Four-Way Microstrip-Based Power Combining for Microwave Outphasing Power Amplifiers

Abstract

Abstract-A lossless multi-way outphasing and power combining system for microwave power amplification is presented. The architecture addresses one of the primary drawbacks of Chireix outphasing; namely, the sub-optimal loading conditions for the branch power amplifiers. In the proposed system, four saturated power amplifiers interact through a lossless power combining network to produce nearly resistive load modulation over a 10:1 range of output powers. This work focuses on two microstrip-based power combiner implementations: a hybrid microstrip/discrete implementation using a combination of microstrip transmission line sections with discrete shunt elements, and an all-microstrip implementation incorporating open-circuited radial stubs. We demonstrate and compare these techniques in a 2.14 GHz power amplifier system. With the allmicrostrip implementation, the system demonstrates a peak CW drain efficiency of 70% and drain efficiency of over 60% over a 6.5-dB outphasing output power range with a peak power of over 100 W. We demonstrate W-CDMA modulation with 55.6% average modulated efficiency at 14.1 W average output power for a 9.15-dB peak to average power ratio (PAPR) signal. The performance of this all-microstrip system is compared to that of the proposed hybrid microstrip/discrete version and a previously reported implementation in discrete lumped-element form.Index Terms-base stations, outphasing, power amplifier (PA), wideband code division multiple access (W-CDMA), Chireix, LINC, load modulation.

Multi-Way Lossless Outphasing System Based on an All-Transmission-Line Combiner

“…The output power P out delivered by the combiner to the load can be easily calculated from (11), and is given by (12) for the four-way combiner of Fig. 1 with halfwavelength transmission-line base length.…”

“…As can be seen from the output power relation derived in (12) and Fig. 5, there exist infinite number of PA outphasing angle pairs [θ, φ] for a given combiner output power P out .…”

“…Four-way implementations of this approach have been demonstrated using all lumped elements in a tree structure [9], [10], and by incorporating microstrip lines with ground-referenced shunt reactive elements [11], [12]. In [12] it was shown that these various realizations of the multi-way combining approach could be related through network transformations and shown to have identical theoretical port relationships. The preferred implementation for a particular application will therefore depend on power level and carrier frequency, and availability of appropriate passive components.…”

“…The ideal voltage sources represent the four branch PAs. mission lines, this structure eliminates the need for reactive elements, reducing parasitic effects due to ground return paths and allowing for more compact layout compared to the radial stub implementation in [12]. The proposed outphasing system is broadly related to those in [8]- [12], although it cannot be directly derived from them and has very different design and control considerations.…”

“…mission lines, this structure eliminates the need for reactive elements, reducing parasitic effects due to ground return paths and allowing for more compact layout compared to the radial stub implementation in [12]. The proposed outphasing system is broadly related to those in [8]- [12], although it cannot be directly derived from them and has very different design and control considerations. The proposed approach shares the benefits of power combining techniques based on nonisolating transmission-line combiners [20], [26], [44], [45] while significantly reducing the reactive loading of the power amplifiers.…”