Techniques for high-efficiency outphasing power amplifiers

Abstract

A trade-off between linearity and efficiency exists in conventional power amplifiers (PAs). The outphase amplifying concept overcomes this trade-off by enabling the use of high efficiency, non-linear power amplifiers for linear amplification. However, the efficiency improvement is limited by the efficiency of the output power combiner. This thesis investigates techniques to overcome this efficiency limit while maintaining sufficient linearity. Two techniques are proposed. The first technique is called the outphasing energy recovery amplifier (OPERA), which recovers the normally wasted power back to the power supply and utilizes a resistance compression network for improved linearity. A 48-MHz, 20-W prototype OPERA system was built which demonstrates more than 2x higher efficiency than the standard outphasing system for a 16-QAM signal. The second technique to improve the efficiency of the outphasing system is asymmetric multilevel outphasing (AMO) modulation. In the AMO system, the amplitude for each of the two outphased PAs can switch independently among multiple discrete levels, significantly reducing the energy lost in the power combiner. Three different AMO prototypes were built, each of which demonstrate between 2x-3x efficiency improvement compared to the standard outphasing system. A 2.4-GHz, 500- mW prototype made in a 65-nm CMOS process achieves an average system efficiency of 28.7% for a 20-MHz 64-QAM signal. To the author's best knowledge, this is the highest reported efficiency for a CMOS PA in the 2-2.7 GHz range for signal bandwidths greater than 10 MHz.