Power Amplifiers For Communications

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BerkeleyPower Amplifiers for CommunicationsProf. Ali M. NiknejadU.C. BerkeleyCopyright c 2014 by Ali M. NiknejadNiknejadAdvanced IC’s for Comm

PA System Level SpecificationsNiknejadAdvanced IC’s for Comm

Power Amplifier SpecificationsPeak Output PowerEfficiencyPdcPower GainAmplifier LinearityPinStability over VSWRAbility to transmit into anunknown/varying loadPower ControlZin 50 ΩStep size,rangeHigh efficiency at back-offNiknejadAdvanced IC’s for CommZout ! 50 ΩPLHeat

Peak Output PowerThe peak output power determines the range for two-waycommunications. When we hit sensitivity limits, the only wayto increase the range is to increase the Tx power.The peak power is specified at the 1-dB compression point orthe maximum output power – the “clipping” point (makes abig difference). 1W for cellular handsets ( 1 km distance) 100mW for W-LAN (100 m) 10mW for W-PAN (Bluetooth) (1-10 m) 1mW for body area networks.In practice, the average power transmitted may be much lowerthan the peak output power due to “back-off”, to obtainlinearity for the amplitude modulation (fast time scale) or forpower control (slow time scale)NiknejadAdvanced IC’s for Comm

Peak EfficiencyPower Added Efficiency (PAE) is a popular metric.Pout is the output power, Pin is the input power,and Pdc is the DC power consumption of the PAFor high power gain systems (Gp ), the efficiencyapproaches the drainηPAE drain efficiency (ηd ), or for a BJT, the “collector”efficiency, or simply the efficiency of the last stageηPAE The efficiency of the PA is an important measure ofthe battery life of the wireless transceiver. Since thePA power dwarfs the power consumption in thereceiver, it is usually the most importantspecifications.For lower power systems (below 10mW), the powerof the entire transmitter chain is important andshould be taken into consideration.NiknejadAdvanced IC’s for CommPout PinPdcPout PoutGpPdcPout(1 Gp 1 )PdcηPAE ηc (1 Gp 1 ) ηc

Average Efficiency with Power ControlFor a constant envelope signal (phase/frequency modulation),the average efficiency is equal to the average efficiency atpeak power.Due to power control, though, we must take into account thestatistics of the transmitted signal. Modern systems usepower control to minimize the impact of a transmitter onnearby systems (interference) and hence only use as muchpower as needed to achieve low error communication with thebase station.Thus the actual average efficiency depends on how theefficiency varies with output powerNiknejadAdvanced IC’s for Comm

Power StatisticsZ ηav η(P)g (p)dP Given the distribution of power levels, or the PDF g (P), wecan calculate the expected value of the efficiencyUnfortunately, for most PAs the efficiency drops at low power.NiknejadAdvanced IC’s for Comm

Envelope StatisticsFor signals with amplitude modulation, the average efficiencydepends not only on the desired power level, but also on thestatistics of the envelope.The amount of power variation is usually captured by thePAR, or the Peak-to-Average Radio.The PAR is a strong function of the type of modulation.Systems with the highest PAR are OFDM systems employingmultiple carriers.NiknejadAdvanced IC’s for Comm

LinearitydBtransmit filter maskIM53f1-2f2IM3f1 f22f1-f2 2f2-f1HD22f1 2f2HD3f3f2-2f1The traditional way to characterize narrowband systemlinearity is with IM3. Since the system may be driven into astrongly non-linear regime, all odd order harmonics should becarefully taken into account to ensure that excessive spectralleakage does not occur.NiknejadAdvanced IC’s for Comm

Sources of Non-LinearityPAs exhibit nonlinear distortion in amplitude and phase. For amulated signal, both sources of distortion are significant.The dominant sources are AM-to-AM and AM-to-PM.Amplitude distortion: AM-to-AM conversionPhase distortion: AM-to-PM conversionFor input: x(t) A(t) cos(ωt φ(t))Corresponding output:y (t) g [A(t)] cos(ωt φ(t) ψ[A(t)])AM-to-AM conversion dominated by gm non-linearity beforeclippingAM-to-PM conversion dominated by non-linear capacitors(phase delay)NiknejadAdvanced IC’s for Comm

