OTA-cmff

3/10/2009Analog & Mixed Signal Center,

Texas A&M University 1

A LowA Low--Voltage Fully Balanced OTA with Voltage Fully Balanced OTA with CommonCommon--Mode Feedforward and Inherent Mode Feedforward and Inherent

CommonCommon--Mode Feedback DetectorMode Feedback Detector

Reference:

A. N. Mohieldin, A.N.,E. Sanchez-Sinencio, J. Silva-Martinez, "A fully balanced pseudo-differential OTA with common-mode feedforward and inherent common-mode feedback detector", IEEE Journal of Solid-State Circuits, Volume: 38 Issue: 4 , Apr 2003 , Page(s): 663 -668



OutlineOutline

IntroductionIntroduction

Pseudo differential OTAPseudo differential OTA

Proposed OTA architectureProposed OTA architecture

Measurement ResultsMeasurement Results

Conclusions Conclusions

Design ConsiderationsDesign Considerations



IntroductionIntroduction

Limited linear input rangeLimited linear input rangeLimited tuning rangeLimited tuning range

Simple Differential OTA With tail Currrent Source

VSS

VDD

Vi+

Vbias

Vout- Vout

+

Vi-

M1 M1

M2 M2

Itail

VDD

Vd

Vbias

Vout- Vout

+

-VdM1 M1

M2 M2

VSS

Gm=gm1

Reasonable CommonReasonable Common--mode gainmode gainReasonable PSRRReasonable PSRR

VDD

Vcm

Vbias

Vout- Vout

+

M1 M1

M2 M2

VSS

Vcm

Rs

Sm

mm Rg

gG1

1

1+=

Differential ModeDifferential Mode Common ModeCommon Mode



Pseudo Differential TransconductancePseudo Differential Transconductance

VSS

VDD

Vi+

Vbias

Vout- Vout

+

Vi-

M1 M1

M2 M2

AdvantagesSuitability for low voltageWider common-mode input range

DisadvantagesPoor common-mode gain ACM=ADM>>1Poor PSRRNeed for fast and strong Extra CMFB Circuit to (1) Fix output common-mode voltage (2) Suppress common-mode signalsSimple Pseudo Simple Pseudo

Differential OTADifferential OTA



What are the Solutions toWhat are the Solutions to

Overcome those Limitations?Overcome those Limitations?



Solution In the LiteratureSolution In the Literature

Pseudo differential OTA With CMFFPseudo differential OTA With CMFF

CMFF is applied to cancel the common mode input signalCMFF is applied to cancel the common mode input signalAdd load to the driving stage, input capacitance doublesAdd load to the driving stage, input capacitance doublesCMFB is still neededCMFB is still needed

_

++

_

Vi+=Vicm+Vd/2

GmVi

-=Vicm-Vd/2

Gm(Vicm+Vd/2)

Gm(Vicm-Vd/2)

GmVd/2

-GmVd/2

+

_Gm

GmVicm

GmVicm

_

Differential Mode OTA

Common Mode OTA

+ VSS

VDD

Vi+

Vout- Vout

+

Vi-

VSS

Vi+ Vi

-

M1 M1 M1 M1

2M2M2M2

2Icm

Icm

Icm



Proposed OTA Block DiagramProposed OTA Block Diagram

CommonCommon--mode detection using the same differential mode detection using the same differential transconductance by making copies of the current transconductance by making copies of the current Input capacitance is not increasedInput capacitance is not increasedCMFF is inherently achievedCMFF is inherently achievedCMFB can be easily arrangedCMFB can be easily arranged

_

++

_

Vi+=Vicm+Vd/2

Gm

Vi-=Vicm-Vd/2

Gm(Vicm+Vd/2)

+

_

Gm(Vicm+Vd/2)

Gm(Vicm-Vd/2)

Gm(Vicm-Vd/2)-GmVicm

-GmVicm

GmVd/2

-GmVd/2

Σ21



How to Implement the How to Implement the

Proposed OTA?Proposed OTA?



