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Large Signal Characterization of GaN HEMT Transistor by
Multi-Harmonic Source & Load Pull Tuner System
Shengjie Gao and Chan-Wang Park
University of Quebec in Rimouski, Rimouski, QC, G5L 3A1,
Canada
Abstract In this paper, a GaN HEMT transistor is
characterized at 3.5 GHz by passive multi-harmonic source &
load pull tuner system in large signal. In order to analyze the
effect on PAE and output power from the source and load impedance
at fundamental, 2nd and 3rd harmonic frequencies, multi-harmonic
source and load pull tuner are used. By using this source &
load pull tuner system, Crees GaN HEMT CGH40010 transistor is
characterized at 3.5 GHz with considering the source and load
impedances at 2nd and 3rd harmonic frequencies. The
characterization result shows that maximum PAE could reach 74.21%
with 39.92 dBm output power.
Index Terms Characterization, harmonic frequencies, power
amplifier, source & load pull tuner.
I. INTRODUCTION
With the development of the modern telecommunication technology,
the requirement for RF (Radio Frequency) PA (Power Amplifier) with
respect to PAE (Power Added Efficiency) and output power is more
stringent. In order to satisfy the increasing demand of the
wireless communication technology, microwave transistor, as a key
active device in PA, should be well characterized. Large signal
characterization for the transistor is essential in order to
estimate the output power and PAE in the non-linear domain. We can
characterize the transistor under large signal excitation to have
desired output power and PAE by using source and load pull tuner
[1]. With this source & load pull techniques, the impedances
both on the input and output of the transistor could be optimized
to have target PAE and output power efficiently. With these optimum
impedances, we can design PA.
For enhancing the efficiency of class F/inverse class F PA, 2nd
and 3rd harmonic frequencies are usually to be considered [2].
Based on this need, the tuner manufacturing companies developed
source/load pull tuner which could control multi harmonic
frequencies [3] [4]. With these tuners, the source and load
impedance at fundamental, 2nd and 3rd harmonic frequencies could be
controlled independently [5].
In order to design a class F/inverse class F PA, the transistor
should be characterized at fundamental frequency, 2nd and 3rd
harmonic frequencies. In this work, a GaN HEMT transistor from Cree
Inc. is characterized by multi-harmonic source & load pull
tuner. In the next section, the setup of source & load pull
tuner system is shown. In section III, Crees GaN HEMT CGH40010
transistor is characterized of at 3.5 GHz with
considering 2nd and 3rd harmonic frequencies by source &
load pull tuner system. The characterization result is
presented.
II. SETUP OF SOURCE & LOAD PULL TUNER SYSTEM
Figure 1 shows the setup for source & load pull tuner
system. We use Rohde & Schwarz SMBV100A vector signal
generator, Agilent E3633A DC power supply, N6705B DC power
analyzer, Agilent MXA signal analyzer and Agilent N1912A P-series
dual channel power meter in this system. These instruments are
connected to a computer by GPIB cables. In order to control the
source & load pull tuner and to obtain the measurement result
from these instruments, corresponding software provided by the
tuner manufacturing company is installed in the computer. The input
block in the source & load tuner system consists of directional
coupler, isolator and bias tee. The output block consists of bias
tee, directional coupler and attenuator. [6] has analyzed the
effect on the tunable region of the tuner at the DUT reference
plane from these accessories in input and output block. Based on
[6], the S11/S22 of the accessories such as directional coupler,
isolator, bias tee and attenuator are chosen for output/input block
at least less than -14 dB.
Fig. 1. Setup of source & load pull tuner system.
With this setup, the maximum tunable at the output of the
transistor is 0.956 at 3.5 GHz, 0.946 at 7 GHz and 0.923 at 10.5
GHz. The maximum tunable at the input of the transistor is 0.984 at
3.5 GHz, 0.926 at 7 GHz and 0.936 at 10.5 GHz.
978-1-4673-4818-8/12/$31.00 2012 IEEE
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III. PROCEDURE OF CHARACTERIZATION
In order to design a class F/inverse class F power amplifier
with Crees GaN HEMT CGH40010 transistor, source & load pull
tuner system is used to characterize this transistor at fundamental
frequency 3.5 GHz and its 2nd and 3rd harmonic frequencies. During
the procedure of CGH40010 transistor characterization, we choose
the impedance which offers maximum PAE while output power is 40 dBm
at each step.
