CMFB

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