RF & Wireless Figure 14: Terminations based on load-pull analysis. The λ/4 shunt transmission line, also used for the drain bias, is high Z for odd harmonics, low Z for even harmonics. The ability to perform source pull and load pull for the device at the second and third harmonics can be used to meet other performance goals. For example, the same kind of optimization can be done to maximize power gain and output power for a particular PA design. The ideal input network for a Class F PA is shown in Figure 11, which in this case consists of transmission lines, plus a shortcircuited stub on one side and an open-circuited stub on the other. The impedance transformations and harmonic terminations of this network closely approximate the values determined by the previous series of sourcepull optimizations. The S11 of that input network is plotted in Figure 12, showing the result at 2, 4, and 6 GHz. If those impedance points are compared with the previous ones that the wizard was predicting, it would be found that they are not exactly the same. This is because physical implementation introduces some differences between the practical network and the ideal impedance points. Figure 13 is the ideal Class F output. The quarter-wave line is also used to provide drain bias for the transistor. In addition, there is an open circuit stub with some transmission line transformation, so that it is an open at the second harmonic and a short at the third harmonic. Looking into the input of the network from the location of the device drain, Figure 14 shows the fundamental, second, and third harmonic impedances presented by the network. Again, going back to the wizard, it can be seen that there are some differences between the practical transmission line-based network and the ideal impedances. In Figure 15, the networks are placed at the input and output of the GaN HEMT device and a simulation of the complete amplifier is run. Figure 16 is the simulated prediction for power gain, output power, and PAE. The results show that PAE reaches a maximum of 84 percent, which is slightly better than the result predicted by the load-pull wizard. Figure 17 shows the drain voltage and current waveforms. Recall what was noted earlier: when source and load pull are done with the Microwave Office Load-Pull Wizard, users will get some degree of waveform engineering. If they tell the wizard that they want to have maximum PAE, the wizard’s optimization algorithms will try to produce the voltage and current waveforms at the transistor that are not only the right shape, but also ideal and anti-phase. The waveform plot shows an approximate square voltage waveform, or as close as we can get by engineering only a few harmonics. The half-sinusoid current waveform is a much better approximation. These waveforms are measured at the device junction, so we do not Using the load-pull data Now that the optimum fundamental, second, and third harmonic terminations have been achieved, the PA design can be implemented. This example is a relatively narrow-band design centered at 2 GHz, using the CGH40010F transistor. Matching networks will be synthesized that as closely as possible transform the 50 ohm input and output to the required device impedances over the entire frequency range. Of course, the practical networks that are produced will emulate the impedances that have been defined, but they won’t exactly match them. Figure 15: The complete PA, with input and output networks, gate and drain bias, and the CGH40010F GaN HEMT. 52 hf-praxis 8/2015
RF & Wireless Figure 16: The simulated prediction for power gain, output power, and PAE. Figure 17: Voltage and current waveforms. have to worry about the effects of parasitics between the device and the model pins. The new Cree GaN HEMT models have additional pins (see Fig 15) that allow us to measure directly at the junction to see the transistor waveforms [7]. Another feature of this design is in the harmonic content as seen at the output of the amplifier (Figure 18). This is not at the drain of the amplifier, but at the output. From this data, it can be seen that the harmonic terminations are doing their job well as there is very little harmonic content coming through the output of the amplifier. Output power varies by 1 or 2 dBm across 200 MHz, so it is an inherently narrow-band design, as are most Class F PAs. Figure 19 shows the PAE versus frequency and it can be seen that it is in the range of 80+ percent over about 150 MHz but drops off quickly on either side at 1.9 and 2.05 GHz. Conclusion In conclusion, switching modes of operation for PAs such as Class F and Inverse Class F are becoming more and more popular as designers focus on improving PAE. This is true for a range of applications from radar to wireless telecom. The Microwave Offi ce Load-Pull Wizard and its ability to inspect transistor voltage and current waveforms helps designers gain confidence in their high performance designs and the process of waveform-engineered PA design. Bibliography 1. P. Colantonio, F. Giannini, E. Limiti, High Effi ciency RF and Microwave Solid State Power Amplifiers, John Wiley & Sons, Ltd, 2009. 2. A. Grebennikov and N.O. Sokal, “Switchmode RF Power Amplifiers”, Newnes, 2007 3. V.A. Borisov and V.V. Voronovich, “Analysis of Switched-Mode Transistor Amplifier with Parallel Forming Transmission Line”, Radiotekhnika Elektronika, vol. 31, pp. 1590-1597, August 1986. 4. M.K.Kazimierczuk, “A new concept of Class F Tuned Power Amplifier”, Proc. 27th Midwest Circuits and Systems Symp., pp. 2007-2012, November 1997. 5. T. He, “Design of Radio Frequency Power Amplifiers for High Effi ciency and High Linearity”, A Thesis Presented to the Faculty of California State University, Chico, 2009. http://csuchico-dspace. calstate.edu/xmlui/bitstream/ handle/10211.4/168/ 10%20 19%202009%20Tien%20He. pdf?sequence=1 6. A. Grebennikov, “Load Network Design Technique for Class F and Inverse Class F PAs”, High Frequency Electronics, pp. 58-76, May 2011 7. R. Pengelly, “The new Cree 6 port intrinsic GaN HEMT large signal models – aids waveform engineering of PAs and deeper understanding of PA operation”, Cree RF Products, October 2013. AWR Group, NI would like to thank Ray Pengelly, Cree RF Products and Mark Saffian, NI for their contributions to this application note. ◄ Figure 18: At the output, there is >23 dB worst-case harmonic rejection, confirming that terminations are working properly. Figure 19: PAE versus frequency. PAE remains relatively constant over 150 MHz, but drops off quickly below 1.9 and above 2.05 GHz hf-praxis 8/2015 53
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