978-1-5090-2586-2/16/$31.00 ©2016 IEEE

High efficiency 8.8-9.6 GHz Class J Power Amplifier Mehdi Forouzanfar, Rouhollah Feghhi, Javad Baseri, and Mojtaba Joodaki.

Dept. of Electrical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran. Email:Mehdi_forouzanfar@yahoo.com

Abstract—This paper presents the first implementation of hybrid 8.8-9.6 GHz X-band class J power amplifier (PA) designed with a GaN HEMT power transistor. High efficiency power amplifiers suffer from nonlinear performance and relatively low bandwidth. The proposed high efficiency class J power amplifier provides larger bandwidth and more convenient linearity performance. The class J PAs should have appropriate matching networks at second harmonic of fundamental frequency which cannot easily realize for high frequency hybrid PAs. In the proposed Class J PA, second harmonic matching network was achieved by using low parasitic GaN HEMT power transistor and precise modeling of transmission lines and passive elements up to 20 GHz. The optimum dimensions of matching networks are determined by employing optimizing algorithm. This PA achieves 14 dB power gain over the frequency range of 8.8-9.6GHz. Drain efficiency and power added efficiency are about 67% and 58% at 9.3 GHZ, respectively that is about 5% higher than class AB X-band power amplifier that is implemented in similar condition.

Keywords—Power amplifier (PA), GaN HEMT, Hybrid, Class J, matching networks, drain efficiency, power added efficiency (PAE).

I. INTRODUCTION

Power amplifier (PA) is one of the most important blocks in high frequency transceivers. Some possible applications of high frequency PAs are mobile communication, wireless systems, pulse radar and medical microwave imaging systems.

GaN HEMT transistor has some unique features such as; high power density, low thermal resistance, small size, sufficient output power, convenient reliability, and small parasitic elements [1]; Therefore, it can be appropriate candidate for realizing high power and high frequency amplifiers.

Several topologies have been used for integration of high frequency transistors [2-4]. Monolithic microwave integrated circuits (MMIC) provide reliable design, occupy small area and thus, exhibit low parasitic elements [2], but they are not suitable for high power applications. Quasi monolithic integrated circuit has been used in [3], which resulted in appropriate frequency response and thermal resistance, but it needs complicated technology for realization. Hybrid approach is another popular integration method which has appropriate thermal performance. Therefore, it can be convenient choice for implementation of high power amplifiers. Furthermore, it can be realized using inexpensive substrate while does not need

sophisticated manufacturing technology. In hybrid design approach, different elements which do not have same substrates can easily integrate, but the frequency response of hybrid circuit can be deteriorated at several tens of GHz due to the large parasitic of interconnects [4]. In the literature, several X-band hybrid PAs was reported [5-7] which provide high output power, appropriate thermal characteristic and good drain efficiency but usually they have lower bandwidth in comparison with MMIC PAs.

Power amplifier must meet several requirements such as wideband frequency response, sufficient gain, adequate drain efficiency, and small area. By using the efficiency enhancement methods, the power dissipation will be reduced and thus, reliability and median lifetime will be increased. Therefore, using of these techniques, is essential for high power applications. Several topologies have been proposed for implementation of high efficiency PAs [8-12]. Reduced conduction angle PAs, in which transistor conducts for part of cycle, provide high efficiency, but have lower output power and suffer from nonlinearity problem[8].

Over-driven PAs is analyzed precisely in [9]. They provide high efficiency but do not have convenient linearity performance. Switching power amplifiers such as class E and class D are well known for their ability to increase efficiency, but they cannot easily realize at X-band hybrid PAs [10]. In harmonic tuned PAs, the dissipated power in transistor will be minimized by shaping drain voltage and drain current. Class-F PA is obtained by using infinite number of harmonics to shape square and half-wave sinusoid waveform at drain voltage and current respectively [11]. For inverse Class-F, drain voltage and current shapes will be inversed. Class-F and inverse class F PAs provide high efficiency, but usually they do not have sufficient bandwidth. In addition, realization of matching network at third harmonic and higher harmonics cannot be practical at high frequencies.

Class J PA was recently presented in [12] , which resulted same theoretical linearity and efficiency as class AB or class B by assumption of linear output capacitor. However, class J drain efficiency will be higher than class AB PA due to the harmonic generation property of the nonlinear output capacitance [13]. In addition, it has broad bandwidth characteristic due to absence of short circuit or open circuit matching at harmonic frequencies. In class J PA, drain voltage and current were shaped as quasi half sinusoidal wave with phase shift of 45 degrees. It was achieved by complex fundamental termination and capacitive harmonic matching.

