In this EDN article Steve Taranovich explores the envelope tracking system as a potential solution to the notoriously inefficient wideband power amplifiers (PAs) used I wireless telecom and other base station transmitters. An ET solution implementing high speed eGaN FETs in a multi-phase buck converter is explored.
http://www.edn.com/design/power-management/4422469/Enhancing-the-inefficiency-of-an-RF-power-amp--The-envelope-tracking--ET--system