A. IV curves
For class F/inverse class F operation, the transistor could be
biased in class AB mode [2]. The IV curve measured by the source
& load pull tuner system is shown in Figure 2. We choose -2.58V
for VGS and 28V for VDS with 200mA drain quiescent current so that
the transistor is biased in class AB mode. Thus, when the
transistor is characterized by source & load pull tuner system,
VDS and VGS are fixed to 28V and -2.58V, respectively.
Fig. 2. Measured IV curve for Crees GaN HEMT CGH40010 transistor
by source/load pull tuner system.
B. Stability circle
For obtaining the stability circle of CGH40010 transistor at 3.5
GHz when the transistor is biased at VDS= 28V and IDS=200mA, the
S-parameter of this transistor is measured by Agilent 8720ES VNA
(Vector Network Analyzer). The setup for measuring the transistors
S-parameter is shown in Figure 3. Bias tees are added to connect to
the DC power supplies.
The S-parameter is measured at the measurement reference plane
of VNA. In order to obtain the S-parameter at DUT reference plane,
the bias tees should be de-embedded. With the de-embedded
S-parameter of transistor, the stability circle at 3.5 GHz on both
input and output of the transistor are outside of the Smith chart
as shown in Figure 3, so the transistor is unconditionally
stable.
Fig. 3. Source and load stability circle for CGH40010 transistor
at 3.5 GHz.
C. Procedure of characterization of Crees GaN HEMT CGH40010
transistor
As a first step of characterization by source & load pull
tuner system, load pull characterization at 3.5 GHz is performed
when source impedance at fundamental frequency is fixed to
3.18-j13.30 Ohm (the source & load impedances at the 2nd and
3rd harmonic frequencies are fixed to 50 Ohm). The maximum PAE
obtained in the load pull characterization is 62.41% with 40.47 dBm
output power when the load impedance at 3.5 GHz is 0.603165.50 as
shown in Figure 4 (a). The maximum output power is 40.73 dBm with
59.14% PAE when the impedance is 0.559173.30 as shown in Figure 4
(b). In order to obtain 40 dBm output and to have higher PAE,
maximum PAE impedance point is chosen for the next step.
Then, as a second step, source pull characterization is done to
find source impedance at fundamental frequency when load impedance
at fundamental frequency is fixed to 0.603165.50. The source pull
characterization result in Figure 5 shows that maximum PAE is
64.33% with 40.07 dBm output power when the source impedance at 3.5
GHz is 0.892-151.70. The maximum output power is 40.32 dBm with
64.09% PAE when the source impedance is 0.861-152.50. In order to
have 40 dBm output power and higher
VDS= 28V
VGS= -2.58V
Drain quiescent current =200 mA
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PAE, maximum PAE impedance point is chosen for further
characterization. From the contour in 3D in Figure 4 and 5, we can
also see that PAE is more sensitive to the impedance variation at
3.5 GHz than output power.
(a)
(b)
Fig. 4. Measured PAE (a) and output power contour (b) in 3D by
load pull tuner at 3.5 GHz.
The next step is sweeping the load impedance at the 2nd and 3rd
harmonic frequencies by source & load pull tuner system.
First, the load impedance at 2nd harmonic frequency is tuned
when the source and load impedances at fundamental frequency are
fixed to the values which are found in the previous steps (Source
impedance at fundamental frequency: 0.892-151.70 and load impedance
at fundamental frequency: 0.603165.50). The load impedance at 3rd
harmonic frequency and source impedances at 2nd and 3rd harmonic
frequencies are fixed to 50 Ohm. Figure 6 shows the measured PAE by
load pull tuner at 7 GHz. PAE is increased to 68.48% with 40.06 dBm
output power by tuning the load impedance at 2nd harmonic frequency
to 0.938146.10. PAE is increased by 4.15% by tuning the load
impedance at 2nd harmonic frequency.
