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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. peak-to-peak

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differential amplifiers
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
Two-Stage, Miller, Differential-In, Differential-Out Op Amp

peak-to-peak

output voltage

≤ 2·OCMR

Output common mode range (OCMR)

= VDD-VSS - VSDPsat - VDSNsat

two stage miller differential in differential out op amp with push pull output
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

differential output folded cascode
Differential-Output, Folded-Cascode

OCMR = VDD -VSS - 2VSDP(sat) -2VDSN(sat)

Quite limited

two stage differential output folded cascode
Two-Stage, Differential Output, Folded-Cascode

M11-M13 and M10-M12 provide level shift

common mode output voltage stabilization
Common Mode Output Voltage Stabilization

Common mode drift at output causes differential signals move into triode region

common mode feedback
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
cause of common mode problem
Cause of common mode problem

Unmatched quiescent currents

Vbb=VbbQ+Δ

Vbb

I2

Vin=VinQ

Vbb=VbbQ

Vo1

Vo2

Vin

I1

Vo1Q

Vo1

Vin=VinQ+ΔVin

actual Q point M2 is in triode

slide11

Vxx

Ix

Vo

Ix(Vo)

VOCM

Vin

Iy(Vo)

Vyy

Iy

Vo

slide12

Basic concept of CMFB:

CM

measurement

Vo+ +Vo-

2

Vo+

Vo-

Voc

-

CMFB

Dvb

e

VoCM

+

desired common mode voltage

slide13

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: err*Aerr(0)/ACMF(0)

example
example

Vb2

CC

CC

Vi+

Vi-

Vo+

Vo-

VCMFB

Vb1

Vo+

VCMFB

Voc

-

Vo-

+

slide15

Example

Voc

?

?

VoCM

Need to make sure to have negative feedback

slide16

VDD

M7A

150/3

150/3

M2A

M2B

300/3

300/3

75/3

M13A

M13B

BIAS4

averager

1.5pF

1.5pF

M7B

75/3

M3B

BIAS3

OUT+

OUT-

20K

20K

M3A

300/2.25

300/2.25

300/2.25

300/2.25

M6C

75/2.25

IN-

IN+

Source

follower

M1A

M1B

M12B

M6AB

M12A

1000/2.25

75/2.25

1000/2.25

200/2.25

BIAS2

M11

M10

M9A

M9B

CL=4pF

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

resistive c m detectors
Resistive C.M. detectors:

Vo.c.

R1

R1

Vo+

Vo-

Vi+

Vi-

slide19
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)
slide20

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.

slide21

Why not:

Vo.c.

Vo+

Vo-

* Initial voltage on cap.

slide22

C1

C2

slide23

Use buffer to isolate Vo node:

gate cap is load

or resistors

switched cap cmfb
Switched cap CMFB

Vo+

Φ1

Φ2

Φ1

VoCM.

Vo-

VoCM.

slide25

To increase or decrease the C.M. loop gain:

e.g.

Vo.c.

Vo.c.d.

Vo.c.

Vo.c.d.

VC.M.F.B.

VC.M.F.B.

another implementation
Another implementation
  • Use triode transistors to provide isolation & z(s) simultaneously.

M1, M2 in deep triode.

VGS1, VGS2>>VT

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.

slide28

Example:

Input state

Vo+

Vo-

Vb

M1

M2

e.g. Vo+, Vo-≈2V at Q & Vb≈1V ,

Then M1&2 will be in deep triode.

slide29

Vo-

Vo+

Vb1

Vb2

VX

M1

M2

slide31

Vo++ Vo-

2

VoCM.

VCMFB

Vo+

Note the difference from the book

accommodates much larger VoCM range

Vo-

slide32

Small signal analysis of CMFB

Example:

IB

IB

VCM

M4

M3

Vo+

Vo-

M1

M2

-Δi

+Δi

+Δi

+Δi

M5

+Δi

-Δi

-Δi

-Δi

VCMFB

Δi=0

2Δi

slide33
Differential Vo: Vo+↓ by ΔVo, Vo-↑ by ΔVo
  • Common mode Vo: Vo+↑ by ΔVo, Vo-↑ by ΔVo
slide34

IB

IB

VCM

M4

M3

Vo+

Vo-

M1

M2

+Δi

+Δi

M5

-Δi

-Δi

VCMFB

Δi=0

2Δi

M7

Δi7

+

-

1

gm6

-2Δi

-2Δi

M6

cmfb loop gain example
CMFB loop gain: example

Vb2

CC

CC

Vi+

Vi-

Vo+

Vo-

VCMFB

Vb1

Vo+

VCMFB

Voc

-

Vo-

+

slide37

-gm5vro2

-gm5vro2gm6

Vo

gm5v

v

Poles: p1

p2

z1 same as before

slide38

VDD

M7A

150/3

150/3

M2A

M2B

300/3

300/3

75/3

M13A

M13B

BIAS4

averager

1.5pF

1.5pF

M7B

75/3

M3B

BIAS3

OUT+

OUT-

20K

20K

M3A

300/2.25

300/2.25

300/2.25

300/2.25

M6C

75/2.25

IN-

IN+

Source

follower

M1A

M1B

M12B

M6AB

M12A

1000/2.25

75/2.25

1000/2.25

200/2.25

BIAS2

M11

M10

M9A

M9B

CL=4pF

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

removing the cm measurement
Removing the CM measurement

Vo+

VoCM

Vo-

VCMFB

Directly connect Vo+, Vo- to the gates of CMFB diff amp.

slide40

VDD=+1.65V

M11

M12

M3

M4

M26

M27

Vo1

Vo2

M21

M22

M23

M24

M1C

M2C

IDC=100υA

VCM

M13

Vi1

M2

Vi2

M1

M14

M51

M52

M25

-VSS=-1.65V

slide41

CMFB with current feedback

M3

M4

IB

Vo+

Voc

CM

detect

VoCM

Vo-

M5

M6

M7

M1

M2