250w hbt doherty

7/30/2019 250w Hbt Doherty

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250W HVHBT Doherty with 57% WCDMA Efficiency

Linearized to -55dBc for 2c11 6.5dB PAR

Craig Steinbeiser, Thomas Landon, Charles Suckling

TriQuint Semiconductor, 500 W. Renner Road, Richardson, Texas 75080 USA

Abstract A 2-way symmetrical Doherty amplifier exhibiting250W saturated power has been developed using High-VoltageHBT (HVHBT) GaAs technology biased at 28V on the Collector.Greater than 57% collector efficiency at 50W (47dBm) averageoutput power has been demonstrated while achieving 55dBclinearized ACPR at 5MHz offset using a 2-carrier-side-by-sideWCDMA input signal with 6.5dB peak to average ratio measuredat .01% probability on the CCDF. At this condition, themeasured overall power-added efficiency is 53%. The HVHBTDoherty exhibits 200W (53dBm) P1dB at 70% efficiency with57% efficiency at 6dB output back-off (OBO) from P1dBshowing a 25 percentage point improvement over class ABoperation.

Index Terms GaAs HVHBT, Doherty, efficiency, WCDMA,digital pre-distortion, power amplifier.

I. INTRODUCTION

Recent developments in High Voltage Heterojunction

Bipolar Transistor (HVHBT) GaAs technology have enabled

significant advancement in WCDMA basestation RF power

amplifier efficiency.

The highest priority and main concern of base-station

designers is focused on RF power amplifier efficiency-

enhancement solutions due to the high efficiency RF lineup

requirement for multi-carrier WCDMA high power base-

stations. Class AB amplifiers are no longer attractive for use

in the final power stage, even with the best crest reduction

algorithms.

To improve WCDMA RF power amplifier efficiency, the

Doherty amplifier has been investigated and is well

documented in the literature [1,2]. As an advantage, the

Doherty amplifier enhances efficiency without need of any

additional complex circuitry. However, the load modulation

of the carrier amplifier may create a challenge for some

simple types of digital pre-distortion linearization.

In this work we present a 250W HVHBT Doherty amplifierthat exhibits WCDMA efficiency rivaling results reported for

more complex envelope tracking solutions utilizing

LDMOS[3] and GaN[4] technology. We observed greater

than 57% collector efficiency at 50W (47dBm) average output

power while achieving 55dBc linearized ACPR at 5MHz

offset using a 2-carrier-side-by-side WCDMA input signal

with 6.5dB peak to average ratio measured at .01% probability

on the CCDF.

In this paper we describe a relatively simple design method

that performed remarkably well, producing a HVHBT

Doherty that exhibits 200W (53dBm) P1dB at 70% efficiency

with 57% efficiency at 6dB output-back-off (OBO) from

P1dB. The HVHBT Doherty amplifier is shown to improve

efficiency by 25 points over class AB at 6dB OBO from

P1dB. In addition the HVHBT Doherty amplifier is shown to

be compatible with industry standard pre-distortion

technology improving adjacent channel ACPR by more than

20dB.

II. PACKAGED DEVICE

Transistor devices in this work are fabricated using

TriQuints proprietary InGaP GaAs High-Voltage

Heterojunction Bipolar Transistor (HVHBT) process. Figure

1 shows a top view of a single-ended HVHBT module that

exhibits 100W (50dBm) at P1dB.

Fig. 1. Photograph of HVHBT Module that exhibits 100W(50dBm) at P1dB. Base (bottom lead) Collector (top chamferedlead) and Emitter (flange).

The module consists of two HVHBT transistor die each

capable of delivering 50W (47dBm) at P1dB. Internal

prematching circuits are fabricated using TriQuints standardGaAs passive process. The output impedance is near 2-j2 at

2140MHz enabling an easily realizable output matching

network.

