Kuhn, WilliamB., "A 400 to 500-MHz CMOS Power Amplifier with

Kuhn, WilliamB., "A 400 to 500-MHz CMOS Power Amplifier with
TU2P-10
A 400 to 500-MHz CMOS Power Amplifier
with Multi-Watt Output
Jeongmin Jeon, Student Member, IEEE and William B. Kuhn, Senior Member, IEEE
Kansas State University, Manhattan, KS, 66502, U.S.A.
Abstract –- A two-stage P-band (400 MHz) CMOS multi-Watt
Power Amplifier (PA) is reported. Four identical 1.25-Watt PA
cells are power-combined to generate approximately 5-Watt
output. The same PA chip is used for a drive-amplifier,
providing a total of 17 dB power gain.
In the measurements, the two-stage five-chip PA produces
5.1-Watt with 17 % PAE. The five-chip PA assembly will be
2
merged into a 7×10 mm single die amplifier in a future
project.
Index terms –- CMOS, UHF, PA, power amplifier
I. INTRODUCTION
Although compound materials, such as GaAs and GaN,
and specially processed LDMOS are preferred over CMOS
in commercial RF power amplifiers (PA) [1], Watt-level
CMOS PA has been intensively researched for decades. As
many components of radio are realized by CMOS
analog/digital circuits, the CMOS PA is needed to be
integrated to achieve a single chip radio.
Watt-level PAs in cellular bands utilize off-chip
components or bondwires in the absence of high Q
inductor/balun [2] [3] [4] [5]. Utilizing a DAT (distributed
active transformer) technique, fully integrated PAs have
been also reported in 900 MHz and 2.4 GHz by Han and
Aoki, respectively [6] [7]. Notably, the Aoki’s PA reported
the highest output power, 2-Watt, by a CMOS PA. Since the
DAT technique is not scalable due to limited 3×3 mm2 die
size at P-band (400 MHz), instead, the authors demonstrated
a fully integrated CMOS 1-Watt PA with high-Q 1:3 turnsratio output balun [8]. However, if the die size is large
enough, the DAT technique is still viable for higher output
power at P-band.
In this paper, we present a 3 to 5-Watt CMOS power
amplifier implemented in Silicon On Sapphire (SOS) and
operating at P-band (400 MHz) by DAT power-combining.
The design is intended for use in a phased array radar where
high levels of integration are needed to decrease mass, and
where the cost is justified by the integration goals. Although
currently implemented by wirebonding multiple dies on a
PCB level, the layout allows for full integration in future
projects.
978-1-4244-2699-7/09/$25.00
©2009 IEEE
978-1-4244-2699-7/09/$25.00
©2009 IEEE
Figure 1. A two-stage five-chip configuration employing powercombining
Figure 2. Schematics of a single PA chip
II. BACKGROUND AND DESIGM METHODS
In order to produce 5-Watt output power, the PA design
leverages an existing 1-Watt PA circuit developed
previously by the authors [8]. The 1-Watt PA was capable
of 30 dBm output with 27 % PAE when operating with 50
Ohm single-ended inputs and outputs. The 5-Watt
extrapolation was created by reworking the I/O circuits
(transformers) to provide 12.5 Ohm secondaries, each with
1.25-W output, that are power combined as shown in the
schematics of Figure 1. A parallel capacitor of 25pF is used
to resonate out leakage inductance in the inter-stage
matching.
Hence, only a single 1.25-Watt design is required for the
prototype – limiting fabrication cost. Four copies of this die
are connected with bondwires to form the 5-Watt output-
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Figure 3. Pout and Pin relation in the five-chip PA simulation
(a)
Figure 4. Pout and frequency relation in five-chip PA simulation
(b)
Figure 5: Photo of (a) the PA unit (b) the five-chip PA assembly
stage design in the current implementation. A fifth copy is
used as a drive-amplifier.
As shown in Figure 2, the PA unit utilizes stacked PMOS
transistors for high breakdown voltage and high-Q on-chip
baluns/transformers at input and output, following the
design methodology in the 1-Watt PA [8]. In conjunction
with the 1:1 balun, output network is tuned for a class-E/Fodd
[9]. Utilizing insulating substrate, Qs of the output balun are
16 and 15 at the primary and the secondary, respectively. C1
and C2 are added for the optimum efficiency. C3 and
bondwires are used at the gates of M3 and M4 for ac and
DC ground. The layout of the core tile is shown in Figure 5a,
and the full five-tile set is shown in Figure 5b. The size of
the core tile is 3.3 × 3.3 mm2.