Figure 1. schematic of the proposed Class J PA

Proposed X-band hybrid class J power amplifier was implemented using bare die GaN HEMT transistor. It has appropriate gain, sufficient output power and good thermal performance. In addition, it has higher efficiency than class AB power amplifier that is implemented in the similar condition. It is the first implementation of X-band class J hybrid power amplifier. The paper is organized as follows. Section II describes the principles of class J power amplifier design. The proposed class J X-band hybrid PA are described in section III and simulation and measurement results are presented in section IV. A summary of the important results and conclusion has been derived in section V.

II. PRINCIPLES OF CLASS J PA DESIGN

There is set of fundamental and second harmonic impedances that provide same efficiency and output power as a class AB PA [14]. This design space is given by equations (1), (2) as follows [13]

Zf0=Ropt(1+αJ) (1)

Z2f0=-jRopt3π/8 (2)

which α variable varies from 1 to -1 that 1, 0 and -1 stated class J*, class B and class J, respectively. Ropt is the optimum fundamental load impedance that should be used at class AB PA to achieve maximum output power. It is given by (3).

Ropt =2(VDC-VK)/Imax (3)

The calculated impedances in (1) and (2) refer to current generator plane and should be transformed to package plane by considering extrinsic parasitic elements of transistor.

Figure 2.Drain current and drain voltage of ideal class J & class J*.

Drain current and drain voltage of ideal class J and class J* is shown in by assumption of linear output capacitor.

However, due to the harmonic generation of nonlinear Cout

especially at second harmonic, phase difference between drain current and voltage can be reduced and therefore, efficiency enhancement will be achieved [13].

In order to have an appropriate X-band class J design, accurate model of transistor's parasitic elements, transmission lines and interconnects should be extracted up to second harmonic frequency (about 20 GHz). The nonlinear characteristic of GaN HEMT output capacitor versus drain-source voltage was shown in Figure 3. Since the transistor output capacitor is sufficiently low, the strict requirement for second harmonic matching, that decreases freedom for high frequency matching, can be relaxed.

III. THE PROPOSED X BAND CLASS J PA

Schematic of the proposed class J PA is shown in Fig.1. It was realized using TGF2023-01 6 watts GaN HEMT transistor. Simulations were done based on the available nonlinear model of transistor and load pull measurement data that is provided by data sheet. Input and output matching networks were implemented on Rogers RO3010 substrate. Fundamental and second harmonic impedance of the proposed PA at current generator plane was shown in Figure 3. Bias network was designed using quarter wave length stub and bypass capacitors. Stability resistor that is placed in the gate bias network reduces low frequency gain and thus improves amplifier stability while it does not deteriorate high frequency performance.

Wire bonds structure has significant effects on amplifier performance and thus, should be modeled precisely using 3d full wave high frequency simulator as shown in . In the proposed PA, two 25 um gold wire bonds were used for

Figure 3. output capacitor of TGF2023-01 GaN HEMT transistor.

Figure 4. Fundamental and second harmonic impedance.

Figure 5 Wire bond simulations.

each pad to reduce the equivalent parasitic impedance of the interconnect structure and thus increase the bandwidth.

In the proposed PA, the numbers of independent variable are large enough which increase the circuit complexity to a level that optimum values cannot achieve manually. Therefore, an automated tool was used to optimize PA performance. Variables are width, length and radius of different Microstrip stubs. The Momentum ADS simulator was used to evaluate the characteristic of solution that were generated by optimization tool. The optimization goals are input and output reflection coefficient, power gain, bandwidth and drain efficiency.

Correct thermal management of power transistor can improve the important device's characteristic such as power gain, output power, reliability and median lifetime. Therefore, modeling of power transistor's thermal behavior is essential for high power applications. As shown in Figure , thermal model of power transistor was obtained using ANSYS software which has good agreement with thermal measurements. By using this model, the effects of different die attach on transistor thermal performance was investigated. Finally, high thermal conductivity silver paste (Sk70) was chosen for attaching transistor to the carrier.

The proposed PA is shown in Fig.7. Carrier of transistor was made by gold plated copper that has good thermal conductivity. Tuning array pads were placed along the matching networks that can use for correction of PA's frequency response if needed.

IV. SIMULATION AND MEASUREMENT RESULTS

Simulated and measured S parameters are shown in Figure 7 that has reasonable agreement with each other. A power gain S21 of 14 dB in the frequency range of 8.8-9.6

Figure 6. Thermal model of power transistor.

Figure 6. The proposed PA.

GHz is obtained. Having Mu greater than one guarantees that amplifier is unconditionally stable[15]. As shown in Figure 8, the proposed PA is unconditionally stable over the all frequency ranges. Drain voltage and current waveforms are shown in Fig.10 when pin sweeps for 20 dBm to 28 dBm.

In order to have a reliable design, sensitivity analyze should be done to check the effects of dimension variations on PA's performance. As shown in Figure 91, dimension variations cannot easily deteriorate the input and reflection coefficient of the proposed PA.

Figure 7.Simulated and measured S parameters of the proposed PA.

