Slide 19/3/2002 S. Xie, V. Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini, U. Mishra, S. Long, M....

9/3/2002

S. Xie, V. Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini,

U. Mishra, S. Long, M. Rodwell

Department of Electrical and Computer Engineering,

University of California, Santa Barbara

Email: [email protected], [email protected] Phone: (805)893-8044 Fax: (805)893-5705

2002 Topical Workshop on Power Amplifiers September 2002, UCSD

High Linearity Class B Power Amplifiers in GaN HEMT Technology

9/3/2002

Outline UCSB

• Introduction

How does Class B PA work?

Why single-ended Class B?

• Highly linear single-ended Class B PA design and simulation

• Measurement results

• Conclusions

S. Xie, V. Paidi

9/3/2002

• Two identical devices working in 50% duty cycle with 180° phase shift. • Half sinusoidal drain current on each device, but full sinusoidal drain voltage. • Even harmonics are suppressed by symmetry => wide bandwidth (limited

by the power combiner). • Class B: Ideal PAE 78.6%; feasible PAE 40-50% (typical GaN HEMT at X-

band); Class A: Ideal PAE 50%, feasible PAE 20-30%.

VinVout = VDS1 – VDS2

0

180

0

180

+Vin

-Vin VDS2

VDS1

UCSBHow does push-pull Class B PA work?V. Paidi, S. Xie

9/3/2002

To obtain high efficiency (78%), a half-sinusoidal current is needed at each drain. This requires an even-harmonic short. This can be achieved at HF/VHF frequencies with transformers or bandpass filters. However,

1. Most wideband microwave baluns can not provide effective short for even-mode. Efficiency is then poor.

2. They occupy a lot of expensive die area on MMIC.

UCSBWhy not push-pull for RF MMIC V. Paidi, S. Xie

RL

VG

Vin

Cbias

Cbias

VD

1:1

1:1

Vin-

Vin+

ID+

ID-

Vout

VDS+

VDS-

9/3/2002

ID1

Vin

-Vin

Vin

+-ID2

= ID

33

2211 inininD VaVaVaI

3

32

21

21

22 ininin

DDD

VaVaVa

III

33

2212 )( ininininD VaVaVaVI

][ 33

221 inininLLDout VaVaVaRRIV

Zero Z at 2fo

RLvi

band passfilter @ fo

voutID

0

180

0

180

+Vin

-Vin ID2

ID1

Push-pull Class B

Single-ended Class B with Bandpass filter

Even harmonics suppressed by symmetry

Conclusion: From linearity point of view, push-pull and single-ended Class B with bandpass filter B are equivalent – same transfer function.

Even harmonics suppressed by filter

UCSBSingle-ended Class B = push-pull

Bandwidth restriction < 2:1

V. Paidi, S. Xie

9/3/2002

Ideal Class B Bias too low: Class C Bias too high: Class AB

UCSBWhy is biasing critical for Class B?

ID1

Vin

Vin

Vin

+

ID2

= ID

Vp

ID1

Vin

Vin

Vin

+ID2

= ID

Vp

ID1

Vin

Vin

Vin

+ID2

= ID

Vp

V. Paidi, S. Xie

9/3/2002

-20

-15

-10

-5

0

5

10

15

20

-4 -3 -2 -1 0 1 2 3 4

Out

put V

olta

ge, V

Input Voltage, V

Class C, nonlinear

Class B, linear

Class AB, nonlinear

Class B is linear given that the current transfer function is linear

UCSBVoltage Transfer Function as a Function of Bias Voltage V. Paidi, S. Xie

9/3/2002

.........*** 33

2210 gsgsgs VIVIVIII

The distortion will be minimum when the amplifier is biased at Class B by using GaN HEMT on SiC.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-5 -4 -3 -2 -1 0

Experimental (A/mm)Modeled (A/mm)

Dra

in C

urr

en

t (A

/mm

)

Gate Bias (V)

Highly linear characteristics of GaN HEMT on SiC

Bias point

UCSBA source of IM3 distortion:Transconductance distortion V. Paidi, S. Xie

The third order term in the Taylor expansion is very small when biased at pinch off.

9/3/2002

RLvi

band passfilter @ fo

vout

UCSB

RLTLIN

R1 L1

L2

C1

RF IN

Vg

Vd

BIASTEE Input

matchingnetwork

Outputmatchingnetwork

Cds BIASTEE

(short at 2fo, 3fo...)

Gate 2

Gate 1

• Lossy input matching network to increase the bandwidth

• Cds is absorbed into the -section output lowpass filter

Single-ended Class B Power AmplifierS. Xie, V. Paidi

Lossy input matching - section lowpass filter

9/3/2002

15

20

25

30

35

40

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30

Out

put p

ower

, dBm PAE (%

)

Input power, dBm

Simulation of class B amplifier @10GHz

Saturated PAE ~48%

Class B bias: Saturated output power ~ 37 dBm,Saturated PAE ~ 48%

UCSBSimulation performance of Class BS. Xie, V. Paidi

10

15

20

25

30

35

40

45

50

0 50 100 150 200

Dra

in V

olt

ag

e, V

time, psec.

0

200

400

600

800

1000

0 50 100 150 200

Dra

in C

urr

en

t, m

A

time, psec.

