Willy Sansen 10-05 081

Fully-differential amplifiers

Willy Sansen

KULeuven, ESAT-MICASLeuven, Belgium

willy.sansen@esat.kuleuven.be

Willy Sansen 10-05 082

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 083

Single-stage OTA

VDD

VSS

M1 M2

M3 M4

+-

VDD

VSS

vOUT

M1 M2

M3 M4

+-

- -+

iCirc iCirc

VB1

VB2

VB1

Willy Sansen 10-05 084

Simple CMOS fully-differential OTA

12

3

VDD

VSS

vOUT+

CL

M1 M2

M3 M4

M5

CL

GBW =gm1

2π CL

Differential pairNo current mirror

vIN+ vIN-

vOUT-Problem:

keep M1-4 in saturation:

Control VOUTCOMControl IDS5

VB1

VB2

Willy Sansen 10-05 085

Simple CMOS fully-diff. OTA with CMFB - 1

12

3

VDD

VSS

vOUT+

CL

M1 M2

M3 M4

M5

CL

vIN+ vIN-

vOUT-

Control VOUTCOM, IDS5

IBVB

2

4

5

Willy Sansen 10-05 086

Simple CMOS fully-diff. OTA withy CMFB - 2

12

3

VDD

VSS

vOUT+

CL

M1 M2

M3 M4

M5

CL

vIN+ vIN-

vOUT-

Control VOUTCOM, IDS5

2VB

4

5

B2 : 1

Willy Sansen 10-05 087

Common-mode feedback equivalent circuit

12

3

VDD

VSS

vOUTCM

CL

M1,2

M3,4

M5

vINCM

Control VOUTCOM, IDS5

2VB

4

5

M5,6

1 : B1

B2 : 1

Willy Sansen 10-05 088

Common-mode feedback CMFB

Avind+

- +- +

-+

CL

voutd

CL

Ad Acvinc

CMFB in unity gain : CMRR = AvCM

Three tasks : 1. Measure the output voltages2. Cancel out the differential signals3. Close the CMFB loop

Willy Sansen 10-05 089

Requirements fully-differential amplifiers

• High speed : GBWCM > GBWDM

• Matching

• Output swing limited by :

• Output swing of differential-mode amp

• Input range of common-mode amp

• Low power PCM < PDM

Willy Sansen 10-05 0810

Load capacitance ?

-vIN CM

+Gm

CS CL

CF

Gm

CS

CF

+

-vOUT

CF CF

CS CS

+

-

-

+

+

+-

-

Willy Sansen 10-05 0811

Load capacitance CIN

vIN CM

CL

CF

CS+

-vOUT

CF

CS

Willy Sansen 10-05 0812

Load capacitance CINDM

vINDM CM

CL

CF

CS+

-vOUT

CF

CS

CINDM = CM +CF + CL + CS

2

Willy Sansen 10-05 0813

Load capacitance CINCM

vINCM CM

CL

CF

CS+

-vOUT

CF

CS

CINCM = 2 (CF + CL + CS) > CINDM

Willy Sansen 10-05 0814

GBWDM & GBWCM

GBWDM =gm

2π 2CL

vOUT-

CL

+-

IBv v

vOUT+R R

vOUT-2CL

+-

IBv v

vOUT+

R R 2CL

CLCM = 0 CLCM = 4 CL

GBWDM =gm

2π 2CL

Willy Sansen 10-05 0815

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 0816

CMFB amplifier with linear MOSTs

CL

+ -M1

M3

M2

VDD

VSS

Vr1

Linear MOSTs:VDS3 ≈ 200 mV

GBWDM =gm1

2π CL

CL

GBWCM = gm3

2π CL

is always smaller !

IDS = β VDS(VGS-VT) gm3 = β VDS3

Willy Sansen 10-05 0817

Fully-differential amp. with linear MOSTs

CL+ -

B : 2 : B

M1

M2 M3

M4

M6

M5VDD

VSS1 : BB : 1

Vbias

Linear MOSTs:VDS5 ≈ 200 mV

Cancel diff. signals

GBWDM = Bgm1

2π CL

CL

GBWCM = gm5

2π CL

is always smaller !even with M5 in wi !

M5 M7

Willy Sansen 10-05 0818

Fully-differential amp. with linear MOSTs

Ref. Choi, JSSC, Dec.83, 652-653

Linear MOSTs:VDS3 ≈ 200 mV

Willy Sansen 10-05 0819

Total amplifier schematic

E.Peeters etal, CICC 1997

Willy Sansen 10-05 0820

Fully-differential OTA with FF

850 MHz1.2 µm CMOS

F. Op’t Eynde, Kluwer Ac. 1993

Willy Sansen 10-05 0821

Transconductor with CDG compen.