The module is routinely measured on a 50 single-ended

test fixture biased Class AB with a quiescent collector current

of 400mA and an operating voltage of 28V. CW P1dB

compression is typically 100W (50dBm) with collector

efficiency of 65%. CW P4dB is typically 140W (51.4dBm)

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with collector efficiency of 72%. Single carrier WCDMA

efficiency is typically 36% at 25W (44dBm) average output

power linearized to 60dBc using an input signal with

PAR=7.5dB. At 44dBm gain is typically 14.5dB.

III.DOHERTY PALLET

represented our

adjustments. The optimum values forc & p

ed in the three follo

Fi

2. Without this phase

adjustment the impedance would have been 1.1-j1.4,

approximate

ig. 4. Output impedance presented at peak power (2-j2) and 6dB

O

carrier amplifier shifted by a small but appreciable amount.

A symmetric 2-way Doherty amplifier was designed to

boost efficiency at 6dB Output-Back-Off from P1dB (OBO)

shown in figure 2.

Fig. 2. Photograph of Doherty Amplifier.

The Doherty amplifier is composed of a main amplifier and

a peaking amplifier employing a pair of 100W (50dBm)

HVHBT GaAs modules. The carrier amplifier is biased Class

AB at 550mA collector current. The peaking amplifier is

biased well below turn-on at 550mV.

A. Design Method

Theory of Doherty operation is well covered in the

literature[5]. In Doherty operation the efficiency boost

achieved is a result of modulating the carrier amplifier load

between Ropt at peak output power and 2Ropt at 6dB OBO.

For our Doherty amp we choose a simple 50 Doherty output

combining network. This allowed re-use of matching

elements previously developed for the 50 single-ended class

AB test fixture. To achieve the desired load modulation at

the carrier amplifier module output reference plane it is

important to have the correct phase offset (c) between theoutput match of the carrier amplifier and the Doherty

combiner. Equally vital for good Doherty performance is to

ensure the peaking amplifier does not load the carrier

amplifier when the output power level is at or below 6dB

OBO. This too can be achieved with an optimized phase

offset (p).

For this design, the general approach was to model the load

presented to the carrier amplifier at and below 6dB OBO. A

model of the Doherty combiner including both device output

matching sections (Zc & Zp) was developed in AWR, see

figure 3. Due to physical PCB layout constraints, additional

wavelength 50 transmission lines were added to the

model. Last, 50 transmission lines of electrical length = c

andp were added to the model. c & p

phase offset

were determin wing steps.

g. 3. Block Diagram of Doherty Combiner AWR Model.

First, recall the class AB target measured at the module

output reference plane, Ropt, was 2-j2. In a Doherty amplifier

operating at 6 dB OBO the carrier amplifier load target should

be 2Ropt, or 4-j2. In the AWR model, the peaking arm was

disconnected at the TEE junction and replaced with an open

circuit stub. Next c was adjusted until the load presented to

the carrier amplifier rotated to 4-j

ly 90 degrees off target.

F

BO (4-j2) plotted on a 4.2 ohm smith chart.

The next step was to account for the effect of the peaking

amplifiers off-state output impedance. The peaking arm was

reconnected to the TEE junction andp was set to equal c.

The peaking amplifier module port was then loaded with the

off-state s-parameters of the 100W (50dBm) HVHBT module.

After simulation it was noted that the load presented to the

c

p

Rt

50

Dohertytw

Zcorkne

Zp

0 1

.0

0

.2

- 0. 2

0

.4

- 0.4

6

0

.8

0

.6

-1

.0

-0.

-0.

8

2 j2

4 j2


purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work

in other works must be obtained from the IEEE


3/4

and edge

im

sted to

provide signal alignment for proper power summing.

B. Prototype Phase Adjustments -

verified providing further confirmation of

de

as adjusted to center Pin/Pout

pe

am

mplifier to the output side

centered AM/PM characteristic.

llector bias at 28V and base-plate temperature near 25-

30C.

A. CW Performance

15-point

put Power for

Doherty and non-Doherty (class AB) bias conditions.

overall power-

ad

quency at 6dB, 7dB, and

8dB Output Back-Off from P1dB (OBO).