Proposed OTA ArchitectureProposed OTA Architecture

VDD

Vi+ Vi

-

VDD

VSS

VX

I1

M1 M1

M2 M2 M2

I2

M3 M3

VoutI12

210

III −=

I2

I1

M2 M2M2

M3 M3

Vout+

I1

221 II +

221 II +

212

01III −

=I2

Inherent commonInherent common--mode detectionmode detectionInherent commonInherent common--mode Feedforwardmode Feedforward



Combine CMFB and CMFFCombine CMFB and CMFF

VDD

Vi+ Vi

-

VDD

VSS

VX

I1

M1 M1

M2 M2 M4M4M4

M4

I2

M3M3 M3 M3

Vout+ Vout

-

I1 I2

VX (from next stage) VX (from next stage)

VY

VDD

VY

Vref M1

M2

'3M '

3M

'4M '

4MVZ

CMFB is arranged exploiting the direct connection of the OTAsCMFB is arranged exploiting the direct connection of the OTAsAvoid using a separate commonAvoid using a separate common--mode detectormode detectorDifferentialDifferential--mode signals and commonmode signals and common--mode signals share mode signals share basically the same loopbasically the same loop



Small Signal AnalysisSmall Signal Analysis

The path from the differential signal to the ouput The path from the differential signal to the ouput encounters one poleencounters one poleThe other path is a commonThe other path is a common--mode pathmode path

nd

m

Zm

mm

d

odm s

gsCg

ggvisg

ω/1)( 1

2

21 +

=+

≅=Z

mnd C

g 2=ω

( )11 /tan ndωωφ −−≅Δ

( ) ( ){ }4321 ,maxmin ovovTPpeakovovTNDD VVVVVVVV +++++=



Simulation ResultsSimulation Results

Output voltage applying common-mode current step (Icm)

_

++_

_

++_1mg 2mgC

Vin+

Vin-

CMFBInformation

Vo+

Vo-

Icm

Icm



Sources of NonlinearitySources of Nonlinearity

Short Channel EffectsShort Channel EffectsMobility degradationMobility degradation

Cross product of differential and commonCross product of differential and common--mode signalsmode signals

Even order harmonicsEven order harmonicsNonlinear mixing components due to CMFBNonlinear mixing components due to CMFB

Due to the nonlinear commonDue to the nonlinear common--mode detectionmode detection



Short Channel EffectsShort Channel Effects

)2()1(8.

)2()1(16.

2

2_

2

2

3ovovov

rmsin

ovovov

Peak

VVVV

VVVVHD

θθθ

θθθ

++=

++≅

L HD3

CLE1

=θ

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛ +++

++≅

⎥⎥⎦

⎤

⎢⎢⎣

⎡=

1

432

12

32_

2_

221...2

).2()1(..3log10log10

m

mmm

movovov

rmsn

rmsin

ggggKTBW

gVVVHDVV

SNRθ

θθMaximize gm1

Maximize Vov

( )TGS VV −+=

θμμ

10

Tradeoff: LinearityTradeoff: Linearity--Frequency responseFrequency response



Cross product of differential and Cross product of differential and commoncommon--mode signalsmode signals

⎟⎟⎠

⎞⎜⎜⎝

⎛+++++=⎥⎦

⎤⎢⎣⎡ ++−−⎟

⎠⎞

⎜⎝⎛= cmdcmcmov

dovdovcm

dTPICMDD

P vvvvVvVvVvvVVVL

WKi 22

22

11 2

4222β

⎟⎟⎠

⎞⎜⎜⎝

⎛−+++−=⎥⎦

⎤⎢⎣⎡ +−−−⎟

⎠⎞

⎜⎝⎛= cmdcmcmov

dovdovcm

dTPICMDD

P vvvvVvVvVvvVVVL

WKi 22

22

12 2

4222β

Differential second harmonic

Common-mode signal allowed at the input of the filter must be lowFor example an HD2 of –50dB and Vov=0.6V, the maximum tolerated common-mode signal is 3.8mVPeak Previous stage should take care of this HD2



Nonlinear components due to CMFBNonlinear components due to CMFB

( )22

22

121

.2.

42.2

dov

dTPICMDD

P

vV

vVVVL

WKii

γβ +=

⎥⎦

⎤⎢⎣

⎡+−−⎟

⎠⎞

⎜⎝⎛=+

Nonlinear CM detectionNonlinear CM detection

VDD

Vo1 Vo2

VDD

VSS

i1

M1M1

M2M2

i2

2M33M

(i1+i2)

Z Z

id/2 -id/2

2

1 dI

DC vA

AI γ+

+

3M

2

1 dI

DC vA

AI γ+

+

( )FHD

VVAAHDHD

ovovCMFB

×=

⎥⎦

⎤⎢⎣

⎡++

+≅

3

33 5.011

11

θθ

Im AZgA ××=



How to use OTAs as CM Detector?How to use OTAs as CM Detector?