VDS1 = RDID ≈ 0.2 V

IDS1 = β1VDS1(VGS1-VT)

gm1 = β1VDS1 is constant

Ref. Alini, JSSC, Dec.92, pp.1905-1915

Willy Sansen 10-05 0822

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 0823

Fully-differential amplifier with resistive CMFB

M2

vin1 vin2vo1 vo2

M1 M1

IB

R R

Av = gm1 (R // ro )

ro = ro1//ro2

Linear resistors RTo cancel differential signals

Willy Sansen 10-05 0824

Fully-diff. amp. with source followers: Diff. mode

Ref. Banu, JSSC, Dec.88, 1410-1414

GBWDM =

gm1

2π CL

CL= 4 pF

Willy Sansen 10-05 0825

Fully-diff. amp. with source followers: CM

Ref. Banu, JSSC, Dec.88, 1410-1414

GBWCM = gm6

4π CL

fndCM =

42πRa(CGS6+Ca)

Ra

Ca

12πRaCa

fz =

Willy Sansen 10-05 0826

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 0827

Fully-diff amp. with error amplifier: Diff. mode

Ref. Ribner, CICC 85; Haspeslagh, CICC 88

GBWDM = gm1

2π Cc

Willy Sansen 10-05 0828

Fully-diff amp. with error amp. : Common mode

Ref. Ribner, CICC 85; Haspeslagh, CICC 88

GBWCM = gm58

4π Cc

Nonlinear !

Willy Sansen 10-05 0829

Class AB fully-differential amplifier

Ref. Lee, JSSC Dec.85, 1103-1113

Willy Sansen 10-05 0830

Comparison

Criterion Linear Error amp Error amp.MOST Source foll. Quad amp.

GBWCM/GBWDM < 0.1 > 1 > 1

Required tol. < 1 % < 6 % < 6 %

Diff.output swing 0.8 VDDSS 0.4 VDDSS 0.4 VDDSSIs limited by cascodes source foll. cm input

Power dissipation 1 amp 3 amps 2 amps

Willy Sansen 10-05 0831

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 0832

Fully-differential amplifier with gain boosting

Willy Sansen 10-05 0833

Low-voltage (1.1 V) DIDO

Gata, JSSC Dec.021670-1678

Willy Sansen 10-05 0834

Linear CM amplifier

Ref. Hernandez, JSSC Aug.05, 1610-1617

VoutCM = VDC + VGS7

Willy Sansen 10-05 0835

Fully-diff.amp. with separate linear trans.CMFB

24 MHz/ 3 pF 3 V/ 5 mA IDS1 = 0.25 mA Comp 4 kΩ/ 2 pF > 20 MHz

Linear MOSTs

Av = 0.3

Ref. Pasch, AICSP, 2000

Willy Sansen 10-05 0836

CMFB over 2 or more amplifiers

-vIN CM

+CL1

vOUT1

+

-

-

+

+

-

-

+CM vOUT2

CL1

CL2

CL2

Efficient use of 2nd amplifier !Ref. Mohieldin, JSSC April 2003, 663-668

vCM

Willy Sansen 10-05 0837

CMFB over 2 pseudo-differential amps

CL+ -

B : 1

M1

M2

CL-

M3

M5

1 : B

vCM

+

vCM

M4

M6M7

Willy Sansen 10-05 0838

Fully-differential amplifier with SC CMFB

Willy Sansen 10-05 0839

Fully-differential amp. with SC CMFB : Φ1

Switches φ1H closedgives CMFBandprecharge C

Willy Sansen 10-05 0840

Fully-differential amp. with SC CMFB : Φ2

Switches φ2H closedgives CMFBandprecharge C

Willy Sansen 10-05 0841

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise

Willy Sansen 10-05 0842

Fully-differental folded cascode with source foll.

IB1

IB2

Vr2

Vr1 Vr1CLCL

RaRa

+ -

+-

1 : 1 1

M1

M2

M3

M4

M6

M5

M7

M4

3

1

2

4

VDD

VSS

M4

M3

M5

M6

2

1

55

Willy Sansen 10-05 0843

Fully-diff. amp. : Specifications

Techn: CMOS Lmin = 0.8 µm ; VT = 0.7 VK'n = 60 µA/V2 & K'p = 30 µA/V2

VEn = 4 V/µm & VEp = 6 V/µm

Specs: GBWDM = 10 MHz CL = 3 pF GBWCM = 20 MHzall PM > 70o

VDD/VSS = ± 1.5 VMaximum Vswingptp = Voutmax - VoutminMinimum Itot

Verify: Slew Rate, Noise, ...

Willy Sansen 10-05 0844

Fully-diff. folded cascode in BICMOS

IB1

IB2

Vr2

Vr1 Vr1CLCL

RaRa

+ -

+-

1 : 1 1

M1

M2

M3

M4

M6

M5

M7

M4

3

1

2

4

VDD

VSS

M4

M3

M5

M6

2

1

55

Willy Sansen 10-05 0845

Fully-diff. amp. : Specifications

Techn: BICMOS Lmin = 0.8 µm ; VT = 0.7 VK'n = 60 µA/V2 & K'p = 30 µA/V2

VEn = 4 V/µm & VEp = 6 V/µmfTn = 12 GHz & fTp = 4 GHz

Specs: GBWDM = 10 MHz CL = 3 pF GBWCM = 20 MHzall PM > 70o

VDD/VSS = ± 1.5 VMaximum Vswingptp = Voutmax - VoutminMinimum Itot

Verify: Slew Rate, Noise, ...

Willy Sansen 10-05 0846

Table of contents

• Requirements• Fully-diff. amps with linear MOSTs• FDA’s with error amp.& source followers• Folded cascode OTA without SF’s• Other fully-differential amps• Exercise