new common-mode level i.e.
only charge sharing between capacitors
•  Switched capacitors to refresh transistor bias
voltages
CMF2
t
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Folded cascode OTA with rail-to-rail common-mode levels
rail-to-railinputstage
Cascodegainstage
VB4
rail-to-railCMFB
VB6
VB3
IN+
IN-
OUT-
OUT+
VB2
VB1
•  NMOS and PMOS input stages in parallel
⇒ rail-to-rail input common-mode range
•  NMOS and PMOS common-mode feedback circuits
⇒ rail-to-rail common-mode detecDon range
VB5
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Fully differenDal Miller compensated two stage amplifier
CMFB-circuit
Inputstage
2nd-stage
2nd-stage
common-mode
sensingcircuit
VB4
VB5
CM
IN-
OUT-
OUT+
VB3
IN+
VB2
VB1
VB1
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Folded cascode input stage
Miller compensated 2nd stage
Common-mode sensing with resistors
Common-mode feedback with differenDal amplifier
•  Limited common-mode range
•  Stability – speed trade-off
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Fully differenDal Miller compensated two stage amplifier
1st stage
VDD
2nd stage
VDD
M4
M5
M6
M7
2nd stage
CMFB
V1
M14
M13
VBB
V2
M37
M35
C1
M8
M9
M2
OUTP
4p
V3
M1
C2
GND
VDD
R4
100k
GND
M16
M17
OUTM
100k
VDD
4p
M36
M12
M36
M15
R3
INM
INP
M10
M11
V4
M3
VBD
GND
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GND
Folded cascode 1st stage
Miller compensated 2nd stage
Current steering common-mode feedback
Common-mode sensing with resistors
•  Stability-speed trade-off in CMFB
(CMFB over both amplifier stages)
•  Limited common-mode range
min{CM} = VGS16 + VDS10
= VT + 2VDS,SAT
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Fully differenDal 2-stage amplifier with double CMFB circuit
Problem:
•  One global CMFB from the output to input stage is difficult to
design fast enough
•  Also stability might be a problem
SoluDon:
•  Separate local CMFB circuits for the input stage and output stage
⇒ Increase complexity and power consumpDon
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