57-65GHz CMOS Power Amplifier Using Transformer-Coupling and

Artificial Dielectric for Compact Design

PA Symposium1/20/09

Tim LaRocca, and Frank Chang

OverviewIntroductionDesign OverviewDifferential Design• Transmission Line Technology• Artificial Dielectric and Output Matching• Differential and Common-Mode Stability

Transformer• Basics• Combine Matching, Bias and Stability Networks

RF PerformanceLayout

60GHz MotivationReleased standards for unlicensed 57-65GHz spectrum:• IEEE 802.15.3c, ECMA, WirelessHD, IEEE 802.11VHT• Very limited success: “Last-mile” efforts, LMDS, 77GHz Automotive,

71-76GHz and 81-86GHz point-to-point • Military (AEHF cross-link) and science applications dominate

New commercial applications• Uncompressed wireless video transfer: “in-room”, Wireless HDMI• Short distance bulk data transfer: “near-field”, <1m

– P2P (Portable-to-Portable), M2M (Machine-to-machine), Proximity Communication, Wireless hard drive backup

Availability of standard digital CMOS process • High ft (>120GHz) for 90nm gate length• Silicon roadmap http://www.itrs.net predicts 37nm ft > 360GHz• Passive element Q’s are reasonable• Do not have to rely on expensive, but high-performance GaAs or InP

Power AmplifierTypical millimeter-wave power amplifiers• Expensive, but high-performance GaAs/InP• Single-ended• Transmission line based with λ/4 structures, such as Lange or

Wilkinson couplers.• Difficult matching impedances, extremely low.

Millimeter-wave CMOS PAs• Limited publications.• Similar architecture to GaAs design; same disadvantages• Low 1.2V supply voltage (knee voltage problematic)• Low ft• Lossy substrate, low-Q passive elements • Single-digit efficiencies

Goals and Achievements• Double-digit efficiencies above 15% and Pout > 12dbm• Compact design: 80% percent reduction from standard design

Schematic

4. Low loss output match network

2. Differential Line with Artificial Dielectric strips

3. No balun

RFout

1. Transformer interstage and input matching

5. Biasing and ground through transformer center tap. Take advantage of virtual grounds.

6. DC block I/O

RFin

Differential Transmission LineCPW (a) and Shield Microstrip (b) are single-ended.GSSG (c) is pseudo-differential• Need 4-port network analyzer• Large Signal testing difficult: magic-T, transitions, amps, etc.• Need off or on-chip balun which is lossy

GS (d)• True differential• Compact , 3dB more power with negligible area increase• Artificial dielectric strips

CMOS Artificial DielectricMethod to artificially increase the dielectric constant, and reduce the wavelength. (1948 for antenna lenses, Dr. Kock)CMOS is a mutiple metal interconnect process (UMC 1P9M 90nm is a 9 metal layer process)Insert floating metal strips directly underneath differentialtransmission line (DTL) to reduce length by increasing εr,eff

Phase ShiftLarge phase shift versus “physical short” and “physical open” differential transmission lines.Result is a 6X increase in the effective dielectric constant.

Simulation (SONNET) is solid line and measurement indicated by circles

L 152μm D 3μm

W 24μm S 0.5μm

G 20μm H 0.5μm

Phase of S21 (deg)

AttenuationMeasured attenuation is similarGreater than 2X benefit in α/β when compared with εr,eff

Artificial Dielectric Output MatchSymmetric short-circuited stub output match.Artificial dielectric used for design and further compact layoutDTL offers less loss than transformer

Output Match Layout

indep(PAE_contours_p) (0.000 to 15.000)

m2

indep(Pdel_contours_p) (0.000 to 55.000)

m1

Artificial dielectric strips are further from S.C. end. ~15% size reduction.

Differential and Common-Mode StabilityDifference between GSSG and GS approach.

2112

2222

211

21

dddd

dddddddd SS

SSK

Δ+−−=

2112

2222

211

21

cccc

cccccccc SS

SSK

Δ+−−=

xxxxxxxxxx SSSS 12212211 −=Δ

( )2212211111 21 SSSSS dd +−−=

( )2414231321 21 SSSSS dd +−−=

( )4232413112 21 SSSSS dd +−−=

( )4434433322 21 SSSSS dd +−−=

( )2212211111 21 SSSSS cc +++=

( )2414231321 21 SSSSS cc +++=

( )4232413112 21 SSSSS cc +++=

( )4434433322 21 SSSSS cc +++=

GS Transformer Stability Factor, K

Unstable

GSSG Transformer (Gnd ring)

Stable

DeviceWg = 2um (nf=16,32,64) for Max. Stable Gain.Source and drain fingers are layered from M1-M2.BSIM4 overlaid with RF layout model (Rg, Cext …)Be careful of gate resistance in foundry BSIM models

DrainGate

SourceM1M2

M7

Layout (nf=16)

BSIM4

Transformer ElementTransformer replaces typical matching network. • Inter-stage impedance matching• Biasing through virtual ground taps• Stability (K-factor)• Compact Layout (no lengthy chokes or matching elements)

Transformer: S-parametersGood agreement between simulation and test differential S-parameters. Qprimary ≈ 10 and k (coupling factor) ≈ 0.6

S11 S21

Transformer: MatchingSimultaneous impedance matching transformation between the output of the nth-1 stage to the nth stage.

