et in pa

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Understand and characterize envelope-trackingpower amplifiersGerard Wimpenny, Nujira Ltd. & Member of OpenET Alliance

1/3/2012 8:50 AM ESTThe process of designing traditional fixed supply power amplifiers (PAs) has been well established formany years. Well defined metrics for performance assessment exist, and the PA designer’s job is ‘simply’ todesign a PA with the best set of performance metrics. In reality of course this is far from a ‘simple’ task, butat least the assessment criteria are well established and well understood. For envelope-tracking (ET) PAs,the situation is more complex and requires the use of more sophisticated characterization techniques.

ET BasicsThe objective of envelope tracking is to improve the efficiency of PAs carrying high peak to average power(PAPR) signals. The drive to achieve high data throughput within limited spectrum resources requires theuse of linear modulation with high PAPR. Unfortunately, the efficiency of traditional fixed supply PAswhen operated under these conditions is very poor. The efficiency of an ET PA is improved by varying thePA supply voltage in synchronism with the envelope of the RF signal. The PA’s fundamental outputcharacteristics (power, efficiency, gain, phase...) now depends on two ‘control’ inputs (RF input power andsupply voltage) and may be represented as 3D surfaces.In a typical envelope tracking system, the supply voltage is dynamically adjusted to track the RF envelopeat high instantaneous power. Here, the PA operates with high efficiency in compression. Its outputcharacteristics are primarily determined by the instantaneous supply voltage. Conversely, when theinstantaneous RF power is low, the supply voltage is held substantially constant and the PA outputcharacteristics are primarily determined by the instantaneous input power (linear region). A transition regionin which both supply voltage and input power influence the output characteristics exists between these twoextremes (see Figure 1)



Figure 1 Instantaneous efficiency vs supply voltage – Isogain shapingET PA Linearity A simple ‘quasi static’ (i.e. memoryless) behavioral model of a PA may be constructed if its AM/AM andAM/PM characteristics are known. These characteristics, along with other key PA metrics such as powerand efficiency are profoundly influenced by the mapping between instantaneous RF envelope and appliedsupply voltage. In an ET system this mapping is determined by the contents of a ‘shaping table’ in theenvelope path (see Figure 2)

Figure 2 Envelope Tracking PA System

A mapping of particular interest is ‘ISOgain’ shaping, in which the instantaneous supply voltage is chosento achieve a particular constant PA gain (see Figure 3).

Figure 3: RF O/P power - Supply voltage mapping - ISOGain shaping

With this mapping, the ET PA system achieves low AM-AM distortion despite operating in compressionover much of the envelope cycle as shown in Figure 4. The equivalent trajectory for fixed supply operationis also shown in Figure 4 – from this it is apparent that ET can actually be used to linearize a PA, reducingACPR and EVM.



Figure 4: ET PA gain characteristics - Isogain shaping

The system trade-off associated with using the shaping table to linearize the PA is a small loss of efficiency(compare Figure 1 and Figure 5) for a substantial improvement in linearity (compare Figure 4 and Figure 6).The choice of shaping function also has a strong influence on the bandwidth requirement of the envelopepath. A smooth transition between the linear and compressed regions results in a lower bandwidthrequirement for the envelope amplifier for modest (1-2%) loss in system efficiency.

Figure 5: ET PA Efficiency - Optimum efficiency shaping



Figure 6: ET PA gain characteristics - Optimum efficiency shaping

When designing a fixed supply linear PA, a great deal of attention must be paid to the achieving adequatelinearity characteristics at maximum output power. Many factors influence the linearity (e.g. fundamentaltechnology characteristics, biasing and RF matching) and it is up to the PA designer to achieve the besttrade-off between efficiency and linearity. For an ET PA, things are different, as the linearity of an ET PAin the compressed region is no longer a self contained PA parameter. The PA still has to be linear in the lowpower, low voltage region, but at higher powers the AM linearity constraint is removed and the PA can bedesigned for optimum ET efficiency without regard to AM linearity. Unlike AM distortion, phase distortionis not directly controlled by the envelope shaping table. However, it is observed that many PAs actuallyshow reduced PM distortion when operated in ET mode.

As a result of this ‘self linearization’, it is possible to push harder into compression at signal peaks than witha fixed supply amplifier, allowing increased output power for given linearity. Figure 7 shows measuredACLR and EVM performance for a PA operated in Fixed Supply and ET modes. In this example the PAoutput power for -40dBc ACLR is 2dB higher in ET than fixed supply mode.



