Switch-Based Variable Length Stubs Network for PA Load Sensitivity Reduction

In Fifth Generation (5G) Multiple-Input Multiple-Output (MIMO) transceivers, Power Amplifiers (PAs) driving antenna arrays experience varying loading conditions. In fact, the input impedance of each antenna element changes within a Smith chart region, a phenomenon attributed to mutual coupling, which will affect PA behavior. Although PA performance is load dependent, its output power and drain efficiency contours are nearly constant along specific straight lines in the Smith chart, a characteristic that can be exploited to improve PA performance in variable load scenarios. This work presents a compression network, featuring switch-based variable length stubs, capable of transforming any load within a 2.1 Voltage Standing Wave Ratio (VSWR) circle into a purely resistive load. This network was designed, implemented, measured and used in the Output Matching Network (OMN) of two Class E PAs being able to compensate the output power variation of the first PA and the efficiency degradation of the second PA under the defined load variation. The innovation of this network is the capability to reduce output power or efficiency load sensibility, which is very much desired in many applications, by using an easily implementable and low loss (0.3dB-0.5dB) network. By properly adjusting the distance between stubs and switches it is also able to transform slightly different load patterns into other optimal lines or curves.