A High Efficiency Doherty Amplifier with Digital Predistortion for WiMAX
نویسندگان
چکیده
The Doherty amplifier architecture is a well known technique offering the potential to improve transmitter efficiency especially for signal protocols that exhibit high peak-toaverage power ratios. Although the Doherty approach has significant efficiency advantages, it generally must be augmented with some form of correction or linearity enhancement in the full transmitter design. Wireless infrastructure applications have demanding linearity and spectral mask specifications. The latest WiMAX standards present a particular challenge with their combination of very high peak-to-average power ratios, 10 MHz or wider channel bandwidths and high linearity standards. This article demonstrates a 2.5-2.7 GHz Doherty amplifier that achieves, when augmented with digital predistortion, 8 watts of WiMAX average output power at greater than 47% efficiency, while satisfying demanding spectral mask specifications. The appeal of the Doherty amplifier configuration is that it involves familiar core amplifier designs connected in a manner that maintains high efficiency over an extended input signal range. Doherty development has been energized by the latest generations of transistors and is well represented in recent literature [1-5]. Applying the Doherty approach to modern WiMAX signals can present unique challenges. In particular, WiMAX signals combine two challenging features: wide video bandwidth of 10 MHz or greater, and large peak-to-average ratios of 10 dB or greater. Additionally, the latest WiMAX standards require that the spectral emissions mask (SEM) at close offsets (1.5 MHz) be at least –45 dB. A typical amplifier (without correction) will exceed this level by 15 to 20 dB, resulting in a significant correction demand for the digital predistortion circuitry. The technologies applicable to this challenge have improved in three areas:
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