One watt gallium arsenide class-E power amplifier with a thin-film bulk acoustic
resonator filter embedded in the output network
Kyle Holzer, Jeffrey S. Walling
University of Utah PERFIC Laboratory, Salt Lake City, UT 84112, USA
E-mail: jeffrey.s.walling@utah.edu
Published in The Journal of Engineering; Received on 7th April 2015; Accepted on 9th April 2015
Abstract: Integration of a class-E power amplifier (PA) and a thin-film bulk acoustic wave resonator (FBAR) filter is shown to provide high
power added efficiency in addition to superior out-of-band spectrum suppression. A discrete gallium arsenide pseudomorphic high-electronmobility transistor is implemented to operate as a class-E amplifier from 2496 to 2690 MHz. The ACPF7041 compact bandpass FBAR filter is
incorporated to replace the resonant LC tank in a traditional class-E PA. To reduce drain voltage stress, the supply choke is replaced by a finite
inductance. The fabricated PA provides up to 1 W of output power with a peak power added efficiency (PAE) of 58%. The improved out-ofband spectrum filtering is compared to a traditional class-E with discrete LC resonant filtering. Such PAs can be combined with linearisation
techniques to reduce out-of-band emissions.
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Introduction
Switched-mode power amplifiers (PAs) offer higher efficiency than
their linear counterparts, however, natively generate a rich spectrum
of harmonics and intermodulation products. This paper presents an
efficient class-E PA with an embedded thin-film bulk acoustic wave
resonator (FBAR, Fig. 1a) filter. The FBAR filter is incorporated
into the class-E output circuit, replacing the traditional LC resonant
tank (Fig. 1b). This provides superior in-band frequency selectivity
and out-of-band spectrum suppression.
Piezoelectric materials have been used for microwave frequency
filtering for decades. Thin film technologies including surface
acoustic wave (SAW) and bulk acoustic wave (BAW) filters have
proven advantages compared to traditional discrete inductor and
capacitor derived counterparts because of the acoustic wavelength
at a given frequency being several orders of magnitude shorter
than the analogous electrical wavelength in a comparable electromagnetic filter medium. The FBAR builds on BAW filter technology with the advantages of improved performance and improved
ease of integration with standard planar device technologies.
FBAR filters do not require exotic materials typical of both SAW
and BAW filters, such as quartz, lithium niobate, ZnO and others.
FBAR [1] can be fabricated in the same process as a typical high
performance amplifier, enabling the integration of a power efficient,
and spectrally pure, switching amplifier design; an integrated extension of the discrete design presented in this paper.
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Design details
The amplifier implementation is fabricated including a custom
printed circuit board with chip-on-board (COB) construction as
shown in Fig. 2. The small size of this implementation shows
future potential for fully integrated PA dies with an FBAR either
integrated into the process technology or bonded directly to the
die in a system-in-package implementation.
2.1 Class-E amplifier design
A broadband TGF2080 gallium arsenide pseudomorphic
high-electron-mobility transistor (GaAs pHEMT) device is chosen
as the power transistor for this switching PA. This device offers
low input capacitance and moderate optimal impedance load magnitude across the target design frequency.
A high pass dual element conjugate match is implemented at the
input to minimise insertion loss and reduce component count. The
J Eng 2015
doi: 10.1049/joe.2015.0058
optimal class-E output shunt susceptance is achieved with a combination of the device’s drain capacitance, C1 and L1 as shown in
Fig. 1 [2]. The device’s output capacitance is larger than required;
hence, finite L1 is used to offset the additional shunt capacitance.
Finite inductance in the drain has also been shown to reduce the
voltage swing at the drain to as low as 2.5 × VDD [3]. Breakdown
voltage for this 0.25 μm GaAs process is in excess of +20 V enabling continuous class-E operation.
Series inductance (L2, Fig. 1) selection optimises transition from
the class-E output into the load impedance as derived in [4]
vL
= 1.1525
R
(1)
The series inductance aligns the voltage and current incident to the
load.
2.2 Class-E with embedded FBAR
Intrinsic to the class-E amplifier output tuning circuit is a series resonant circuit to filter undesired harmonic content that arises as a
result of the periodic pulsed operation of the amplifier. In the
classic implementation, this series resonant circuit can be designed
with any quality factor (Q) and performs well even for relatively
low Q. This series resonant circuit can be replaced by any bandpass
filter or matching network, and can be comprised of more than two
components. Hence, the implementation is a trade-off between the
required out-of-band (OOB) filter rejection and the number of filter
poles. A higher number of filter poles increases both the amplifier
size and its insertion loss.
In this design, we replace these traditional LC tanks with an
FBAR filter that increases the OOB rejection, significantly, while
minimising the insertion loss associated with a high-order LC
filter. The size is comparable to that of a single LC tank filter
while providing significantly better OOB rejection. Owing to the
operating voltage and desired output power, the 50 Ω FBAR filter
provides the real part of the impedance termination for the PA;
hence no additional matching circuits are required. Using the previous equation, L2 is optimised using the FBARs input impedance, R
= 50 Ω. Subsequently, in the case where a traditional LC tank is
used, the selection of L2 does not change for operation at the resonant frequency. Given the antenna input impedance closely matches
50 Ω.
