X-Band GaN High-Power Amplifier Using Hybrid Power Combining Technique for SAR Applications

An X-band high-power amplifier (HPA) based on gallium nitride (GaN) high electron mobility transistors (HEMTs) has been developed for synthetic aperture radar (SAR) applications. A hybrid power combining technique, including microstrip circuits and waveguides, is used to design the HPA. For reducing the size, four 50 W GaN HEMTs cascaded with one 1-to-4 power divider and one 4to-1 power combiner form a 4-way power combined PCB circuits. For combing the high power and driving an antenna, two PCB circuits are combined by magic-T waveguides. The transmission efficiency of the power combining is approximately 80%. In the 10% duty cycle (pulse width 100 us), the output power of the HPA is over 200 W across the band of 9.5–9.8 GHz. The maximum output power is 230 W at 9.5 GHz, and the power gain is 8.3 dB at 46.1°C.


Introduction
An RF power amplifier plays an important role in many radio transmitters. It is used to convert a low power signal into a high power signal to feed the antenna of transmitters. Because of the required power higher than 1 KW, traveling wave tube amplifiers (TWTAs) have been utilized for many radar systems, such as weather radar, synthetic aperture radar (SAR), and so on. However, it is pointed out that TWTAs have high maintenance costs due to their short lifetime, occupy significant area, and have large weight [1]. Hence, there is a strong desire for solid-state power amplifiers (SSPAs) that are superior in long-term reliability and serviceability to replace conventional TWTAs.
In the decade, SSPAs using gallium nitride (GaN) high electron mobility transistors (HEMTs) are reported in [2][3][4][5][6][7][8][9][10]. A GaN HEMT is attractive for high power applications due to particular capabilities such as high output power, great efficiency, and smaller parasitic capacitances. Moreover, it is capable of covering wide bandwidth because of its high input and output impedance. Nowadays, engineers pay attention to design micro satellites (10-100 Kg) for reducing the production costs and launch them into the space easily. A key enabling technique for SAR applications is using hybrid high power combining based on GaN HEMTs. This topology provides large power, compact size, wideband performance, and light weight. After the technique is developed successfully, TWTAs on the satellites can be replaced by GaN high-power amplifiers (HPAs).
In [5], a 4-way power combiner with isolators is used to combine four 80W GaN HEMTs. The low isolation between nearby ports makes the isolators needed and the size large. In [6], the performance of a coaxial waveguide spatial power combining structure is good for large-scale power combing over multi-octave bandwidths. In this paper, we propose a GaN HPA using hybrid power combining technique. It is composed of eight GaN HEMTs with PCB circuits and two magic-T waveguides for size reduction and high power handling. The GaN HPA is operated from 9.5 to 9.8 GHz for SAR applications.

Block diagram of the amplifier
The block diagram of the proposed X-band GaN high-power amplifier is shown in Fig. 1  microstrip circuits. To combine high output power and connect to an antenna, low-loss waveguides are utilized. Fig. 2 shows the corresponding geometry of the proposed main amplifier. In Fig. 2(a), 1-to-2/ 2-to-1 power dividers/combiners are designed by magic-T waveguides. 1to-4/4-to-1 power dividers/combiners are made by three branch-line couplers. Four 50 W GaN HEMTs cas caded with one 1-to-4 power divider and one 4-to-1 power combiner become a 4-way power combined PCB circuits. Moreover, two PCB circuits mounted on each side of heat sinks are combined by magic-T waveguides and cooled by air blasts. The geometry of hybrid 8-way high power combining is illustrated in Fig. 2(b)(c). Commercial CREE GaN HEMTs are used in the HPA design [11]. The measured reflection coefficient of the GaN HEMT is −5 dB. The measured results of the output power and the power gain are shown in Fig. 3

