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CMFB

Or

Matching N and P MOS currents

ID

VGS

VDS

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CMFBWhat is the effect of mismatch between the PMOS

current source and an NMOS current sink?

Effect of mismatch between N and P MOS devicecurrents

ID

VGS

VDS

Verr due to the transistor mismatches, in ID1 and ID2 , is

given by:

Vo=(Ip - In) ro = IRo

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Amplifier Review

single sided

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Amplifier Review

pseudo differentialVDD

Vin Vin- Vin Vin- Vin+

VB

Vin-

VB

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Amplifier Review

differential

Vin Vin- Vin Vin- Vin+

VB

Vin-

VB

Vin+Vin-

Vout-

Vout+

VB4

VB1'

VB4'

VB1VB1

Vcmfb

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Common Source Amplifier

& CMFB

Fully differential (FD) circuits REQUIRE common source(CS) amps and common-mode feedback (CMFB) for

proper operation. Setting the DC value of the high gain

(typically) output nodes.

FD amplifiers typically consist of CS circuits configured as

differential pairs. They model with similar properties of their

single-sided counter parts plus CMR & CMRR.

CS and CMFB amps are a key aspect of FD amplifiers.

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Why CMFB?

For the case of a FD amplifier with current source loadsimplemented by PMOS transistors the common-mode level

is ill defined.Ill defined

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Why CMFB?Ill defined

CM Qpts depend on how close IDM3a and IDM3b match BOTH each

other and ITail/2.

Why Mismatch

ITail/2. is implemented by a NMOS source and M3a and M3b by a

PMOS sink.

Given that we are expecting delta I >gm Vos = (IDxKPxW/L)

AVT/(WL) encompassing the bias generator and OTA. There is littlehope of practical matching even excluding PVT temperatures.

As a result either one of both if M3 or Mtail enter triode! WHY?

Common Mode Level

M3a M3b

ITailITail

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Why CMFB?Ill defined

The high impedance node DC Q points are difficult/impossible to set.Single-Ended & FF Op Amps and OTAs Amplifiers

Small mismatches, temperature, process shift etc.

VQ approach VDD or VSS, Tail transistor in triode or M3s in triode.

Solution, negative feedback use in single ended circuits.

CMFB circuits are singled ended

CMFB feedback circuitry fixes the DC Qpt.

Common Mode Level

M3a M3b

ITailITail

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ZF

ZI

VCM_SET

VCM_SET

VB3

Common Mode Level

CMFBSolution, negative feedback use in single ended circuits.

CMFB circuits are singled ended

CMFB feedback circuitry fixes the DC Qpt.

ITailITail

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VODIF

VIN+ - VIN-

VCM_SET

VB3

Desired Common Mode Set

Level or Qpt.

Max GainMax Signal range

CMFB

I/0 characteristics of a FD Amplifier.

Vocm

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CMFB

Basic Operation

1) Sensing the output CM

level,Vocm

= Vo+ + Vo-

= [ Vdiff+ + Vcmo] +[ Vdiff- +

Vcmo ]

= Vcmo

2) Comparison with a voltage

reference i.e.,

Vcmo VCM_SETFeed back error correcting

level to the amplifier bias

circuitry.

3) System Check for stabili ty

across corners.

Vocm is set by adding negative (Correction

Amp & Sense Ckt) feedback circuit to

maximize gain & Signal swing ofFD OTA.

CM Level

Sense

Ckt

VCM_SET

FD Amp

Correction Amp

Vo

Conceptual CMFB Architecture

Stability - CL PHASE MARGIN for(H1(s)H2(s)H3(s) < 70

Avol Application driven

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CMFB

Basic Operation

1) Sensing the output CM

level,Vocm

= Io+ + Io-

= [ Idiff+ + Icmo] +[ Idiff- + Icmo ]

= Icmo

2) Comparison with a current

reference i.e.,

Vcmo VCM_SET

Feed back error correctinglevel to the amplifier bias

circuitry.

3) System Check for stabili ty

across corners.

Vocm is set by adding negative (Correction

Amp & Sense Ckt) feedback circuit to

maximize gain & Signal swing ofFD OTA. 12

Conceptual CMFB Architecture

Stability - CL PHASE MARGIN for(H1(s)H2(s)H3(s) < 70

Avol Application driven

IREF

Transconductance

Approach

Vocm is set by adding negative (Correction

Amp & Sense Ckt) feedback circuit to

maximize gain & Signal swing ofFD OTA.

IREF

Transconductance

Approach

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CMFBCM Sense Circuits.

1 2

1 2

vodifavodifavodifaVcmo321

++=

a1 = 1 for All, a2 = , a3 =

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CMFBCorrection Amp Circuits

Structure ADC ACMADM THD %

1 plus 1 gmn/(2gmp) 0.051 plus 2 gmn/(2gmp) 0.221 plus 3 gmn/(2gmp)

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CMFB -example

Diff Amp Correction and CM Sense Amp

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CMFB -example

Diff Amp Correction and CM Sense Amp

Vin+

Vout

-

Vout+

Vin-

VB4

VB4

VB1

As Required

Identical Arch.Double Boosting

NOT REQUIRED

VB4'

VB1'

Vcmi =

VB2p

Vout+

VB3

Local Bias

Generator

IinVB1'

VB4'

VB4

VB2

Vin+Vin-

Vout-

Vout+

VB4

VB1'

VB4'

VB1VB1 Vcmfb