Design Article
Understand and characterize envelope-tracking power amplifiers
Gerard Wimpenny, Nujira Ltd. & Member of OpenET Alliance
1/3/2012 8:50 AM EST
Characterization Techniques
The ‘standalone’ performance of ET PAs cannot be measured unless the shaping table is first defined. This requires measurement of the PA’s fundamental characteristics (Pout, Efficiency, Gain, Phase) over the full range of supply voltage and input power. In principle this characterization could be carried out using a CW network analyzer and a variable DC supply, but results are typically poor due to thermal effects, ranging errors and drift in phase measurements. It is also far too slow to allow load pull techniques to be used. An alternative approach is to use a pulse characterization using ATE controlled standard test equipment. This avoids the need for a high bandwidth low impedance supply and is sufficiently fast for Load Pull to be viable, but has the drawback that it is difficult to make accurate phase measurements. The last approach is to use real waveforms and to vary the shaping table to allow all combinations of input power and supply voltage to be measured. This requires a supply modulator, but is very fast, allows accurate phase information to be gathered and can also be used to characterize memory effects.
Figure 8: ET PA Characterization bench (Click figure to download larger version in PDF)
A ‘basic’ ET PA characterization can be used to create a quasi-static (i.e. memoryless) data model of the PA having output power, phase and efficiency as outputs and input power and supply voltage as inputs. Once the shaping table is defined, the model can be used to predict PA system performance parameters such as ACPR, EVM and efficiency for standard test waveforms.
In addition to being used for PA device level characterization, the same hardware can be used for direct verification of PA system performance using the defined shaping table (see Figure 9).
Figure 9: ET PA AM/AM and AM/PM after shaping table definition
For higher bandwidth waveforms, PA memory effects can be a significant source of non linearity. The PA output parameters (AM, PM, efficiency) now depend on time (i.e. signal history) in addition to instantaneous input power and supply voltage. Memory effects show up in the PA characterization as a ‘broadening’ of the AM/AM and AM/PM characteristics and can result from electrical time constants in input or output bias circuits, thermal time constants associated with local die heating, or technology specific ‘charge storage’ effects.
Next: Efficiency Optimization
The ‘standalone’ performance of ET PAs cannot be measured unless the shaping table is first defined. This requires measurement of the PA’s fundamental characteristics (Pout, Efficiency, Gain, Phase) over the full range of supply voltage and input power. In principle this characterization could be carried out using a CW network analyzer and a variable DC supply, but results are typically poor due to thermal effects, ranging errors and drift in phase measurements. It is also far too slow to allow load pull techniques to be used. An alternative approach is to use a pulse characterization using ATE controlled standard test equipment. This avoids the need for a high bandwidth low impedance supply and is sufficiently fast for Load Pull to be viable, but has the drawback that it is difficult to make accurate phase measurements. The last approach is to use real waveforms and to vary the shaping table to allow all combinations of input power and supply voltage to be measured. This requires a supply modulator, but is very fast, allows accurate phase information to be gathered and can also be used to characterize memory effects.
Figure 8: ET PA Characterization bench (Click figure to download larger version in PDF)
A ‘basic’ ET PA characterization can be used to create a quasi-static (i.e. memoryless) data model of the PA having output power, phase and efficiency as outputs and input power and supply voltage as inputs. Once the shaping table is defined, the model can be used to predict PA system performance parameters such as ACPR, EVM and efficiency for standard test waveforms.
In addition to being used for PA device level characterization, the same hardware can be used for direct verification of PA system performance using the defined shaping table (see Figure 9).
Figure 9: ET PA AM/AM and AM/PM after shaping table definition
For higher bandwidth waveforms, PA memory effects can be a significant source of non linearity. The PA output parameters (AM, PM, efficiency) now depend on time (i.e. signal history) in addition to instantaneous input power and supply voltage. Memory effects show up in the PA characterization as a ‘broadening’ of the AM/AM and AM/PM characteristics and can result from electrical time constants in input or output bias circuits, thermal time constants associated with local die heating, or technology specific ‘charge storage’ effects.
Next: Efficiency Optimization
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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.
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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.
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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.
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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.
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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.
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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.
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