An Efficient, Linear, Broadband Class-J-Mode PA Realid Using RF
Waveform Engineering
Abstract — Results from a fully implemented class-J RFPA (RF power amplifier) are prented for the first time, which demonstrate this mode’s high efficiency potential across a substantial bandwidth. Using a commercially available 10W GaN (gallium nitride) HEMT device, and the high power waveform measurement and active load-pull capability at Cardiff University, class-J operation has demonstrated drain efficiencies between 60-70% across a 1.35-2.25GHz (50%) bandwidth whilst delivering 10 Watts of power at the 2dB compression point. Realisation of the design has confirmed that the optimum harmonic load impedances of the class-J amplifier are more practically realisable than conventional Class-AB modes, with better compromi between power and efficiency tradeoffs over a substantial RF bandwidth.
Index Terms — Bandwidth, class-J, high efficiency, power amplifiers.
I. I NTRODUCTION
Until recently, PA design for wireless communication has been focud on specified RF bandwidths of
5% or lower, due mainly to the very tight spectrum allocations. Future systems, including WiMax, 4G and beyond, will likely require larger bandwidths, not just due to wider spectral allocations, but the ba bandwidth of the signals themlves which may well extend up to, and ultimately exceed, 100MHz.
To date, other RFPA applications, such as radar and ECM have not profited from advances in power and efficiency due to their much wider RF bandwidth requirements. Reported results on very high efficiency PAs, typically operating above 75% efficiency, tend to rely heavily upon preci multi-harmonic impedance terminations at the DUT (device-under-test) as well as very high levels of device gain-compression. Both of the factors lead to narrowband frequency performance limitations (less than 10%) and non-linear operation respectively. A newly prented mode of operation - class-J [1] - has shown the theoretical potential of obtaining linear RFPAs that have the same efficiency and linearity as conventional Class-AB designs but do not require a band-limiting harmonic short.
The high power measurement and characterisation capability at Cardiff University [2] has, for the first time, been utilid in the development of a broadband PA design methodology. Coupled with the theoretical backing for class-J in [1], significant investigations into the realisable performance of t
he class-J mode of PA are prented for the first time using a GaN HEMT power transistor. Specifically, analysis into the achievable high bandwidth-efficiency of a class-J PA from a waveform engineering-bad process is prented. Waveform and systematic load-pull measurement data has been ud in the development stage of the design procedure, whilst also being ud to analy the extent to which optimum broadband high-efficiency operation has been achieved in this mode. Measurements have been performed across a bandwidth of 1.3 to 2.3GHz. Device output parasitic de-embedding has been applied to the waveforms captured at the calibrated measurement plane of the device package in order to confirm class-J behaviour from the output voltage and current waveforms at the device plane [3].
Design and realisation of PA matching networks has yielded a broadband amplifier operating at high efficiency. Finally, linearity has been confirmed through ACP (adjacent channel power) measurements.
II.C LASS-J M ODE OF O PERATION
A. Class-J harmonic load terminations
Class-J has been described by Cripps [1], who acknowledges some earlier work by Raab [4]. Class-
J is defined as a mode where the voltage has harmonic components which make it tend asymptotically towards a half-wave rectified sine-wave, which in practice can be ufully approximated by a suitably phad cond harmonic component. The key difference between Class-J and most other high efficiency modes is the requirement for a reactive component at the fundamental. This is necessary to maintain a cond harmonic voltage phasing that is beneficial to efficiency, whilst maintaining a physically realisable impedance termination at the cond harmonic. In this way, a higher fundamental component can significantly outweigh the loss in power implied by the reactive load, a counter-intuitive result that has been discusd in detail elwhere [1].
A Class-J design thus displays approximate half-wave rectified sinusoidal output current and voltage waveforms, with a pha overlap between the two (Fig. 1). This mode of operation lends itlf very well to the process of waveform engineering, as described previously by some current authors [2]. Independent bias control and active multi-harmonic load-
Peter Wright, J. Lees, P. J. Tasker, J. Benedikt and Steve C. Cripps Cardiff School of Engineering, Cardiff University, Cardiff, UK
阅读时间wrightp@cardiff.ac.uk
pull are ud to engineer the shape of the current and voltage waveforms respectively.
The class-J output voltage waveform - assuming two-harmonics - is outlined in Table I [1]. From this the fundamental and cond-harmonic impedances (1) and (2) respectively can be found, defining the matching criteria.
In order to optimi efficiency, the GaN device requires some compromis in tting the design values for the fundamental (‘load-line’) resistance value, due to the effect of the DC ‘knee’-voltage offt [5].