AM-AM and AM-PM Non-LinearityFor a narrowband signal, we can partition the non-linearityinto an amplitude-amplitude (AM-AM) component and anamplitude-phase (AM-PM) componentThis behavioral model can be used to run system levelsimulations to see the effect of non-linearity on a modulatedwaveformNiknejadAdvanced IC’s for Comm

Adjacent Channel Power (ACP)1.25MHzP1P230 kHz885kHzACP P1(dBm)-P2(dBm)For modern communication systems, the IM specificationsleave a lot to be desired since they are only based on two-toneexcitation. Increasingly, the actual modulation waveformneeds to be tested with the PA.To ensure proper etiquette, the amount of power leaking intoan adjacent channel is carefully specified.NiknejadAdvanced IC’s for Comm

Unwanted emissions are caused by a number of factors including non-linearity in the28Chapter 2 Transmitter FundamentalsTransmit Mask Specssystem, noise resulting from interference with other circuits or spurious tones created bymask. Non-idealities in the transmitter, which distort the signal, will bring the modulatedclocks or frequency synthesizers. Because these non-idealities affect the in-band signalsignal higher and thus closer to violating the spectral mask. These non-idealities will bethey can also have an effect on the modulation accuracy. The in-band spectral maskdiscussed in more detail in the following section.requirement for DCS1800 is illustrated in Figure 2.6.1000-20-20-30PSDRelative Power (dBc)-10-40-40-50-60-60-70-8001234567-400Frequency O ffset (MH z)-2000200400Frequency (KHz)Figure 2.6 GSM in-band spectral mask requirement.Figure 2.7 GSM spectral mask at low offset frequencies with GMSK modulated signal.Every standard therefore has a transmitmask specificationIn addition to in-band requirements, out-of-band spectral emissions requirementsthat must be met. This limits spectral regrowth, noise, andother spurious transmissions in the band and in nearby bands.Above examples are for GSM.Typically, one of the most difficult portions of the spectral mask requirement isclose to the carrier at. For example, Figure 2.7 shows the same GSM spectral mask atalso have an affect on transmitter design. While the out-of-band emissions don't affectrelatively low offset frequencies. In addition a GMSK modulated signal has also beenthe modulation accuracy or other transmitters in the same band, the limits are often lowerillustrated in the same plot. Notice that the modulated signal is always below the spectralthan the in-band limits, making them more difficult to satisfy. For example, one of themost difficult requirements in the DCS1800 standard is the emissions requirement thatfalls in the DCS1800 receive band.NiknejadAdvanced IC’s for Comm

FCC LimtsEIRP Spectral Density 901990-31003100-10600 10600-60-65-70-75-8011.52345Frequency (GHz)EIRP(dBm/MHz)-75.3-53.3-51.3-41.3-51.3678 9 10While the transmit mask is standard specific, every transmittermust comply with FCC limits (in the US). The above mask isfor an unlicensed device meeting part 15 requirements.NiknejadAdvanced IC’s for Comm

Error Vector MagnitudeChapter 2 Transmitter Fundamentals26Chapter 2 Transmitter Fundamentalscos( /4- /2)IdealPhase ErrorQActual SignalError Vectorsin( /4 /2)Ideal Signalsin( /4- /2)/2/2Phase Error/4Ideal PhaseIcos( /4 /2Figure 2.5 Graphical representation of the error vector and phase error.Figure 2.10 Quadrature phase error constellation.While the ACP is a good way to measure how much the PAsignal will deteriorate a neighboring channel signal, the EVMGenerally, phasemodulationare owwhichmuchthePAwithbasebandbutuseininterferesthis test, the basebandinputs itself.are low frequency sinusoids given byFor certain types of modulation the difference is measured by the errorvector common and useful graphical representation of accuracy of a transAnothermagnitude (EVM) while in other systems only the phase error is thesignalcriticalmetric.altering the baseband input. Typically modulated data is appliedinvolvesphase error as the metric and NCE modulation schemes use EVM. For example forThe EVM measures the systematic deviation Iof(t ) thecos(2 f t )constellation points from the ideal positions due to amplifierphase error of less than 5 degrees.andnon-linearity.bbGMSK modulation used in DCS1800 cellular phones, the standard requires an RMSQ (t )2.4.2 Spectral Emissionssin(2 f bb t ) .It might be expected that the up-converted spectrum would contain two frequenciNiknejadIC’s for CommIn addition to modulation accuracy, it is critical that a transmitteronly emit a Advancedspecified

