The statistics of typical high Peak to Average Power Ratio (PAPR) signals are such that an ET PA typically spends most of its time operating with relatively low supply voltage, with only occasional high voltage excursions on high power peaks. It makes sense therefore to optimize the PA matching to achieve best efficiency with the target high PAPR signals rather than simply designing for best efficiency at peak power / max supply voltage, as would be the case for a fixed supply PA. It can be seen from Figure 10 that the PA matching should be altered to increase efficiency around the peak of the signal probability density function, even if this necessitates a slight compromise in the peak power efficiency.
Figure 10: Influence of signal statistics on Efficiency of an envelope tracking PA
To fully optimize the efficiency of an envelope tracking PA the device characterization can be extended to include sweeping the load impedance (fundamental or harmonic load pull) in addition to input power and supply voltage. Analysis of the large dataset produced by such a characterization can be automated (e.g. using MATLAB) to predict the average PA efficiency when operating with a specific set of ET parameters. For example, using this characterization methodology it is possible to predict how a PA’s average efficiency varies with shaping function, output voltage swing range, back off from maximum power and waveform statistics when operated in ET mode (see Figure 11).
Figure 11: ET average Efficiency and PA Output power load pull contours
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