Design of a Microstrip Broadband LDMOS Class-E Power Amplifier

By Yingjie Xu, Jingqui Wang, and Xiaowei Zhu Southeast University, Nanjing, China

he power amplifier base stations due to their mature technology This paper describes a 10 (PA) consumes a and relatively low cost. However, the large watt power amplifier for Tmajority of electri- output capacitance of an LDMOS transistor 1400-1600 MHz range, using cal power in a base sta- limits its use for higher switching frequencies, an LDMOS FET device tion, thus high-efficiency especially in broadband designs. operating in Class-E, with amplification is neces- Another technique to design a broadband broadband matching sary to reduce power con- class-E PA using LDMOS transistor is based sumption. High-efficiency on a modified load network [9]. It also works switch mode PA design (class-E, class-F, etc.) at a low frequency band of VHF, like the has attracted a heated discussion. amplifier mentioned in [8]. The topology of class-E high-efficiency PA In this article, the potential of using an was first introduced by N.O. Sokal and A.D. LDMOS transistor for broadband class-E PA Sokal in 1975 [1], the simple structure of has been exploited. A class-E PA working at which makes itself well accepted in high-effi- the frequency band from 1400 MHz to 1600 ciency amplification design. Many papers MHz was designed and implemented through have been reported to analyze the operating an approach based on microstrip lines and principles of class-E PA [2-5]. Class-E working broadband matching techniques. Test results condition requires high load impedance at all show that the peak in-band PAE is 64.3% and the harmonics, which makes a wide-band the maximum output power reaches 40.3 dBm class-E amplifier easier to design than a wide- (10.7 watts). band class-F amplifier [6]. Therefore, class-E is a good candidate to design a high-efficiency Design Considerations broadband PA. Figure 1 shows the circuit schematic of the Some researchers have made contributions proposed broadband class-E PA. The large sig- that improve the bandwidth characteristic of a nal model of the LDMOS transistor normal class-E PA. In [7], A. Grebennikov pro- MRF21010 was obtained from the manufac- posed a method called “reactance compensa- turer (Freescale). Two kinds of broadband tion technique” to broaden the bandwidth of matching networks are used, a two-section class-E switch mode amplifier. This technique Chebyshev for the input, and multi-section provides a constant admittance angle over a quarter wave networks for the output match- wide bandwidth at the fundamental frequency ing circuit. Both of the networks are based on where the transistor is operating as a switch. microstrip lines. Further research implemented this kind of The input impedance of LDMOS transistor technique. In [8], an LDMOS-based broadband MRF21010 is obtained through S-parameter class-E PA was designed, which indicated the simulation using Agilent’s Advanced Design feasibility of the “reactance compensation System (ADS). In this approach, a two-section technique.” The shortcoming of this design is Chebyshev low pass character broadband its low frequency range (VHF band). LDMOS matching network (MN) is used to transform devices have been widely used in PA design for the transistor’s conjugate input impedance

44 High Frequency Electronics High Frequency Design CLASS-E AMPLIFIER

The high Q series LC resonance circuit in the classic class-E topology is the main reason for the con- strained bandwidth of class-E PA [1]. In this proposed approach, instead of the LC resonance circuit, a two-section quarter wave broadband MN is used to transfer the conjugate load impedance 1.292+j*0.807 ohms to 50 ohms. This MN presents a band-pass character, the return loss of which is always better than 15 dB (VSWR <1.45). Figure 1 · Circuit schematic of the broadband class-E PA. ADS’s Harmonic Balance simulation is carried out after the whole topology determination of this broadband class-E PA. Figure 2 shows the simulation frequency response of the class-E PA. Simulation results show PAE greater than 56% and output power is greater than 39.6 dBm through the frequency band of 1400- 1600 MHz. The peak PAE is greater than 70%, and the output power vari- ation is less than 0.8 dB. Figure 3 · Photo of broadband Figure 2 · Simulated PAE and out- LDMOS class-E PA. Measurement Results put power versus frequency for this In order to validate the design broadband LDMOS class-E PA with approach, a testing board of broad-