Example: GSM Transmitter Non-Ideality42FundamentalsChapter 2 TransmitterChapter 2 Transmitter FundamentalsQ110.50.50Q-0.5-10-0.5-1IdealPhase noise-1-0.500.51-1IFigure 2.20 GMSK constellation diagram with phase noise.IdealTherm al Noise-0.500.51IFigure 2.18 GMSK constellation diagram with thermal noise.The first example shows the impact of phase noise whereasIn addition to affecting the modulation accuracy, phase noise also Inchangestheto thermal noise, flicker noise can also negatively impact a tranadditionthe second plot shows the impact of noise (both amplitudewith thermalmodulated spectrum as illustrated in Figure 2.21. Much like the case signal.Flickernoise,noise is present in CMOS transistors and the mean-square drain cuand phase).phase noise can raise cause spectral mask violations. In this examplegiventhe phaseby noise islarge enough to cause spectral mask violations close to the carrier. However, unlikethermal noise, phase noise decreases as the frequency of interest moves away from theLO frequency. Even with this property, phase noise is still a majorconcern at largeoffset IC’s for CommNiknejadAdvancedid2KI Daff

above approximately 300 kHz and below -300 kHz, the noise is clearly evident leading toTransmittera spectralNoisemask violations.0PSD (dB)-20-40W ith noise-60W ithout noise-80-600-400-2000200400600Frequency offset (KHz)Figure 2.17 GMSK signal with thermal noise.Transmitter noise is important for two reasons. First the noiseThe effectsof thermalnoise on the constellationa GMSKsignal are shownlevel shouldnotsignificantlyimpact oftheEVM/BERof inthewill clearlycause error in themagnitudephase of theFigure 2.18. oiseandleakingintootherbandsmust meet specs. This is especially problematictransmittedsignal.in FDD systems that transmit and receive at the same time.NiknejadAdvanced IC’s for Comm

Digital and Analog ModulationBoth digital and analog modulation schemes involve amplitudeand/or phase modulation: Vo (t) A(t) cos(ωt φ(t))Linearity specs of PA determined by envelope variationMost spectrally efficient modulation schemes have largeenvelope variationsAnalog FM (AMPS) uses constant envelope can useefficient non-linear power amplifiers (60%-70%)GMSK (GSM) uses constant envelope as wellπ/4 DQPSK (IS54/136) has 3dB peak to average ratio (PAR)QPSK (CDMA base station) has 10dB of PAR, OQPSK(CDMA handset) has 3dB of PAR802.11g OFDM at 54 Mbps (52 sub-carriers) has about 6-8dB of PARNiknejadAdvanced IC’s for Comm

Modulation SchemesKey specifications are the peak-to-average radio, the peakpower, and the power control range.Constant modulation schemes much easier (GSM, AMPS).WiFi uses OFDM, which is the hardest ! LTE up-link uses asingle carrier to ease the PA back-off requirements.NiknejadAdvanced IC’s for Comm