S11(loaded transformer)

Q2 Load Pull CirclesΓOPT,L

S22(loaded transformer)

Q3 Avail. Gain Circles, ΓGa,MAX

Transformer: Matching Path

L1=imag(Z11)/w

R1=imag(Z11)

L2=imag(Z22)/w

R2=imag(Z22)

M=imag(Z12)/w

Transformer: Stability

10 20 30 40 50 60 700 80

1234

0

5

freq, GHz

Sta

bFac

t1

2M 2M

Transformer

No Transformer

Q and inductive high-pass network provided by the transformer stabilizes large device peripheries

Transformer RequirmentsWidth determined by power handling capability (RMS current), and low loss [5um and 10um]Turn ratio is determined ~ device periphery ratio (2).Load-pull and S11 determine L1 and L2 (self-inductances)Metal thickness increased by combining M8-M9.Minimum spacing for max. couplingSelf-resonance frequencies >> 60GHz Q1,2 >10-12

Small-Signal Performance

Wideband Small-Signal Response

Gain centered at 61GHz.Good agreement between simulation and test.Gain greater than 15dB

Sim = circleTest = solid

Swept Power Performance

0

5

10

15

20

25

-15 -10 -5 0 5Input Power (dBm)

Output Power

Gain

PAE

Gain (dB) PAE(%)

Pout(dBm)

Saturated Power above 12dBmEfficiency greater than 19%.

Large Signal Performance across Band

0

2

4

6

8

10

12

14

56 58 60 62 64 66Frequency (GHz)

Psa

t (dB

m)

Chip#1Chip#2Chip#3

0

5

10

15

20

25

56 58 60 62 64 66Frequency (GHz)

PA

E (%

)

Chip#1Chip#2Chip#3

Peak EfficiencySaturated Power

57-65GHz PAE and Psat performanceThree different chips

Comparison to Prior Art

Reference This Work [13] [11] [16] [15]

Technology 90nm 90nm CMOS 90nm 90nm CMOS 90nm CMOS

PSAT (dBm) 12.5 8.4 12.3 10.6 8.4

PAESAT (%) 19.3 7 8.8 ~1 5.8

GainSAT (dB) 11 10.3 2.3 1 8.4

GainLIN (dB) 15 15.2 5.5 8 17

VD (volt) 1.2 0.7 1.0 1.2 na

PDC(mW) 84 89 87 228.6 54

Area (mm2) 0.15 0.18 0.26* 0.97* 0.99*

Highest reported efficiency and power to-date.

LayoutCompact layout with core area 0.15mm2

16.7% the area of original single ended version.• 7-8dB higher gain, and 3.5dBm higher output power

0.3mm

0.5mm

Original

New Design

Test Set-upAgilent 8731E Network Analyzer, SOLT calibrationAgilent 83640A synthesized sweeper, 83557A 50-75GHz source module, NGC GaAs MMIC ampPower measurements calibrated and tested to standard

Conclusion

60GHz differential CMOS transformer-based power amplifier design validated.Highest reported efficiency and saturated power to date.Compact size achievedAcknowledgements• UMC Foundry• Northrop-Grumman Corp.

Process VariationTT,FF,SS corners for BSIM4 Model• F = Low Threshold, high leakage and driving current

20% Capacitive Variation

Atmospheric AbsorptionO2 resonance

RLC Model for Artificial Dielectric

No effect on Inductance Factor 5-6 for capacitance

Electric FieldE-field confined between artificial dielectric strips and DTL (does not shield H-field)

Short-Circuited Stub EffectNo difference between “physical short” and “physical open” S.C. stub elements

Characteristic Impedance( )( )ll

ββ

tantan

,0

,00

LX

XLin jZZ

jZZZZ

++

=( )lβtan,0_ XSHORTin jZZ =

( )lβcot,0_ XOPENin jZZ −=

0

011 ZZ

ZZSin

in

+−

=

Case A. “Physical Open” Differential Line: Z0, PhyOpen = 65Ω, βℓ = 10o

Case B. “Physical Short” Differential Line: Z0, PhyShort = 30Ω, βℓ = 22.5o

Transformer: Differential ModeExtract differential and common mode S-parameters from electromagnetic simulationMeasurements match Differential Mode Simulation

Effective Dielectric ConstantShort/Open Stub

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

−

−

o

inshort

o

inopen

ZZimag

ZZimag

1

1

tanh1

coth1

l

l

β

β

2/

⎟⎟⎠

⎞⎜⎜⎝

⎛=

ωβ

εcshop

eff

Q TransformerQ is approximately 10

Transformer: Differential Mode[1,0] mode, or ±1V and 0V consists of both even and odd mode.

[1,0] [1,0]

[1,0] mode does not follow measurements above 20GHz

VGA SchematicCascode, Transformer-CoupledLayout is VGA + PA (0.95mm x 0.3mm)

VGA Test Results24dB Peak Gain8dB Variation; 7-22mA

PA Gain

02468

101214161820

56 58 60 62 64 66Frequency (GHz)

Gai

n (d

B)

Chip #1Chip #2Chip #3

GaAs MMIC (ALH382)

PA

-8 -6 -4 -2 0 2 4 6 8 10-10 12

5

10

15

20

25

0

30

P_RF

PAE

0.35V

1.0V0.9V

0.6V0.7V

Vgs=0.4V

0.5V

PA Output Power

-8 -6 -4 -2 0 2 4 6 8 10-10 12

-5

0

5

10

-10

15

Out

put P

ower

(dB

m)

Input Power (dBm)

Conduction Angle Increase

Vgs=0.35V

1.0V

0.6V

Class B

Class A

Vgs Slope0.35V 0.990.6V 0.850.9V 1.02

PA Test Set-up