I. Introduction
Power amplifier (PA), a key component in a transmitter, is in the process of rapid development because of the quick advancements of wireless techniques nowadays. For instance, communication schemes such as long-term evolution advanced (LTE-A) and the upcoming 5G generally rely on complex modulations that have very high peak-to-average power ratios (PAPRs). PAs are therefore required to have high efficiency at the back-off region. Because of this, load-modulated PA types, represented by Doherty and outphasing, are becoming more and more popular recently [1]–[5]. At the same time, solutions for large data throughput, such as channel bandwidth expansion and carrier aggregation, put forward increased requirement on PA bandwidth. Besides, the coexistence of old and new radio standards significantly encourages transmitters that can support wideband or multiband operation to increase hardware configurability and save system cost. Driven by these factors, various wide-band/multiband load modulated PAs have been developed in recent years [6]–[11]. In particular, significant progress has be done to the Doherty PA, for which a fractional bandwidth over one octave or triple-band operation have even become possible [10], [11].