Design Article
Understand and characterize envelope-tracking power amplifiers
Gerard Wimpenny, Nujira Ltd. & Member of OpenET Alliance
1/3/2012 8:50 AM EST
Efficiency Optimization
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 (Click figure to download larger version in PDF)
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 (Click figure to download larger version in PDF)
|
|
|
|
|
Navigate to related information
WKetel
1/4/2012 4:32 PM EST
Envelope tracking reminds me of the two modulated stages in an AM transmitter with class "C" driver and output stages. Much more efficient than anything linear. But more demanding on the output filtering section.
Sign in to Reply
psi123
1/4/2012 7:00 PM EST
WKetel: Envelope tracking does not have to have Class "C" drivers and setting back the clock of the system. I do not want it to be represented as one of the unfinished ideas.
Sign in to Reply
psi123
1/5/2012 2:29 PM EST
The objection of the envelope tracking transmitters is to improve efficiency of the output signal.
It is collecting the input signal data and replaces it with something that we do not know at the time.
Sign in to Reply
XRF
1/10/2012 12:28 PM EST
The surface response of an ET PA is more complex than a simple 3D RF-input vs battery-supply. Phase pushing and pulling will occur in such a 3D ET control solution.
One comment on the transistion region to be noted; it is the same break-point as system modulation pre-distortion uses. There are two other break-points needed to arrive at the most effective ET modulation efficiency which are not mentioned in the text.
Sign in to Reply
WKetel
1/12/2012 10:09 AM EST
I agree that an envelope-tracking amplifier does not have to be a "class C" amplifier, although that would provide the greatest efficiency, it could easily cause some distortion. The other comment is certainly valid, which is that it takes a great deal more effort to design an amplifier that will deliver a satisfactory level of distortion at various operating voltages. Phase modulation certainly does happen and it must be compensated for in order for the distortion level to be acceptable, and designing the correct compensation will certainly add a lot to the design effort.
Sign in to Reply
JeremyHendy
1/16/2012 9:26 AM EST
Almost all ET PAs are used in Class AB mode. And while the AM:PM distortion can't directly be controlled by the supply voltage, our experience with the vast majority of handset PAs is that correcting the AM:AM using IsoGain ET also brings the phase response into line. And simple memoryless correction of AM:PM ("DPD-Lite") is an increasingly common capability of the latest handset chipsets.
Sign in to Reply