(a)
(b)
Fig. 5. Measured PAE (a) and output power contour (b) in 3D by
source pull tuner at 3.5 GHz.
Fig. 6. Measured PAE contour in 3D by load pull tuner at 7
GHz.
As a fourth step, load impedance at the 3rd harmonic frequency
is tuned by load pull tuner. Source impedance at fundamental
frequency is 0.892-151.70 and load impedance at fundamental
frequency is 0.603165.50. Load
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impedance at 2nd harmonic frequency is fixed to 0.938146.10 and
source impedances at 2nd and 3rd harmonic frequencies are fixed to
50 Ohm. The measured PAE in Figure 7 shows that when the impedance
at 3rd harmonic frequency is 0.91842.60, PAE reaches the maximum as
a 69.42% with 40.01 dBm output power. By considering 3rd harmonic
frequency at load, PAE is increased by 0.94%.
Fig. 7. Measured PAE contour in 3D by load pull tuner at 10.5
GHz.
After tuning the impedance at 3rd harmonic frequency, source
impedance at 2nd harmonic frequency is tuned. Source and load
impedance at fundamental frequency are fixed to 0.892-151.70 and
0.603165.50, respectively. Load impedances at 2nd and 3rd harmonic
frequencies are fixed to 0.938146.10 and 0.91842.60, respectively.
Source impedance at 3rd harmonic frequency is fixed to 50 Ohm. The
measured PAE contour in Figure 8 shows that when source impedance
at 2nd harmonic frequency is 0.900-38.50, the maximum PAE 73.08%
can be obtained with 39.83 dBm output power. In this step, PAE is
increased by 3.66%.
Fig. 8. Measured PAE contour in 3D by source pull tuner at 7
GHz.
At last, source impedance at 3rd harmonic frequency is tuned. By
tuning the source impedance at 3rd harmonic frequency, the
characterization result in Figure 9 shows that when source
impedance at the 3rd harmonic frequency is 0.880-68.6, the maximum
PAE 74.21% can be obtained with 39.92 dBm output power. In the last
step, PAE is increased the by 1.13% by tuning the source impedance
at 3rd harmonic frequency.
Fig. 9. Measured PAE contour in 3D by source pull tuner at 10.5
GHz.
IV. CONCLUSION
In this paper, a characterization procedure for GaN HEMT
CGH40010 transistor at fundamental frequency 3.5 GHz, 2nd and 3rd
harmonic frequencies by source & load pull tuner system is
shown. The final result shows that maximum PAE 74.21% can be
obtained when the output power is 39.92 dBm.
REFERENCES
[1] W. Liu, and C. Tsironis, Load pull characterization system
for differential devices, 2003 ARFTG Microwave Measurement
Symposium, pp. 201-204, 4-5 December 2003.
[2] D. Y.-T. Wu, and S. Boumaiza, 10W GaN inverse class F PA
with input/output harmonic termination for high efficiency WiMAX
transmitter, 2009 IEEE Wireless and Microwave Technology
Conference, pp. 1-4, 20-21 April 2009.
[3] F. De Groote, O. Jardel, J.-P. Teyssier, T. Gasseling, J.
Verspecht, V. Mallette, and C. Tsironis, On-wafer time domain
load-pull optimization of transistor load cycle with the new
multi-harmonic MPT tuner, 2007 ARFTG Microwave Measurement
Conference, pp. 1-6, 8 June 2007.
[4] Maury Microwave, Multi-harmonic automated tuners, Maury
Microwave Inc., Ontario, CA, 2012.
[5] Focus Microwave, iMPT-1818-TC, Focus Microwave Inc.,
Montreal, QC, Canada, 2007.
[6] S. Gao, Z. Wang, and C.-W. Park, Contour method to shift the
tunable region of source/load pull tuners in power amplifier
characterization, 2012 Asia-Pacific Microwave Conference, 4-7
December 2012.
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