TABLE I新东方口语培训
I DEAL C LASS -J L OAD T ERMINATIONS , A SSUMING T WO
H ARMONICS
Harmonic
Normalid voltage component
1 1.41 ∠ 45° 2
0.50 ∠ –90°
L L f R j R Z ⋅+=0 (1)
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L f R j Z ⋅⋅
−=8
3002π
(2) This required loading can be realid with the inclusion of the device output capacitance within a PA matching network design, which also has the tendency to provide a short to the higher harmonics. Figs. 1 and 2 illustrate measured class-J waveforms with the harmonic terminations, and with the third harmonic also optimid for efficiency (specified in Table II). The bias point imple
mented here was a deep A/B.
time, nc
RF I / Amps
R F V / V
Fig.1. Measured class-J waveforms on a 10W GaN HEMT.
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chf0.00.20.40.60.81.01.21.41.61.82.0-0.2
2.2RF V / V
R F I / A m p s
Fig. 2. Measured class-J dynamic load-line on a 10W GaN HEMT, overlaid with device DC-IVs.
TABLE II
C LASS -J I GEN . (C URRENT G ENERATOR )-P LANE L OAD
T ERMINATIONS
Frequency
I gen.-Plane Load Impedance
f 0 43.8 + j45.2 Ω 2f 0 1.6 – j52.0 Ω 3f 0
2.4 – j49.7 Ω
B. Waveform engineering very high efficiency class-J operation in a GaN HEMT
By optimising the fundamental and harmonic load impedances up to 3f 0, applying the class-J reacti
ve components through active load-pull, very high efficiency device operation has been measured at a fundamental frequency of 1.8GHz. The power sweep for this optimum emulated ca is shown in Fig. 3. This shows the output performance of the DUT in a class-J loading configuration, operating with a bias point in class-C. A peak drain efficiency of 83% has been measured in this state with just below 10W device output power and approximately 3.5dB of gain compression. However when looking at the device output performance 6dB backed off from this point, the measured drain efficiency is still above 60%. Device output power is slightly less than may be obrved in an optimid class-F/class-F -1 PA mode within the same boundary conditions [6].
P o u t , G a i n / d B m , d B
20
18
1614
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12
Pin / dBm
Drain Efficiency / %
Fig. 3. Power sweep showing very high efficiency class-J operation at 1.8GHz.
Following this initial performance characterisation of the class-J mode, a 10W broadband PA design was initiated, with a goal of above 60% efficiency over a 50% bandwidth.
II. I NVESTIGATING A B ROADBAND C LASS -J PA D ESIGN In order to begin the design stage of the class-J PA the GaN transistor to be ud in the design was put under test and harmonically load-pulled using the three-harmonic active load-pull t-up at Cardiff University [2]. This was carried out at veral frequencies between 1.3 and 2.3GHz. The device was in the same state of gain compression (approximately P2dB) in each ca. Working with device measurement data at the calibrated package-plane (i.e. without output parasitic de-embedding applied) the resulting load-pull sweep data was
ud to map out the drain efficiency as a function of fundamental load impedance on the Smith chart.
Fig. 4 indicates the 70% drain efficiency contour obtained from each fundamental load sweep, indica
ting the movement of the load at the device package-plane as a function of frequency. The cond and third harmonic impedances were t at the same respective load impedances for each fundamental load-pull sweep; I gen.-plane 2f 0 = 0.95 ∠ –90°, 3f 0 = 0.95 ∠ –170° (as indicated in Fig. 4).
A corresponding output power contour plot was extracted following the load-pull sweep which provided an ‘area’ of load impedances for which to aim a matching circuit design to provide a compromid class-J PA output power and drain efficiency.
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Fig. 4. Measured load-pull data for GaN HEMT indicating load impedance contour for 70% P2dB drain efficiency between 1.5-2.3GHz.
Fig. 5. Realid load matching network schematic [1].
The original target frequency band for this design was 1.5-2.5GHz. In designing a suitable matching network over this extended bandwidth, some compromis have to be made. The initial strategy on
the first design iteration (prented here) was to give higher priority to the fundamental impedance and allow the cond harmonic more latitude. This particular device has a low output capacitance (approximately 1.5pF) which at 2GHz is itlf quite clo to the optimum reactive termination at the cond harmonic. Thus the network itlf mainly synthesis the required fundamental load over the
extended bandwidth, but through a shunt inductive stub increas the effective cond harmonic capacitive reactance for lower frequencies. The output matching network is shown schematically in Fig. 5. For purpos of comparison between the load-pull data and the realid PA performance, the same 50-Ohm input impedance environment was applied for the PA input.
IV. R EALISED C LASS -J PA P ERFORMANCE R ESULTS A. Efficiency and output power vs. frequency
The realid class-J amplifier is shown in Fig. 6. Power sweeps over 12dB were carried out on the PA across a frequency range of 1.2 to 2.6GHz whilst also calculating the drain efficiency of the device within the PA across this bandwidth. Results of this sweep are shown in Fig. 7. The efficiency performance at the P2dB compression state is displayed; this is customarily ud as an ‘e
donghangnd-point’ for uable high-end efficiency performance in high PAR (peak-to-average ratio) signal applications.
