Asynchronous modulator for linearization and switch-mode RF power amplifier applications

An implementation of an asynchronous modulator which encodes a modulated RF signal into a two level pulse train is described. The modulator is designed for RF switch mode amplifier applications and feedback linearization using conventional linear power amplifiers. The modulator is implemented in 0.18 µm SiGe BiCMOS and operates over a frequency range from 750 MHz to 2.2 GHz. The design includes the implementation of a fourth order complex bandpass filter and an encoder to synthesize a binary pulse train with asynchronous zero crossings. Experimental results are shown for a power amplifier module where the modulator is configured in a feedback loop linearizing a Doherty power amplifier while processing a three carrier WCDMA input signal.