17.6 Envelope modulator for multimode transmitters with AC-coupled multilevel regulators

Modern wireless communication systems, such as high-speed uplink packet access (HSUPA) or long term evolution (LTE), employ highly spectral-efficient modulations with inherent non-constant envelope signals having high peak-to-average power ratio (PAPR). These signals require the RF Power Amplifiers (PAs) to be backed off from saturation to satisfy the stringent linearity requirements. Unfortunately, linear operation leads to very low overall system efficiency. The envelope tracking (ET) technique has been identified as one possible solution to improve the global efficiency of the RF transmission chain. This technique has raised interest in the optimization of the fast supply envelope modulator (EM), the most critical component in the system. Several topologies for the EM exist such as fast buck converters, multilevel buck converters [1] and parallel hybrid structures with a class-AB amplifier AC-coupled to a buck converter [2].