Waveforms of drain voltage and current

Saturated output power ~37 dBm

9/3/2002

Best IM3 suppression is achieved at Class B and Class A

UCSBSimulation of Intermodulation Suppression and PAE @10GHz S. Xie, V. Paidi

36

38

40

42

44

46

48

50

20

25

30

35

40

45

50

-5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5

Sat

urat

ed P

AE

(%

)

IMD

3 @6dB

back_off (dB

c)

Gate Bias, V

Class B bias: C/IMD3~44dBcPAE ~ 48% Class A bias

C/IMD3~42dBcPAE ~ 35%

Class AB bias

Class C bias

9/3/2002

0

5

10

15

20

25

30

35

1.00E+00 1.00E+01 1.00E+02 1.00E+03

UPG

H21

Ft~55 GHz

RF Performance

UCSBDevice Performance of GaN HEMTs

Performance of 12 fingers (1.2mm) device:

• Lg ~ 0.25um

• Idss ~ 1A/mm

• ft ~ 55 GHz (~ 50 GHz for dual gate)

• Vbr ~ 40V (~ 55V for dual gate)

DC I_V curve

~1.2 A

V. Paidi, S. Xie

9/3/2002

Chip photograph of Class B power amplifier

(Approximately 6mmX1.5mm)

Air bridges

Source

Drain

Gate 1Gate 2

UCSBV. Paidi, S. Xie

9/3/2002

Measurement setup

Signalgenerator_1

Signalgenerator_2

Poweramplifier_1

Poweramplifier_2

Powercombiner Coupler

Powermeter

Bias T DUT

Coupler

Bias T

- 20 dB - 20 dB

Coupler 50 OhmLoad

Spectrum Analyzer

CH_A

CH_B

- 20 dB- 20 dB - 20 dB

UCSB

• Single tone from 4 GHz to 12 GHz;

• Two-tone measurement at f1 = 8 GHz, f2 = 8.001 GHz;

• Bias sweep: Class A (Vgs = -3.1V), Class B (Vgs = -5.1V, Class C (Vgs = - 5.5 V) and AB (Vgs = -4.5 V).

Measurements:

S. Xie, V. Paidi

9/3/2002

Class B power amplifier measurement result

Gain vs. frequency

-5

0

5

10

15

20

2 4 6 8 10 12 14 16

Gain

, dB

Frequency, GHz

Class AB

Class B

3 dB bandwidth: 7GHz - 10GHz

S. Xie, V. PaidiUCSB

9/3/2002

Class B bias @Vgs = - 5.1V

Single tone performance @ f0 = 8GHz:

Two tone performance @ f1=8GHz, f2=8.001GHz :

15

20

25

30

35

40

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25 30

Out

put

pow

er,

dbm

PA

E

Input power, dbm

-50

-40

-30

-20

-10

0

10

20

30

0

0.05

0.1

0.15

0.2

0.25

-15 -10 -5 0 5 10 15 20

Out

put

pow

er,

dBm

PA

E

Input power, dBm

f1,f2

2f1-f2, 2f2-f1

PAE (maximum) ~ 34%

Saturated output power 36 dBm

Good IM3 performance: • 40dBc at Pin = 15 dBm, and• > 35 dBc for Pin < 17.5 dBm


9/3/2002

Class A bias @Vgs = - 3.1V

-10

0

10

20

30

40

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-20 -10 0 10 20 30

Out

put

pow

er,

dBm

PA

E

Input power, dBm

-30

-20

-10

0

10

20

30

40

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-20 -15 -10 -5 0 5 10 15 20

Outp

ut

pow

er,

dB

m

PA

E

Input power, dBm

f1,f2

2f1-f2, 2f2-f1

Single tone performance @ f0 = 8GHz:

Two tone performance @ f1=8GHz, and f2=8.001GHz :

Saturated output power 36 dBm

Good IM3 performance at low power level but becomes bad rapidly at high power levels

Saturated output power each tone ~ 33dBm


PAE (maximum) ~ 34%

9/3/2002

Summary of IM3 suppressions

10

20

30

40

50

60

0 5 10 15 20 25 30 35

IM3 c

om

pre

ssio

n,

dB

c

Pout, dBm

Class BClass A

Class C

Class ABPsat

• Low output power levels (Pout < 24 dBm), Class A and Class B both exhibit good linearity (Class B > 36 dBc, Class A > 45 dBc).

• Higher output power levels, Class A behaves almost the same as Class B.

• Class AB and C exhibit more distortion compared to Class A and B.


9/3/2002

Class B vs. Class A

10

20

30

40

50

0

0.05

0.1

0.15

0.2

0.25

0.3

-5 0 5 10 15 20 25 30 35

IM3

supp

resi

on,

dBc P

AE

, twoto

ne

Output power, dBm

Class B

Class A

IM3 suppression and PAE of two-tone

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

10 15 20 25 30 35 40

PA

E,

sing

le

Output power, dBm

PAE of single tone

Class BClass A

While maintains the same IM3 suppression as Class A, Class B can get more than 10% of PAE.


9/3/2002

• For a less than octave bandwidth, a push-pull Class

B amplifier can be replaced by a single-ended Class B amplifier with bandpass or lowpass filter.

• A single-ended Class B MMIC power amplifier in GaN HEMT is designed and 36dBm of saturated power and 35dBc of IM3 suppression are obtained.

• Class B is better than Class A because it can get good IM3 performance comparable to that of Class A, while providing PAE ~10% higher than that of Class A.

UCSBConclusions S. Xie, V. Paidi

9/3/2002

This work was supported by the ONR under grant (N00014-00-1-0653)

Special thanks to Dr. Walter Curtice, who provide us the C_FET3 model for simulation;

Thanks to L.-Y. (Vicky) Chen and Likun Shen, who helped us for the measurement.

UCSBAcknowledgementsS. Xie, V. Paidi

Slide 19/3/2002 S. Xie, V. Paidi, R. Coffie, S. Keller, S. Heikman, A. Chini, U. Mishra, S. Long, M....

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