Switching versus “Linear” PATwo general classes of PA: Linear andNon-Linear“Linear PAs” preserve amplitude andphase information while “Non-linear PAs”only preserve phase mod. Typically (notstrictly), linear PAs employ transistors ascurrent sources (high Z), non-linear PAsemploys transistors as switches (low Z)Linear PAs can drive both broadband andnarrowband loads.Non-linear PA usually drive a tunedcircuit loadAmplitude information in a non-linear PAcan be recovered by:Oversampling, duty cycling, or varyingthe supply voltageNiknejadAdvanced IC’s for Commvi (t)io (t)vi (t) A(t) cos(ωt φ(t))io (t) Gm A(t) cos(ωt φ(t))VDD A0 cos(ω0 t φ(t))A0 cos(ω0 t φ(t))

Clipping: A “Linear” PA is ImpossibleAll amplifiers eventually clip,that is the output cannot behigher than some multiple ofthe power supply. Note thatthe peak amplifier outputcan be arbitrarily large, butthe average output powerwill limit.If we “back-off” sufficientlyfrom the peak so that theamplifier never clips, then wecompromise the efficiency.Limited OutputIdeal ClippingPointWe can generally make acompromise and choosesufficient back-off to meetthe EVM specs.NiknejadAdvanced IC’s for Comm

Power Back-offIn applications requiring a linear PA due to PAR, we mustback-off from the peak power point to avoid clipping thewaveform.For 10 dB of PAR that means operating the PA at 10 dBlower power (or power back-off).An OFDM 802.11g system that needs 20 dBm at antenna andhas a PAR of about 17 dB. That means to transmit 20 dBmaverage power, the PA should be capable of transmitting 37dBm !!!In practice the peak amplitude is a rare event and the PAshould be allowed to clip. A 6-7dB back-off is typical.NiknejadAdvanced IC’s for Comm

Power ControlMost wireless systems have power control. Power control isimportant to limit transmit power to the lowest possiblesetting. This saves battery power and limits the amount ofinterference to other nearby users.There are two power control loops to consider: (1) Mobilepower control loop and (2) Basestation to mobile powercontrol loopThe mobile unit must transmit a given output power with acertain resolution. In GSM the output power can be off by 2dB.in CDMA systems, the noise level is actually set by thisinterference so all users are required to back-off to make thesystem as a whole more efficient.NiknejadAdvanced IC’s for Comm

Closed Loop Power ControlThe mobile power control loop can be a closed loop or openloop system. In an open loop system, the power of the outputpower of the hand-held is measured and calibrated for eachDAC setting. Then an open loop system is used estimatedbased on a one-time calibration.In a closed-loop system, the output power is estimated using adirectional coupler, a voltage measurement, or a currentmeasurement.NiknejadAdvanced IC’s for Comm

Stability over VSWR45501.00.9551. 601.61.82.0060.50.440.7050.4203.00.6VSWR 3 COEFFICIENT IN DEGREESLE OFANGISSION COEFFICIENT INTRANSMDEGRLE OFEESANG0.0.60.047( 20.3900.150.35400.2100.10.420o)jB/YE .440.4190.-25-750.6- IC’s for —RD LOATOWATHS0.47ENGVELWA —-900.40.10.4500.2200.20.2RESISTANCE COMPONENT (R/Zo), OR CONDUCTANCE COMPONENT (G/Yo)500.0 — WAVELENGTHS TOWARD0.00.49GENERA0.48TOR—0.47 0.140.3631R,Oo)SWR 1 x jy SWR 1Niknejad0.370.On the Smith Chart any load lyingon a constant VSWR circle is avalid load, or any impedance suchthat0.130.3819A system with a VSWR of 3:1 candrive any load with magnitude aslarge as 3 50Ω or as small as50Ω 3 17Ω.0.12Yo)jB/E ( PA generally must be able todrive a varying load. The ability todrive a given range of loads isspecified as the VSWR, e.g. aVSWR of 3:1

Power Control LoopsPower AmplifierFilterV Directional Coupler V V AnalogPowerControl V Z0 V Power DetectorReferenceA directional coupler is one of the more accurate methods tomeasure the power delivered to the load (antenna). Thepower reflected from the antenna due to a mismatch is notcomputed. But the directionality of the coupler is key.NiknejadAdvanced IC’s for Comm