Figure 8. simulated Mu factor of the proposed PA.

Figure 10. Drain voltage and current waveforms.

Figure 91. Monte Carlo simulation of the proposed PA.

Large signal measurement results are shown in Figure 102. The drain bias voltage is 32 V and gate bias voltage is pulsed from -3.3 to -5 with duration of 20µs and duty cycle of 10%. It achieves 37.2 dBm output power with power added efficiency of 58% and drain efficiency of 67% at 9.3 GHz. It is about 5% higher than power added efficiency of the PA that was implemented in Class AB with similar condition.

Figure 10 Measurements results of the proposed PA@ 9.3GHz.

V. CONCLUSION

A 8.8-9.6 GHz class J hybrid PA is designed and fabricated using GaN HEMT transistor. Thermal model of power transistor is obtained using Ansys software that has

good agreement with measurement data. Proper matching at second harmonic become feasible by using low output capacitance GaN HEMT transistor and accurate modeling of matching networks up to 20 GHz. Nonlinear behavior of transistor’s output capacitor, produces second harmonic voltage which decreases the phase shift between drain voltage and drain current waveforms. As a result, the proposed PA exhibits very high efficiency, moderate bandwidth and convenient linearity performance. In order to have the best performance, the value of different components were optimized by using optimization tool.

References

[1] S. Rezaei, L. Belostotski, F. M. Ghannouchi, and P. Aflaki, "Integrated design of a class-J power amplifier," IEEE Transactions on Microwave Theory and Techniques, vol. 61, pp. 1639-1648, 2013.

[2] B. Kim, M. Greene, and M. Osmus, "Broadband high efficiency GaN discrete and MMIC power amplifiers over 30–2700 MHz range," in 2014 IEEE MTT-S International Microwave Symposium (IMS2014), 2014, pp. 1-3.

[3] M. Joodaki, G. Kompa, and H. Hillmer, "An enhanced quasi-monolithic integration technology for microwave and millimeter wave applications," IEEE transactions on advanced packaging, vol. 26, pp. 402-409, 2003.

[4] P. Saad, H. M. Nemati, K. Andersson, and C. Fager, "Highly efficient GaN-HEMT power amplifiers at 3.5 GHz and 5.5 GHz," in Wireless and Microwave Technology Conference (WAMICON), 2011 IEEE 12th Annual, 2011, pp. 1-4.

[5] G. C. Barisich, S. Pavlidis, C. A. D. Morcillo, O. L. Chlieh, J. Papapolymerou, and E. Gebara, "An X-band GaN HEMT hybrid power amplifier with low-loss Wilkinson division on AlN substrate," in Microwaves, Communications, Antennas and Electronics Systems (COMCAS), 2013 IEEE International Conference on, 2013, pp. 1-4.

[6] C. F. Campbell and M. Poulton, "Compact highly integrated X-band power amplifier using commercially available discrete GaN FETs," in Asia-Pacific Microwave Conference 2011, 2011, pp. 243-246.

[7] H.-C. Jeong and Y. Kyung-Whan, "A Design of X-Band 40 W Pulse-Driven GaN HEMT Power Amplifier," IEICE transactions on electronics, vol. 96, pp. 923-934, 2013.

[8] G. Gonzalez, Microwave transistor amplifiers: analysis and design vol. 2: Prentice hall New Jersey, 1997.

[9] D. M. Snider, "A theoretical analysis and experimental confirmation of the optimally loaded and overdriven RF power amplifier," IEEE Transactions on Electron Devices, vol. 14, pp. 851-857, 1967.

[10] H. L. Krauss, C. W. Bostian, and F. H. Raab, Solid state radio engineering: John Wiley & Sons Inc, 1980.

[11] F. Khan, F. Mohammadi, and M. Yagoub, "A GaN HEMT Class-F amplifier for UMTS/WCDMA applications," in 2008 IEEE International RF and Microwave Conference, 2008.

[12] S. C. Cripps, P. J. Tasker, A. L. Clarke, J. Lees, and J. Benedikt, "On the continuity of high efficiency modes in linear RF power amplifiers," IEEE microwave and wireless components letters, vol. 19, pp. 665-667, 2009.

[13] J. Moon, J. Kim, and B. Kim, "Investigation of a class-J power amplifier with a nonlinear for optimized operation," IEEE Transactions on Microwave Theory and Techniques, vol. 58, pp. 2800-2811, 2010.

[14] S. Preis, D. Gruner, and G. Boeck, "Investigation of class-B/J continuous modes in broadband GaN power amplifiers," in Microwave Symposium Digest (MTT), 2012 IEEE MTT-S International, 2012, pp. 1-3.

[15] M. L. Edwards and J. H. Sinsky, "A new criterion for linear 2-port stability using a single geometrically derived parameter," IEEE Transactions on Microwave Theory and Techniques, vol. 40, pp. 2303-2311, 1992.