Switch-Mode RF PAs Using Chireix Outphasing
Robin Wesson, Mark van der Heijden
It is well known in the wireless infrastructure domain that the final stage RF power amplifier consumes a significant proportion of the total radio base station power budget. Today, two-way LDMOS Doherty amplifiers are widely used in conjunction with digital pre-distortion (DPD) to provide greater than 40% drain efficiency for the final stage of the power amplifier. The Doherty amplifier architecture is part of a wider class of load-modulation architectures. The Doherty architecture, consists of two amplifiers that modulate each other’s load depending on the required output power. There is another PA architecture that consists of two amplifiers, known as an outphasing amplifier. The simplest version of an outphasing architecture is known as a LINC amplifier (Linear Amplification using Non-linear Components). A LINC amplifier uses an isolating combiner with saturated amplifier stages to achieve high linearity with good peak efficiency. Due to the isolating combiner there is no load modulation effect in a LINC amplifier. With this amplifier, high peak to average (PAR) signals suffer from reduced average efficiency since the individual amplifiers operate at a constant power even when the required signal output power is low.
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