Class-G based envelope tracking system for high-efficiency transmitters in mobile communications
Final Report Abstract
Within this project a class-G system for the 2 GHz frequency range was developed and evaluated for wideband telecommunication signals. The project has covered the full range of topics from GaN-HEMT optimization, power amplifier design and class-G modulator development to full system evaluation including digital predistortion with an improved model for discontinuous systems. The results have significantly advanced the state-of-the-art in class-G systems and are competitive with alternative solutions such as Doherty. A first system analysis predicted that PAs with two or three supply voltage levels can achieve power added efficiencies beyond 50% for high peak-to-average power ratio LTE signals. Evaluation of the available transistor structures showed that the FBH standard layout with gate-drain distance of 4 µm and source connected field-plate overlap of 1.0 µm, which was developed for PA applications in general, yields best results for class-G as well. This was verified by measurements and TCAD simulations. An asymmetric gate cross-sections leads to an interesting power improvement in general but not specifically for class-G. As expected the class-G driver turned out to be the most critical component. Significant progress has been obtained in the course of the project. Investigation of different circuits led to improved modulators that allow switching supply voltages of up to 60 V using a GaN- HEMT. A maximum measured modulator power efficiency of 93% was obtained. There is still room for improvement in terms of efficiency and switching frequency when further optimizing components and by integrating the driver. As overall result, a dynamically operated class-G system is presented which achieves an average efficiency of over 50% for a 20 MHz modulation bandwidth LTE-like signal with a high peak-to-average power ratio of 9 dB. The system is based on a highly efficient modular class-G modulator, operating with two to four supply voltage levels and switching frequencies up to 100 MHz. This system demonstrates that using class-G modulation it is possible to achieve state-of-the-art results in terms of efficiency and modulation bandwidth. The latest class-G system built uses a three-level class-G modulator based on GaN diodes and HEMTs, operating with frequencies up to 200 MHz. With this system measurements with up to 100 MHz modulation bandwidth were performed with an efficiency improvement of 13% points compared to the fixed-supply case. The high modulation bandwidth proves also the potential of class-G modulation for future telecommunication systems (e.g. 5G). It is also seen that the modulator and RF PA have to be integrated to fully exploit the capabilities of the concept. All work-packages have been successfully completed. The overall system analysis has shown the feasibility of power efficient class-G modulated PA systems (AP1). Existing transistors were evaluated with the conclusion that the present standard is the best also for class-G operation (AP2). Initial designs of modulators and RF PAs showed excellent potential (AP3, AP4). Improved designs yielded state-of-the-art performance (AP5, AP8, and AP9). The measurement systems were successfully implemented at both project partners (AP6). Finally control circuitry and full system tests were successfully dynamically implemented with digital predistortion (AP10, AP11, AP12). The project results not only prove that the class-G concept is a competitive solution for the present high-efficiency broadband transmitters, they also reveal the feasibility of class-G systems for the coming very large bandwidth systems (<200 MHz IQ BW). The following measures were identified to be the major steps towards this goal: Further transistor improvement for discrete operation; Gate driver development for the class-G switch in the modulator; Integration of RF PA GaN-HEMT and GaN switch parts (transistor and diode) in MMIC technology.
Publications
- (2017) 100-MHz GaN-HEMT Class-G Supply Modulator for High-Power Envelope-Tracking Applications. IEEE Trans. Microwave Theory Techn. (IEEE Transactions on Microwave Theory and Techniques) 65 (3) 872–880
Wolff, Nikolai; Heinrich, Wolfgang; Bengtsson, Olof
(See online at https://doi.org/10.1109/TMTT.2016.2633987) - “Concept for a GaN Based Class G Modulator for Mobile Communications with High Signal Bandwidth,” in Micro- and Millimetre Wave Technology and Techniques Workshop 2014
M. Schmidt, J. Pfeiffer, M. Grözing, and M. Berroth
- “Linearity analysis of a 40 W class-G-modulated microwave power amplifier,” in 2015 European Microwave Integrated Circuits Conference (EuMIC) Dig., pp. 365–368
N. Wolff, O. Bengtsson, M. Schmidt, M. Berroth, and W. Heinrich
(See online at https://dx.doi.org/10.1109/EuMC.2015.7345988) - “A Novel Model for Digital Predistortion of Discrete Level Supply-Modulated RF Power Amplifiers,” in IEEE Microw. Wireless Compon. Lett.,vol. 26, no. 2, pp 146-148, 2016
N. Wolff, W. Heinrich, O. Bengtsson
(See online at https://doi.org/10.1109/LMWC.2016.2517445) - “A Three-Level Class-G Modulated 1.85 GHz RF Power Amplifier for LTE Applications with over 50% PAE,” in IEEE MTT-S International Microwave Symposium, San Francisco, USA, 2016
N. Wolff, W. Heinrich, M. Berroth, O. Bengtsson
(See online at https://dx.doi.org/10.1109/MWSYM.2016.7540389) - “Discrete Gate Bias Modulation of a Class-G Modulated RF Power Amplifier,” in 2016 European Microwave Conference Dig., pp. 827-830
N. Wolff, W. Heinrich, O. Bengtsson
(See online at https://dx.doi.org/10.1109/EuMC.2016.7824471)