Vds = 22 V and Pin = 26 dBm. and fourth harmonic signal and band LDMOS class-E PA was fabri- therefore realize this requirement. cated and measured. The substrate is The λ/4 feed line at the gate further selected as RF-35 from Taconic Co., ε towards 50 ohms. By referring to the assists to short-circuit the even har- which has a dielectric constant r of tables of Chebyshev impedance monic voltage components. 3.5 and a thickness of 30 mil. A photo transforming networks [10], a refined The output capacitance of of the constructes class-E PA is shown broadband MN is designed. The MRF21010 works as the shunt capac- in Figure 3. return loss of this MN through the itor of class-E PA, so there is no need Figure 4 presents the measured designed frequency band is better of any external shunt capacitor. PAE and output power characteristics than 15 dB (VSWR <1.45). The capac- According to Raab’s design equations according to frequency with an input itor C1 in the MN compensates the [2], different frequencies require dif- power of 26 dBm, gate-source voltage inductive part of the input impedance ferent values of the shunt capacitor of 3.4 V and drain-source voltage of 22 which is partially formed by the and it brings difficulty to design a V. As shown in the figure, PAE is packaging effects of the transistor. broadband switch mode class-E PA. greater than 56%, output power is Also, this capacitor works as a DC Load-pull simulation is adopted in greater than 38.9 dBm and transduc- block component. this paper to help overcoming this er power gain is greater than 12.9 dB Ideally, a class-E PA should be shortcoming. A load impedance of for frequencies at the interested band driven by a square wave signal at the 1.292 –j0.807 ohms is obtained by the from 1400 MHz to 1600 MHz (relative gate of the transistor [2]. In the input load-pull method, regardless of the bandwidth over 13.3%). The peak in- network, we take steps to shape the value of the shunt capacitor. The band PAE is 64.3% and the maximum gate voltage drive waveform. Two determined load impedance ensures in-band output power is 40.3 dBm open stubs of two-section Chebyshev the class-E PA achieves high PAE (10.7 watts). Over the entire 200 MHz network, which are designed as elec- and high output power simultaneous- bandwidth, the output power varia- trical length of λ/8 and λ/16 respec- ly through the whole designed fre- tion is 1.37 dB. tively, help to short-circuit the second quency band. The simulated and measured

46 High Frequency Electronics High Frequency Design CLASS-E AMPLIFIER

Table 1 · Comparison of simulated and measured results.

Figure 4 · Measured PAE and out- results are compared in Table 1. put power versus frequency for this According to this table, the results of broadband LDMOS class-E PA with experiment show good agreement Vds = 22 V and Pin = 26 dBm. with the simulation. More measurement results of this broadband LDMOS class-E PA are MHz with PAE above 56% and output shown in Figure 5. In Figure 5(a), the power greater than 38.9 dBm. The input power is swept with Vds = 22 V peak in-band PAE is 64.3% and the and Vgs = 3.4 V. PAE and output maximum output power is 40.3 dBm power are strictly monotonic, increas- (10.7 watts). The output power varia- ing with input power. When the input tion is 1.37 dB. power is driven at 23 dBm, the output power can go up to 37.6 dBm with Acknowledgment a maximum transducer power gain of The authors would like to 14.6 dB. acknowledge Scott Li, Senior In Figure 5(b), the drain-to-source Engineer in the Freescale Co., for voltage is swept with input power valuable discussions and opinions equal to 26 dBm and Vgs = 3.4 V. The about LDMOS nonlinear model. output power monotonically and lin- This work was supported in part early increases when Vds increases. by NSFC under Grant 60621002 and This character can be utilized in sup- in part by the National High-Tech ply tracking applications [11]. Project under Grant 2009AA011801. Figure 5 · Measured PAE and output In Figure 5 (c), the gate-to-source power versus (a) input power, (b) voltage is swept with the input power References drain-to-source voltage, (c) gate- equal to 26 dBm and Vds = 22 V. 1. N. O. Sokal and A. D. Sokal, to-source voltage for this broad- When the gate of the transistor is “Class E-A new class of high-efficien- band class-E LDMOS PA with f = biased at approximately 3.4 V, the cy tuned single ended switching 1510 MHz. amplifier achieves high PAE and power amplifiers,” IEEE J. Solid- high output power simultaneously. State Circuits, vol. SC-10, no. 3, pp.