Both p & c were optimized until the load presented to the

carrier amplifier returned to near 4-j2 and b

pedance was grouped tight with minimal slope.

Finally the input Wilkinson phase offset was adju

A prototype was built. First performance in non-Doherty

configuration was measured (that is, both amplifiers biased

Class AB). CW Pin/Pout versus frequency showed non-

Doherty performance to be as expected, confirming the

matching elements, Wilkinson splitter, and output combiner

were properly aligned. In addition single ended performance

of each channel was

vice matching.

In Doherty operation, CW Pin/Pout showed efficiency

peaked near 2110MHz indicating the carrier phase offset was

too long. An empirical adjustment method, similar to the

modeled approach was employed. First the peaking arm was

disconnected from the TEE junction and the input to thecarrier amplifier disconnected from the Wilkinson, re-routed

to a 50 port connector. Next, the phase offset at the output

of the carrier amplifier w

rformance at 2140MHz.

The peaking amplifier was then reconnected to the output

combiner. To maintain phase balance, the phase length of the

peaking amplifier input Wilkinson arm was increased by the

ount that the carrier amplifier output phase was decreased.

Efficiency measured to be centered at 2140MHz, however,

AM/PM remained centered at 2110MHz getting worse at

2170MHz. Transferring the phase offset from the Wilkinson

arm at the input of the peaking a

IV.DOHERTY PERFORMANCE

The Doherty amplifier was measured under both single-tone

CW and WCDMA single and multi-carrier signals with and

without linearization. The Doherty bias condition sets the

carrier amplifier to Class AB at 550mA collector current and

the peaking amplifier to well below turn-on at 550mV. The

Non-Doherty bias condition sets both amplifiers Class AB at

400mA collector current. All measurements were performed

with co

CW gain and efficiency versus output power curves for

Doherty and non-Doherty bias conditions at 2140MHz are

shown in figure 5. CW saturated output power is 54dBm

(250W) and P1dB-Compression at 53dBm (200W). Notice at

6dB OBO, output power is 47dBm (50W) and corresponding

Doherty efficiency is greater than 57%, a 25-point

improvement compared to non-Doherty efficiency of 32% at

2140MHz. At 10dB OBO, output power is 43dBm (20W)

and corresponding Doherty efficiency is near 35%, a

improvement over non-Doherty efficiency of 20%.

0

1

2

3

4

5

6

7

8

9

10

1112

13

14

15

30 32 34 36 38 40 42 44 46 48 50 52 54

Gain(dB)

0

5

10

15

20

25

30

35

40

45

50

5560

65

70

75

CollectorEfficiency(%)

Gain_2140MHz_AB

Gain_2140MHz

Ceff_2140MHz_AB

Ceff_2140MHz

Fig. 5. Gain and Collector Efficiency versus Out

As shown, HVHBT Doherty gain is between 11dB and

12dB at 6dB OBO. At 50W (47dBm) the

ded-efficiency is above 53% at 2140MHz.

Doherty collector efficiency versus frequency at 6dB OBO,

7dB OBO, and 8dB OBO is shown in Figure 6. Notice the

efficiency curve is well centered within the band from

2110MHz through 2170MHz and changes by less than 3-

po s across band.int

Fig. 6. Collector Efficiency versus Fre

Amplitude and Phase versus Output Power is an important

indication of Digital Pre-Distortion compatibility shown in

Fig 7. Notice phase and amplitude remain relatively

unchanged until peaking amplifier begins to turn on (near

46dBm). Minimizing the amount of change in phase over a

Output Power (dBm)

40

45

50

55

60

2100 2110 2120 2130 2140 2150 2160 2170 2180 2190 2200

6dB-OBO

7dB-OBO

8dB-OBO

Efficiency(%)

Frequency (MHz)





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indication that digital linearization

B. WCDMA Performance -

digital pre-distortion test system provided by PMC

Si

ted while achieving

s

both with and without digital pre-distortion at 2140MHz are

shown in Figure 9.