4-to-1 Power Combiner
A 4-to-1 power combiner composed of three branch-line couplers in two stages is shown in Fig. 4. The couplers are designed in circle shapes in order to reduce transmission losses. The 4-to-1 power combiner is printed on a single piece of Taconic RF35 substrate demonstrating a dielectric constant (εr) of 3.5 and thickness of 20 mil. Fig. 5 illustrates the simulated and measured results of the 4-to-1 power combiner. In Fig. 5(a), the reflection coefficient is below −20 dB and the insertion loss is about −7.3 dB from 9.5 to 9.8 GHz. The isolation among output ports is greater than 25 dB in Fig. 5(b). Due to some effect of fabrication, SMA connectors, and solders, the maximum variation of phase difference between nearby output ports is approximately 6° between simulation and measurement results. Moreover, we use the method of back-to-back to measure the transmission efficiency of the 4-to-1 power combiner. In Fig. 6, the measured insertion loss is about −1.7 dB. Hence, the transmission efficiency of the 4-to-1 power combiner is approximately 82%. The measured and simulated results are highly consistent. Fig. 7 illustrates a geometry of the 2-to-1 power combiner. It is made by a low-loss magic-T waveguide with a waveguide termination. Three SMA-to-waveguide adapters are designed for measurement. Fig. 8 shows the simulated results of the 2-to-1 power combiner with three SMA-towaveguide adapters. The reflection coefficients of three ports are lower than −25 dB. The insertion losses are approximately −3.02 dB and the isolation is higher than 34 dB. Moreover, the measured reflection coefficient is lower than −23 dB and the insertion loss is approximately −3.13   dB. Hence, the transmission efficiency is higher than 99%.

2.2.3.
Waveguide-to-Microstrip Transition Fig. 9 shows a structure of the waveguide-to-microstrip transition with a SMA-to-waveguide adapter. It is utilized to connect the 4-to-1 power combiner and the 2-to-1 power combiner. We design a back-to-back probe to measure the performance of the waveguide-to-microstrip transition as shown in Fig. 10. The measured reflection coefficient is lower than −15 dB and insertion loss is −0.5 dB. Moreover, two waveguide-to-microstrip transitions with the back-toback 4-to-1 power combiner is plotted in Fig. 11. The simulated reflection coefficient is lower than −25 dB and the insertion loss is −1.6 dB. Hence, the transmission efficiency of one waveguide-to-microstrip transition with a 4-to-1 power combiner is approximately 83%.

DC Module
In the space, the power source is obtained from the solar panels. In our designed satellite, the supplied voltage of solar panels is 28 V, which is different from that GaN HEMTs need. Hence, we have to design a DC module of the main amplifier, including DC-to-DC converters and regulators, to transform the voltages. A block diagram of the DC module is shown in Fig. 12. The positive voltage is obtained by using DC-to-DC converters with voltage divided circuits to convert 28 V to 40 V. The negative voltage is got by utilizing a DC-to-DC converter (TPS84250) and a CMOS voltage converter (LMC7660) to convert 28 V to approximately -2.7 V.

Experimental results
An equipment of X-band amplifier is used as the preamplifier. The accepted input power is -5 dBm and pushed upon for driving the main amplifier. Fig. 13 depicts the fabricated 2-to-1 and 4-to-1 power combiners with eight GaN HEMTs to form the X-band GaN high-power amplifier.
The RF input port is in the lower left. The overall size is approximately 19 cm × 13.7 cm × 17.7 cm. The measured small signal parameters are plotted in Fig. 14. According to the low S11 of the GaN HEMT, the measured reflection coefficient is higher than -10 dB from 9.5 to 9.8 GHz. The gain is approximately 7.6 dB at 9.65 GHz. Fig. 15 shows the measured peak output power, power added efficiency (PAE), and power gain at 9. 5, 9.65, 9.8 GHz, respectively. In the 10% duty cycle (pulse width = 100 us, period = 1 ms), the maximum output power is 53.6 dBm (230 W) when the input power is 45.3 dBm at 9.5 GHz (The drain voltage is 40 V and the total drain current is 4.3 A.).The power gain is 8.3 dB and the power added efficiency is 16.5% at 46.1°C. Although much of the overall size is used for the heat sinks,  the operating temperature is still above the room temperature, which results the low PAE. Fortunately, in the future, heat pipes with heat sinks on the satellite will replace the used heat sinks with air blasts. The PAE will be higher than the measured results nowadays.

Conclusion
In this paper, we present a GaN high power amplifier using hybrid power combining technique, which is composed of PCB circuits and magic-T waveguides. By using PCB methods, the combining structure of 4-way GaN HEMTs can be compact. Moreover, two PCB circuits mounted on each side of heat sinks are combined by magic-T waveguides and cooled by air blasts. The 3D structure make the performance of power handling increased. The proposed GaN HPA exhibits the output power over 200 W and is a successful prototype for SAR applications. On the satellite, a compact 2500 W HPA can be made by combined several sets of the proposed HPA.

Acknowledgements
This work was supported by the National Space Organization (NSPO), Taiwan, under Contract NSPO-S-104094. This work was also partially supported by the