Investigating the linearity versus efficiency tradeoff of different power amplifier modes in an envelope tracking architecture

Envelope tracking (ET) power amplifiers achieve wideband efficiency over dynamic range by modulating supply voltage in response to the input envelope signal magnitude. Shaping functions relate input envelope magnitude to dynamic supply voltage and can be defined to achieve optimum performance in terms of not only efficiency and power, but also linearity. Using an active harmonic load-pull measurement system, this paper explores the trade-off between linearity and efficiency in terms of the intrinsic current and voltage behavior within an emulated ET environment, and as determined by different envelope shaping functions. The analysis compares the behavior of conventional class-AB and high-efficiency class-F modes in an ET architecture, using a 50V LDMOS device. Experimental results at 900MHz show that simple shaping functions with only second order terms are able to generate linear relationships between the fundamental output current and input voltage envelopes. In the case of class-F, this is achieved while maintaining high-efficiency, and thus raises the attractive prospect of using an ET architecture to reduce required linearizer complexity - an important factor in small-cell base station power amplifier (PA) design.