A A 22ndnd Order Filter is used as an exampleOrder Filter is used as an exampleExploit direct connection of the cascaded OTAs in the filterExploit direct connection of the cascaded OTAs in the filterDifferential OTA used as CM detector alsoDifferential OTA used as CM detector also

_

+

+_

_

+

+_

_

+

+_

_

+

+

_1mg 2mg

C

Vin+

Vin-

VBP1+

VBP1-

VLP1-

VLP1+

CMFBInformation

1mg 1mg

CMFBInformation

C



Filter ArchitectureFilter Architecture

CommonCommon--mode level sensed only once per output mode level sensed only once per output CommonCommon--mode level is fixed only once per nodemode level is fixed only once per node

_

+

+_

_

+

+

_

_

+

+_

_

+

+

_

_

+

+

_

_

++_

_

+

+

_

_

+

+_

CMFF

Amg 1 Amg 1 Amg 1

Bmg 1Bmg 1Bmg 1

Amg 2

Bmg 2

1LCVin

+

Vin-

VBP1+

VBP1-

VBP2-

VLP1-

VBP2+

VLP1+

VLP2-

VLP2+

CMFF+CMFB

CMFF+CMFB

CMFF+CMFB

Common ModeInformation

CMFFCMFF+CMFB

CMFF

CMFF

1LC

1LC

1LC

2LC

2LC

2LC

2LC



Chip MicrographChip Micrograph

X=350μm, Y= 450μm



Measurement Results: Measurement Results: Frequency responseFrequency response

Magnitude ResponseMagnitude Response Phase ResponsePhase Response--3dB cutoff frequency is 100MHz3dB cutoff frequency is 100MHz



LinearPhaseFilter

Network analyzer

Vin

Bias

CB

CB

Rg

Rg

VoutC

C

R

R

Measurement Setup: Measurement Setup: Frequency responseFrequency response



Measurement Setup: Measurement Setup: Intermodulation distortionIntermodulation distortion

LinearPhaseFilter

Spectrum analyzer

Vin

Bias

CB

CB

Rg

Rg

VoutC

C

R

R

Signal generatorPower

combinerVF1

VF2



Measurement Results: Measurement Results: Frequency responseFrequency response

Magnitude ResponseMagnitude Response Phase ResponsePhase Response--3dB cutoff frequency is 100MHz3dB cutoff frequency is 100MHz



Group delay ripple <3% up to 100MHz

Measurement Results: Measurement Results: Group delayGroup delay



IMIM33≤≤40 dB over the whole baseband for twin tones of 350mV40 dB over the whole baseband for twin tones of 350mVpp--pp

Measurement Results:Measurement Results: IMIM33



Comparison with previously published workComparison with previously published work

* * MMaximum differential input is 500 mVp-p for 1% THD

JSSC-1997 JSSC-1999 This WorkFilter Type and Order 7th Order 0.050

Equirriple7th Order 0.050

Equirriple +filter boost4th Order Bessel

Cut- Off Frequency 50 MHz 100 MHz 100 MHzRipple on Group Delay < 2% @ f < 1.5f3dB < 5% @ f < 2f3dB < 3% @ f < f3dB

Max Input Signalfor 0.5% THD

200 mVp-p 100 mVp-p 350 mVp-p*

THD -46 dB -46 dB -46 dB

Output Noise Level 1.7 mVrms N/A 700 μVrms

Dynamic Range@ THD=-46dB

32 dB > 40 dB 45 dB

Supply Voltage 3 V 3 V 3.3 VCurrent Consumption 27 mA 40 mA 26 mA

Technology 0.72 μm CMOS 0.29 μm BiCMOS 0.5 μm CMOS



ConclusionsConclusionsA pseudo differential fully symmetric fully balanced A pseudo differential fully symmetric fully balanced OTA architecture has been presentedOTA architecture has been presentedThe OTA has inherently the commonThe OTA has inherently the common--mode detector, mode detector, hence CMFB is economically implementedhence CMFB is economically implementedCMFF and CMFB are combined to exploit the direct CMFF and CMFB are combined to exploit the direct connection of the cascaded OTAs in a filterconnection of the cascaded OTAs in a filterDesign tradeDesign trade--offs have been demonstratedoffs have been demonstratedThe OTA achieves The OTA achieves --43dB THD@900mV43dB THD@900mVpppp and and 9.8nV/9.8nV/√√Hz noise spectral densityHz noise spectral densityThe filter achieves group delay ripple of 3% up to The filter achieves group delay ripple of 3% up to 100MHz, 45dB of DR@THD=100MHz, 45dB of DR@THD=--46dB in 0.546dB in 0.5μμm CMOSm CMOS

OTA-cmff

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