EDNE July 2012 - (Page 29)

power amplifiers over temperature to be worse than that of their fixed-supply counterparts. The reverse situation is true, however. An envelope-tracking power amplifier’s performance is more sensitive than that of a fixed-supply amp to changes in the supply-voltage characteristics than to changes in gain of the RF chain driving the power amplifier. Because you can better control the characteristics of the supply voltage over temperature than the variation of the RF gain, little variation in linearity occurs for extreme temperature variations (Figure 12). In a handset environment, the power amp receives an uncontrolled load impedance due to reflections from nearby objects, which can result in the amplifier’s having to work into load mismatches with a VSWR (voltage-standing-wave ratio) as high as 3-to-1. The envelope-tracking power amplifier’s self-linearization principle also applies under highVSWR conditions, and this operation can result in significantly better adjacent-channel power ratio and error-vectormagnitude performance than that of an amplifier operating in fixed-supply mode (Figure 13). a key aspect is the definition of the shapinG taBLe. once you define the shapinG function, you can measure efficiency and Linearity usinG a system-characterization Bench. The system-efficiency benefit of operating a power amplifier in envelope-tracking mode is well-known. However, it also offers other useful system benefits, such as increased output power, improved operation into mismatched loads, and insensitivity to temperature variations. In contrast to fixed-supply power amplifiers, the performance of an envelope-tracking power amplifier requires the gathering of substantially more data to predict system performance and the use of a test environment that allows sweeping of the supply voltage and the input power. A key aspect is the definition of the shaping table, which defines the relationship between supply voltage and RF power. Once you define the shaping function, you can directly measure efficiency and linearity using an appropriate system-characterization bench.EDN author’s biography Gerard Wimpenny is the chief technology officer of Nujira Ltd and a member of the OpenET Alliance. He has more than 20 years of RF and signal-processing experience and has been responsible for strategic R&D, design-process definition, and top-level technical support for business-development activities. Wimpenny has been instrumental in delivering numerous wireless products to semiconductor vendors and infrastructure and handset manufacturers. He has a master’s degree from Cambridge University (Cambridge, UK). www.edn-europe.com JULY 2012 | EDN EUROPE 29 http://ad.doubleclick.net/clk;256714400;80512663;z?http://www.digikey.com/europe http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE July 2012

Cover
Contents
International Rectifer
Microchip
FTDI
Masthead
Comment
International Rectifer
Pulse
Analog Devices
Digi-Key
Analog Devices
Test & Measurement
Agilent Technologies
Baker's Best
Messe München
Understand and characterize envelope-tracking power amplifiers
Digi-Key
Cover Story
Coilcraft
Digi-Key
Signal Integrity
Altera
USB 3.0 : Bringing SuperSpeed connectivity to mobile devices
Digi-Key
Developing high-frequency integrated circuits for test and measurement
Design idea
Product Roundup
Tales from the cube

EDNE July 2012