However, when Vgs increases above 168–176, Jun. 1975. tions on Circuits and Systems, vol. 34, 3.6 V, PAE drops, although output 2. F. H. Raab, “Idealized operation pp. 941-944, Dec. 1994. power still increases. of the Class E tuned power amplifier,” 5. N. O. Sokal, “Class-E switching- IEEE Trans Circuits Syst., vol. 24, pp. mode high-efficiency tuned RF Conclusion 725–735, Dec. 1977. microwave power amplifier: In this paper, a broadband 3. C.-H. Li and Y.-O. Yam, improved design equations,” IEEE LDMOS class-E PA was presented “Maximum frequency and optimum MTT-S, pp. 779-782, 2000. based on two microstrip broadband performance of Class E power ampli- 6. Saad Shahrani, Design of Class- MNs. The load-pull technique was fiers,” IEEE Proceedings-Circuits, E Radio Frequency Power Amplifier, used to determine the load Devices and Systems, pp. 174-84, Jun. Blacksburg, Virginia, July, 2001. impedance of this PA. The measured 1994. 7. A. Grebennikov, RF and results showed the design approach 4. M. J. Chudobiak, “The use of Microwave Power Amplifier Design. successfully. This amplifier works at parasitic nonlinear capacitors in New York: McGraw-Hill, 2004. the frequency band of 1400-1600 Class E amplifier,” IEEE Transac- 8. N. Kumar, C. Prakash, A.

48 High Frequency Electronics Grebennikov, and A. Mediano, “High- Author Information Research Group, University College efficiency broadband parallel-circuit Yingjie Xu obtained a Bachelor Dublin, Dublin, Ireland. Her research class E RF power amplifier with reac- Degree of Information Engineering interests include linearization and tance-compensation technique,” from Southeast University in 2008. system-level modeling of RF/micro- IEEE Transactions on Microwave He is currently undergoing a mas- wave PAs with emphasis on digital Theory and Techniques, vol. 56, pp. ter’s program in Electrical Magnetic predistortion and field-programmable 604-612, Mar. 2008. and Microwave technology at State gate-array (FPGA) hardware imple- 9. F.You, S. B. He, T. Cao and X. H. Key Laboratory of Millimeter Waves, mentation. Tang, “Class E power amplifier Southeast University, Nanjing, Xiaowei Zhu received the B.E., design with a modified load network,” Jiangsu, China. His research interest M.E., and Ph.D. degrees in radio International Conference on MTT is on high efficiency power amplifier engineering from Southeast Univer- Proceedings, vol. 1, pp. 120-123, Apr. for communication systems. HE can sity, Nanjing, China, in 1984, 1996, 2008. be contacted by e-mail at: and 2000, respectively. Since 1984, he 10. G. L. Matthaei, “Tables of [email protected] has been with Southeast University, Chebyshev impedance–transforming Jingqi Wang received the B.E. Nanjing, China, where he is current- networks of low-pass filter form,” degree in communication engineering ly a Professor with the School of Proceedings of the IEEE, vol. 52, pp. from the Nanjing University of Information Science and Engineer- 939-963, Aug. 1964. Aeronautics and Astronautics, ing. His research interests include 11. C.-C. Chen, H.-Y. Ko, Y.-C. Nanjing, China, in 2003, the M.E. RF and antenna technologies for Wang, H.-W.Tsao, K.-Y. Jheng and A.- degree in electronic engineering from mobile communications and Y. Wu, “Polar transmitter for wireless Southeast University, Nanjing, China, microwave and millimeter-wave theo- communication system,” Proceedings in 2006, and is currently working ry and technology. Dr. Zhu was the of International Symposium on toward the Ph.D. degree at Southeast recipient of the 1994 First-Class Intelligent Signal Processing and University. From September 2007 to Science and Technology Progress Communication Systems, pp. 613- March 2009, she was a visiting Ph.D. Prize presented by the Ministry of 616, Dec. 2005. student with the RF and Microwave Education of China.