This is an open access article published by the IET under the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0/)
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Fig. 1 Schematic of
a Class-E PA with embedded FBAR filter and
b Traditional class-E PA
2.3 Class-E with traditional LC tank
For comparison to the embedded FBAR resonator a single LC tank
resonator is designed for the centre frequency of operation. High
frequency multi-layer ceramic components are chosen to maximise
the realisable network Q. To aid the comparison between the two
amplifiers, no impedance transformation is used, hence, the Q of
the LC tank is limited by the load resistance, which is 50 Ω, and
yields a 20% fractional bandwidth.
2.4 Amplifier input match optimisation
Owing to amplifier reverse isolation, S12, the input match can be
affected with changes to the load impedance. This was verified
through simulation of input impedance comparing a traditional
LC tank and the FBAR filter resonant elements. The previously
designed high pass dual element input amplifier match proves sufficient in both cases.
Fig. 2 PCB layouts for
a Class-E PA with FBAR filter and
b Class-E PA with LC tank
2.5 COB implementation
To minimise parasitics associated with packaging, the amplifier die
is COB bonded directly to the PCB. This provides additional advantages of smaller size compared to packaged surface mount devices
as well as a lower junction to board thermal conductivity.
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Measurement results
The PA was fabricated and its performance characteristics measured. A TQP3M9035 driver amplifier is included on the printed
circuit to lower the required power level for input test signals.
In order to validate the FBAR tuned class-E PA (Fig. 2a), measurements are included for a class-E with LC tank PA (Fig. 2b).
Both amplifiers use the same TGF2080 device and PCB with
only modifications to the output network components (e.g.
FBAR, LC tank).
The peak output power of 29.95 dBm is achieved by the class-E
FBAR PA at 2.64 GHz (Fig. 3). The passband ripple is a characteristic of the FBAR filter. The class-E LC tank PA has the highest
average output power across the passband because of the small insertion of the single LC tank compared to 2.5 dB insertion loss for
the FBAR.
The PAE of each amplifier configuration is plotted against frequency in Fig. 4. The low insertion loss of the single LC tank
Fig. 3 Measured output power against amplifier configuration
Fig. 4 Measured PAE against amplifier configuration
This is an open access article published by the IET under the Creative Commons
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J Eng 2015
doi: 10.1049/joe.2015.0058
Fig. 5 Measured harmonics against amplifier configuration: (horizontal
bar line) = class-E FBAR harmonics 2–6 and (dashed line) = class-E LC
tank harmonics 2–6
Fig. 7 Measured class-E PA FBAR Pout against VDD (plus symbol 5 V,
circle symbol 6 V, asterisk symbol 7 V and multiplication sign 8 V)
wider range of voltages is plotted in Fig. 7. Increasing the drain
voltage beyond 8 V offers minimal effect on output power as the
drain current is at saturation. Decreasing the drain voltage reduces
the output signal swing and power as expected without significantly
affecting load match performance.
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Fig. 6 Measured class-E PA output spectrum comparison
clearly yields the highest PAE. The peak PAE for the Class-E
FBAR PA is only moderately lower at 58%. The PAE can be significantly improved with integration onto a single die because of
a combination of lower insertion loss through the FBAR filter optimised for target filter rejection and minimised signal transition
losses between component interfaces.
The distinct advantage of FBAR integration is the superior filtering capability in a small footprint. The two amplifier configurations
harmonic content are measured and plotted against frequency in
Fig. 5.
The class-E LC tank PA that offers a PAE advantage is also
unable to adequately suppress harmonic content. The FBAR filter
shows similarly strong harmonic rejection for both the linear
mode and the class-E mode, as expected.
The PA output is plotted over one octave of spectrum in Fig. 6
and demonstrates the adjacent channel rejection afforded by the
FBAR filter, in this case nulling among others the adjacent ISM
2.4 GHz band used by WLAN and Bluetooth. The LC tank offers
virtually no spectrum shaping. This application of harmonic filtering has a performance against size advantage over alternate
approaches, such as tuned distributed element filters [5]. Active harmonic suppression [6] requires considerable stability optimisation
for mitigated harmonic improvement.
In order to provide linear operation, the class-E PA must be linearised via external means. One example is envelope elimination and
restoration (EER); to accurately recreate the amplitude envelope
modulation at the amplifier load [7]. The optimal amplifier load impedance will vary with the drain voltage. A drain voltage of 8 V was
used for device design and optimisation. The performance over a
J Eng 2015
doi: 10.1049/joe.2015.0058
Conclusions
An integration of a class-E PA and an FBAR filter is shown to
provide high PAE in addition to superior out of band spectral
content suppression. The PA is designed using a broadband discrete
GaAs pHEMT device configured for class-E operation in concert
with the ACPF7041 FBAR filter (2496–2690 MHz). The PA provides up to 1 W of output power with a peak PAE of 58%.
Superior out of band spectrum filtering is shown in comparison
to discrete LC resonant filtering.
5
Acknowledgments
The authors wish to acknowledge Triquint Semiconductor for their
donation of the PA driver and transistor dies and Avago
Technologies for their donation of the FBAR Filters. The authors
also wish to thank L-3 Communications for their measurement
support.
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This is an open access article published by the IET under the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0/)
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