Fig. 9. PSD with and without Digital Predistortion at 2140MHz.

esigners seeking a

simple efficiency-enhancement solution.

discussions and independent verification of measured results.

[2]

[3]

[4]

[5]Efficiency, IEEE Int. Microw. Symp. Workshop, 2004.

WCDMA Power Spectral Density (PSD) measurement20dB output power dynamic range is important. Notice the

change in phase is highest at 2170MHz but shows less than 10

degrees of relative change over a 20dB range. This

characteristic is a first

should function well.

0

2

4

6

8

10

12

14

32 34 36 38 40 42 44 46 48 50 52 54

Gain(dB)&Phase(De

g)

Gain-2110

Phase-2110

Gain-2140

Phase-2140

Gain-2170

Phase-2170

Output Power (dBm)

Fig. 7. Amplitude and Phase versus Output Power. V.CONCLUSIONS

We have shown a 2-way symmetrical Doherty amplifierbuilt using HVHBT GaAs transistor devices. A simplistic

design approach combining modeling and empirical

adjustments was used and found to be successful. The

HVHBT Doherty amplifier produces 250W of CW saturated

power at 72% collector efficiency. We reported observing

57% WCDMA efficiency at 50W average power while

linearized to 55dBc using an input signal with two-carriers

side by side with PAR=6.5dB. This solution provides an

interesting alternative to basestation d

WCDMA linearity was characterized both with and without

Digital Pre-Distortion. The WCDMA input signal comprised

of 2 carriers side-by-side with peak to average ratio of 6.5dB

measured 0.01% on the CCDF. This waveform takes full

advantage of the Doherty efficiency boost at 6dB OBO.

Linearization was accomplished using a PALADIN-15

adaptive

erra.

Figure 8 shows Doherty Collector Efficiency and ACPR

versus Output Power both with and without linearization.

Notice greater than 57% collector efficiency at 47dBm (50W)

average output power has been demonstraVI.ACKNOWLEDGEMENT

The authors would like to thank Andrzej Haczewski,

Sandro Lanfranco, and Jukka Holster of Nokia Siemens

Networks Oulu Finland for their insightful technical

55dBc linearized ACPR at 5MHz offset.

5

10

15

20

25

30

35

40

45

50

55

60

65

Efficiency(%)&PeakPower(dBm)

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

ACPR(dBc)

CollectorEfficiency

Peak Output Power0.01% on CCDF

REFERENCES

[1] S. C. Cripps, RF Power Amplifier for WirelessCommunications, Norwood, MA: Artech House, 1999.F.H.Raab, Efficiency of Doherty RF power-amplifier

ACP5(not Linearized)

ACP10(not Linearized)

47dBm(50W) systems,IEEE Trans. Broadcast., vol.BC-33, no. 3, pp. 77-83,

Sep.1987.P. Draxler, S. Lanfranco, D.Kimball, C.Hsia, J.Jeong, J. Van de

Sluis, and P.M.Asbeck, High Efficiency Envelope TrackingLDMOS Power amplifier for W-CDMA, IEEE

ACP5Linearized

ACP10Linearized

MTT-S Int.Microw. Symp. Dig., Jun. 2006, pp. 1534-1537.D.F. Kimball, J. Jeong, C. Hsia, P. Draxler, S. Lanfranco, W.Nagy, K. Linthicum, and L.E. Larson, High-EfficiencyEnvelope-Tracking W-CDMA Base-Station Amplifier UsingGaN H

30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Averag e Pout (d Bm)

Fig. 8. ACPR-5MHz, ACPR-10MHz, efficiency, and peak output

power at 0.01 on CCDF vs. average output power with and withou FETsIEEE MTT, Vol. 54, No. 11, Nov. 2006, pp. 3848-3856.B. Kim, Microwave Doherty Amplifier for High Linearity and

t

linearization for WCDMA 2-carrier side-by-side with PAR=6.5dB.

250w hbt doherty

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