Wireless communication transmitters have very low efficiencies due to the use of linear radiofrequency power amplifiers. Several techniques have been proposed over the years to improve the efficiency of these systems. One of the most promising is called the envelope tracking technique, which is based on using a fast switching mode power supply to provide a varying voltage to the power amplifier that tracks the envelope of the transmitted signal. The amplifier can, thus, operate continuously near its theoretical maximum efficiency, greatly improving the overall efficiency of the communication system. This paper proposes a multilevel digitally controlled power supply suitable for this application. It is shown to perform very well, achieving very high efficiency, highoutput power capability and tracking bandwidths above 50 kHz. This paper also shows that the proposed system is able to produce a 15% overall increase in efficiency in a complete envelope tracking system.Index Terms-DC-DC power conversion, energy management, HF amplifiers, switched mode power supplies.

The efficiency of linear power amplifiers can be improved by modulating its supply voltage according to the peak value of the signal that is being amplified in a technique usually known as Envelope Tracking. High bandwidth DC/DC converters called Envelope Amplifiers (EA) are used to carry out the supply modulation in an efficient manner. In the case of linear Radio Frequency Power Amplifiers, the bandwidth requirements over the EA strongly depend on the signal being amplified: in modern telecommunication applications it can range from several hundred kHz to a few tens of MHz. The path to achieve such bandwidths usually goes through increased switching frequencies and/or reduced ratio of switching frequency to converter bandwidth. However, these may lead to lower converter efficiency and increased distortion, making the design of Envelope Amplifiers a challenging task. This paper addresses the analysis and design of the output filter for Buck converters operating as EA. The output filter must provide enough attenuation at the switching frequency and harmonics to guarantee a desired level of distortion while enabling tracking of widebandwidth envelope signals with high converter efficiency. This paper focuses on the use of high-order filters (up to sixth-order Bessel-Thomson, Butterworth and Legendre-Papoulis filters) in a Buck DC/DC converter working as an Envelope Amplifier. The filters are analytically described and characterized, and a design procedure that takes into account the major design constraints is proposed, allowing the selection of the appropriate filter for a given application. The proposed analysis and design method are supported by simulation results, as well as by experiments obtained using a 1 MHz Buck converter operating as an EA. Pablo F. Miaja (S'07 M'13) was born in Oviedo, Spain, in 1984. He received the M.S. degree in telecommunication engineering from the University of Oviedo in 2007 and in 2012 the PhD. degree from the same university. Since December 2007 he works as a researcher at the Electronic Power Supply Systems Group of the University of Oviedo. His research interests include dc/dc conversion, digital control of switched converters and power-supply systems for RF amplifiers. Alberto Rodríguez (S'07) was born in Oviedo, Spain, in 1981. He received the M.S. degree in telecommunication engineering in 2006 from the University of Oviedo, Gijón, Spain, where he is currently working towards the Ph.D. degree. In 2006, he has been a Telecommunications Engineer with the Government of the Principality of Asturias and an Assistant Professor with the Department of Electrical Engineering, University of Oviedo. Since 2007 he has been working in University of Oviedo at full time. His research interests are focused on multiple port power supply systems, bidirectional DC-DC power converters and wide band gap semiconductors. Miguel Rodríguez (S'06, M'11) was born in Gijón, Spain, in 1982. He received the M.S. degree and the Ph.D. degree in telecommunication engineering from the University of Oviedo ...

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Abstract-The use of techniques such as envelope tracking (ET) and envelope elimination and restoration (EER) can improve the efficiency of radio frequency power amplifiers (RFPA). In both cases, high-bandwidth DC/DC converters called envelope amplifiers (EA) are used to modulate the supply voltage of the RFPA. This paper addresses the analysis and design of a modified two-phase Buck converter optimized to operate as EA. The effects of multiphase operation on the tracking capabilities are analyzed. The use of a fourth-order output filter is proposed to increase the attenuation of the harmonics generated by the PWM operation, thus allowing a reduction of the ratio between the switching frequency and the converter bandwidth. The design of the output filter is addressed considering envelope tracking accuracy and distortion caused by the side bands arising from the nonlinear modulation process. Finally, the proposed analysis and design methods are supported by simulation results, as well as demonstrated by experiments obtained using two 100-W, 10-MHz, two-phase Buck EAs capable of accurately tracking a 1.5-MHz bandwidth OFDM signal.

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