It is important to note that the layout was done to allow a
fully-integrated 5-Watt PA to be developed from the same
design in later fabrications. The design employs five copies
of a core 1.25 Watt tiles with bondwire interconnects only to
save prototyping cost.
III. SIMULATED AND MEASURED RESULTS
The circuit in Figure 1 was simulated in Agilent ADS and
the results are shown in Figure 3 and 4. The PA generates
36.6 dBm (4.6-Watt) output power with PAE of 37 % at 400
MHz, using 20 dBm input power. 80 MHz 1-dB bandwidth
and 180 MHz 3-dB bandwidths are obtained.
Figure 6: Load-pull of the drive-amplifier and compensation for
connectors and transmission-line
Figure 5 shows pictures of the fabricated die bondwireconnected on a PCB. The chip on the left hand side in
Figure 5b drives the four-chip output-stage on the right hand
side in the same picture. Capacitors between the stages are
small-valued and can be integrated with ease when a 7×10
mm2 die is granted for the whole PA integration. Figure 5a
shows a closeup of the single tile.
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(a)
(a)
(b)
Figure 8: Measured performance of the two-stage five-chip PA
including the drive-amp. (a) Output power and PAE. (b)
Frequency sweep.
(b)
(c)
Figure 7: Measured performance of the output stage when it is
driven by the external PA. (a) Output power and PAE. (b)
Frequency sweep. (c) Power supply sweep.
Prior to building up the full five-chip PA, the drive-stage
and the output-stage were tested separately. In load-pull test
of the drive-amplifier, the PA generated 28.8 dBm output
power at Z1 = 269 + j116 Ohms as shown in Figure 6. The
plot can be misleading since the output power increases as
magnitude of the load increases. However, when an electric
length of connectors and transmission-line between a tuner
and the PA, equivalent to 153º phase rotation, is
compensated, the load impedance at the edge of the drivePA chip Z2 is 8.5 − j15.4 for the maximum power, which is
close to the initial design assumption, 12.5-Ohm driving.
Secondly, the four-chip output-stage was tested with an
external power amplifier and 4.5 V power supply. The
external input is set at 28 dBm and the PA generates 4.3Watt at 520 MHz with 19 % drain efficiency and 17 % PAE.
Gain of the PA is 8.4 dB. When Pin is 30 dBm, Pout is 5.1Watt as shown in Figure 7a with 21 % drain efficiency and
17 % PAE. Figure 7b shows that its 1dB BW is greater than
20 %. The 120 MHz frequency offset from the intended
400MHz center frequency is caused by a layout mistake, but
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the reason of two dips at 450 MHz and 550 MHz are not
known. It is possible that they will disappear once the
frequency offset is corrected. However, more investigation
is required. Figure 7c depicts linearly increasing output
power in the power supply sweep, suggesting feasibility of
power control by supply modulation.
Finally, preliminary measurement results from the whole
five-chip PA in Figure 5 were obtained. As shown in Figure
8a, the output power of this prototype was 5.1-Watt (37.1
dBm) at 480 MHz with 17 % PAE when 20.6 dBm was
provided to the drive-amplifier. Figure 8b shows that the PA
has approximately 75 MHz (16 %) 1-dB bandwidth.
However, the PAE was inferior to the simulated data. We
found that the impairment resulted not because of unsound
design but because of damaged circuits by bonding in the
output-stage. The dense bonding penetrated soft passivation
layer in SOS and caused shorted circuit. Although we could
not obtain the same level of the efficiency, output power of
5.1-Watt with 16.5 dB gain and the identification of the
problem, implies that the design is sound and should be
capable of meeting design goals in a future full-integration
effort.
IV. CONCLUSIONS AND FUTURE WORK
A two-stage five-chip PA was fabricated in SOS CMOS
and assembled to demonstrate 5-Watt output capability by
power combining.
To the author’s knowledge, the 5.1-Watt output power is
the highest ever reported by an integrated CMOS PA.
However, the center frequency is off-tuned and PAE is low.
The frequency offset is an easy fix in layout but the low
efficiency problem needs further investigation. It is
suspected that part of bias circuitry is damaged by
inaccurate wire-bonding. We believe that bonding with
smaller contact area will solve the problem. Finally, the
bonding is not an issue after all if 7×10 mm2 die is given for
full integration in the future project.
ACKNOWLEDGMENT
This work was supported by a contract with California
Institute of Technology and Jet Propulsion Laboratory. The
authors would like to thank Dan Nobbe at Peregrine
Semiconductor for providing loadpull measurements.
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