As en in Fig. 7, the measured P2dB drain efficiency for the realid class-J amplifier is between 60-70% between 1.35GHz and 2.25GHz; a 50% bandwidth about a centre frequency of 1.8GHz. Within this bandwidth output power from the amplifier is between 9 and 11.5Watts.
A comparison between the realid PA results and simulated efficiency from the non-linear device model, t in the same impedance environment, is also shown in Fig. 7.
Fig. 6. Realid class-J 10W amplifier – output matched only.
B. PA Linearity and ACPR characteristics
Very high efficiency PAs can be prone to very non-linear characteristics which prent urs with a difficult, if not impossible, task of pre-distorting the PAs to meet communication system standards. ACPR, without any form of pre-distortion, was measured across a range of drive powers for the realid class-J PA, focusing on the centre frequency of 1.8GHz. Using a WCDMA signal of 5MHz channel bandwidth and 8.51dB PAR, a spectrum of the output signal from the PA operating, with a dr
ive sufficient to cau 2dB peak compression, is shown in Fig. 8. Average efficiency and ACPR is displayed in Fig. 9 with increasing drive power to the PA. ACPR of –32dBc, whilst operating at 39% average
DUT incl. output parasitics
DC bias feed
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PA Output
C ds
Fig.7. P2dB drain efficiency, P out performance, and device model-predicted efficiency for the realid class-J PA across a bandwidth of 1.2-2.5GHz.
efficiency, has been measured at a centre-frequency of 1.8GHz. At the extremities of the measured PA bandwidth (1.4 and 2.2GHz), the averaged ACPR is –30dBc and –35dBc respectively, operating at 2dB peak compression.
Fig. 8. Output spectrum from class-J PA, measuring ACP for a 5MHz WCDMA signal at centre frequency 1.8GHz, 39% average efficiency.
The symmetry of the upper and lower ACP sidebands, as in Fig. 8, implies minimal memory effects and good potential pre-distortability.
A v g . P o u t , G a i n , E f f . / d
B m , d B , %
26
24
22
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14
12
Avg. Pin / dBm
-50
-40-30-20-100
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ACPR / dBc
Fig.9. Average class-J PA performance with a power-swept 5MHz WCDMA signal at centre frequency of 1.8GHz.
V. C ONCLUSION & D ISCUSSION
This paper demonstrates the fully realid performance potential of the class-J mode of PA in terms of efficiency, bandwidth, and linearity. Using waveform engineering techniques, including active harmonic load-pull, on a packaged 10W GaN HEMT power transistor, an efficient, broadband PA design has been obtained and realid. The bandwidth achieved with this PA operating at a drain efficiency of between 60-70% was 1.35 to 2.25GHz (50% at 1.8GHz centre frequency). At the same time, fundamental output power was up to 11W whilst at P2dB compression. At an ACPR for a 5MHz WCDMA signal of –30dBc, average output power was 35.5dBm and efficiency was 42%.
A CKNOWLEDGEMENT
Rearch reported in this paper has been supported by Milmega Ltd., for financing the rearch and
CREE Inc. for supply of devices.
R EFERENCES
[1] Cripps, S. C., “RF Power Amplifiers for Wireless
Communications”, 2nd Edition, Artech Hou Publishers, 2006. [2] J. Benedikt et al., “High Power Time Domain Measurement
System with Active Harmonic Load-pull for High Efficiency Ba Station Amplifier Design,” IEEE MTT-S Int. Microwave Symp. Digest, June 2000, pp. 1459-1462. [3] A. Sheikh et al., “The Impact of System Impedance on the
Characterization of High Power Devices,” Proceedings of the 37th European Microwave Conf., Oct 2007, pp. 949-952. [4] Raab, F. H., “Class-E, Class-C, and Class-F Power Amplifiers
Bad upon a Finite Number of Harmonics,” IEEE Trans. Microwave Theory & Tech., Vol. 49, No. 8, August 2001, pp. 1462-1468. [5] Chris Roff et al., "Analysis of DC-RF Dispersion in AlGaN/GaN
HFETs Using RF Waveform Engineering," Accepted for IEEE Trans. Microwave Theory & Tech.
[6] Wright, P. et al., “Highly Efficient Operation Modes in GaN
Power Transistors Delivering Upwards of 81% Efficiency and 12W Output Power,” IEEE MTT-S Int. Microwave Symp. Digest, June 2008, pp. 1147-1150.
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E f f i c i e n c y , P o u t / %, d B m
2.50
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1.901.801.70
1.60
1.50
1.40
1.30
1.20
Frequency / GHz
80
7060504030
Drain Efficiency, Pout / %, dBm
