A Limited Bandwidth Envelope Follower for Efficiency Enhancement in a Linear Power Amplifier in Broadband Transmitters

An efficiency enhancement scheme for transmitters using a linear Power Amplifier (PA) for non-constant amplitude modulation is proposed. Various current and emerging wireless standards require envelope modulation with wide bandwidth. It is difficult to accommodate this in PA supply-modulation schemes using an efficient switched regulator (or S-modulator) to follow the envelope signal, as the regulation efficiency declines with the increase in bandwidth. In the proposed scheme a linear PA is used, having a switched power supply that is modulated with a limited bandwidth, while an amplitude modulated signal, which may be predistorted, is fed to the PA input. The presented real-time signal processing algorithm generates a limited bandwidth power supply modulation signal to minimize the average headroom in the PA, thus increasing its efficiency, while also allowing the switched-regulator to retain high modulation efficiency. Implementation area and power consumption are estimated for 0.18 mu m CMOS.