Figure 7: ET PA / Fixed supply PA linearity comparison

Characterization TechniquesThe ‘standalone’ performance of ET PAs cannot be measured unless the shaping table is first defined. Thisrequires measurement of the PA’s fundamental characteristics (Pout, Efficiency, Gain, Phase) over the fullrange of supply voltage and input power. In principle this characterization could be carried out using a CWnetwork analyzer and a variable DC supply, but results are typically poor due to thermal effects, rangingerrors and drift in phase measurements. It is also far too slow to allow load pull techniques to be used. Analternative approach is to use a pulse characterization using ATE controlled standard test equipment. Thisavoids the need for a high bandwidth low impedance supply and is sufficiently fast for Load Pull to beviable, but has the drawback that it is difficult to make accurate phase measurements. The last approach is touse real waveforms and to vary the shaping table to allow all combinations of input power and supplyvoltage to be measured. This requires a supply modulator, but is very fast, allows accurate phaseinformation 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 PAhaving output power, phase and efficiency as outputs and input power and supply voltage as inputs. Oncethe shaping table is defined, the model can be used to predict PA system performance parameters such asACPR, EVM and efficiency for standard test waveforms.In addition to being used for PA device level characterization, the same hardware can be used for directverification 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 PAoutput parameters (AM, PM, efficiency) now depend on time (i.e. signal history) in addition toinstantaneous 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 ininput or output bias circuits, thermal time constants associated with local die heating, or technology specific ‘charge storage’ effects.

Efficiency OptimizationThe statistics of typical high Peak to Average Power Ratio (PAPR) signals are such that an ET PA typicallyspends most of its time operating with relatively low supply voltage, with only occasional high voltageexcursions on high power peaks. It makes sense therefore to optimize the PA matching to achieve bestefficiency 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 PAmatching 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 toinclude sweeping the load impedance (fundamental or harmonic load pull) in addition to input power andsupply 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 efficiencyvaries with shaping function, output voltage swing range, back off from maximum power and waveformstatistics when operated in ET mode (see Figure 11).

Figure 11: ET average Efficiency and PA Output power load pull contours (Click figure to downloadlarger version in PDF)

ET PA parameter variation sensitivityIt is commonly expected that the performance of ET PAs over temperature will be poorer than their fixedsupply counterparts. In fact the reverse is true - unlike a fixed supply PA, an ET PA’s performance is muchmore sensitive to changes in the supply voltage characteristics than to changes in gain of the RF chaindriving the PA. As the characteristics of the supply voltage can be much better controlled over temperaturethan RF gain, little variation in PA linearity is observed for extreme temperature variations (See Figure 12).

Figure 12: ET PA ACLR vs Temperature (105C range)

In a handset environment, the load impedance presented to the PA is not well controlled due to reflections



from nearby objects, which can result in the PA having to work into load mismatches as high as 3:1VSWR. The ET PA’s ‘self linearization’ principle previously described is also applicable under highVSWR conditions and this can result in significantly improved EVM and ACPR performance comparedwith the same PA operated in fixed supply mode (see Figure 13)

Figure 13: Comparison of EVM vs backoff for ET and fixed supply PAs operating into 3:1 VSWRload mismatch

SummaryThe system efficiency benefit of operating a PA in ET mode is widely known. However, it also offers otheruseful system benefits, such as increased output power, improved operation into mismatched loads, andinsensitivity to temperature variations.In contrast to fixed supply PAs, the performance of an ET PA is not ‘self contained’ and requiressubstantially more data to be gathered to predict system performance. This requires use of a testenvironment which allows the supply voltage in addition to input power to be swept. A key aspect is thedefinition of the ‘shaping table’ which defines the relationship between supply voltage and RF power. Thisdefines many key PA metrics. Once the shaping function is defined, efficiency and linearity can be directlymeasured using an appropriate ‘system characterization’ bench.

About the authorGerard WimpennyChief Technology Officer, Nujira Ltd. & Member of OpenET AllianceGerard has over 20 years of RF and Signal Processing experience, and has been responsible for strategicR&D, design process definition, and top level technical support for business development activities. He hasbeen instrumental in delivering numerous wireless products to semiconductor vendors, infrastructure andhandset manufacturers and holds an MA degree from Cambridge University.

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