Power DevicesNiknejadAdvanced IC’s for Comm

RF Power TransistorsIn a BJT there is a direct trade-off between the breakdownvoltage and thefT of the device. Some people define this as a metric for thetransistor.Thus we should employ the lowest tolerable fT device for ourPA. That’s because such a device can swing the largestvoltage.Unfortunately, the trend in technology is the opposite, mostlyfor digital and RF applications, giving us over 100 GHz fT andonly 1-2 volts to operate with. This is good for digital.CMOS devices also require a large Cox (small Tox )(gate control) for short channels, and thus gate oxidebreakdown is a big issue. Punchthrough is another breakdownmechanism.NiknejadAdvanced IC’s for Comm

BJT Device Power GainFor 300mA of current need20,000 mm2 of Si areaOperating at frequencies fT /10 (say 2.5 GHz in 25GHz process)Device parasitics dominateimpedance levels. Don’tforget Temp!!!Package parasitics limit gainby providing feedbackGain determined by (M.Versleijen et. al.):GP ωT 2ω1CCjcBRCCcsrbTypical TerminalImpedance Levels (CE)Zin 40 j -50Zout 3 j -30RERSLELGE1RL int ) ωT RL CBC ωT LE RE RB (1 ωT RL CBCNiknejadAdvanced IC’s for CommS1CBC LEω2

CMOS Device LossesFor each finger of the CMOS device, we must carefully extractthe capacitance and resistive parasitics. The layout will have alarge impact on these parasitics.NiknejadAdvanced IC’s for Comm

Power FET g1gated1g1g1GATEPower FETs are typically very large (millimeter size) and thelayout is broken into sub-cells. Each sub-cell is broken intomulti-fingered transistors and the gate/drain lines are delayequalized.The layout metals introduce significant resistance andcapacitance, which needs to be carefully modeled.NiknejadAdvanced IC’s for Comm

CMOS Device Power GainGNeed a good model of device, especiallyresistive paraistics. To predict power gainneed to know the gate/source/drainresistance, including the interconnectparasitics (vias, metal, poly), and the gateinduced parasitics.SR3CjdR2R5BDrainCgdextGateNote that the point the device looksdistributed depends on the slow wavevelocity due to large gate and drain cap.NiknejadR1R4Substrate parasitics will also limit powergain and needs to be extracted accurately.Include inductance for high frequencies orvery large “distributed” device.DCjsAdvanced IC’s for CommLgRgCgsextLdCdbRdRdbRbbRsRsbCsbLsSourceBulk

Substrate Parasiticsperpenicular body contactR WWR 1WverticalcontactNotice that the placement of the substrate contacts has animportant impact on the overall substrate loss (the outputimpedance of the device). Increasing the device finger widthW will decrease the “parallel resistance” component butincrease the “perpendicular resistance”.NiknejadAdvanced IC’s for Comm

MOS CV CurveCggClassicalCoxQuantumPoly DepletionVF BVT HVGBCapacitors need to be large signal accurate over voltageswing. Voltage swings maybe negative and move device intoaccumulation (due to inductors).Make sure the CV model is accurate (BSIM capmod 2).Include quantum effects and poly depletion.NiknejadAdvanced IC’s for Comm

Typical Multi-Stage PAOff-chip OutputMatching NetworksOn-chip Matching NetworksPower Loss 2dB10mW100mW1W1mWPo 1W 30dBmPower Gain 13dBNeed 1W at antenna and about 30 dB of power gainEach amplifier stage has about 13 dB of power gainInterstage matching networks have an insertion loss of about2 dBIf you cannot afford loss at output stage, must off-chipcomponents (preferably in the package to keep them close andparasitics minimal).NiknejadAdvanced IC’s for Comm

Count your dBsIn RF r

Peak E ciency Power Added E ciency (PAE) is a popular metric. P out is the output power, P in is the input power, and P dc is the DC power consumption of the PA For high power gain systems (G p), the e ciency approaches the drain drain e ciency ( d), or for a BJT, the \collector" e ciency, or simply the e ciency of the last stage

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