RF Power Amplifier Design

1 RF Power Amplifier DesignMarkus Mayer & Holger ArthaberDepartment of Electrical Measurements and Circuit DesignVienna University of TechnologyJune 11, 20012 Contents~Basic Amplifier ConceptszClass A, B, C, F,hHCAzLinearity AspectszAmplifier Example~Enhanced Amplifier ConceptszFeedback, Feedforward, ..zPredistortionzLINC, Doherty, EER, ..3 Efficiency Definitions~Drain Efficiency:~ Power Added Efficiency: DCOUTDPP= = =GPPPDDCINOUTPA11 4 Ideal FET Input and Output Characteristics0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0V=VGSPV=0 GSOhmicSaturationBreakdowngmDDKDDVVV = 5 Maximum Output Power Match0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0V=VGSPV=0 GSOhmicSaturationBreakdowngmmKDSOPTIVVR =max6 Class A0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ7 Class A Circuit VDDRLDGS48%dB) 14 ( 50%PA == = ADGG8 Class B0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ9 Class C0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ10 Class B and C Circuit VDDRLDGSf0 Class BClass C%65dB) (8 6dB-%78PA == = ADGG%01%100PA GD11 Influence of Conduction Angle12 Class F ( harmonic controlled Amplifier )0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ13hHCA(halfsinusoida llydriven HCA)

2 0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ14 Class F andhHCA Circuit VDDRLVDSID Ze(n)0, n=eveninf, n=even Zo(n)0, n=1inf, n=oddClass FhHCA%87dB) (9 5dB-0%10PA == = ADGG%96dB) (15 1dB0%10PA =+= = ADGG15hHCA Third Harmonic Peaking0 VGSIDSIm2 VPVPVDSmaxVDDVKVDS0 VGSVDS2ppQIDSIm02ppQ16 Third Harmonic Peaking Circuit VDDRLDGSf03f0%87dB) ( =+= = ADGG17 Linearity Aspects18 Linearity Aspects~Class AB~Class C~Class A~Class B19 Linearity Aspects~Ideal strongly nonlinear model~Strong-weak nonlinear model20 Amplifier Design An Example ~Balanced Amplifier ConfigurationPort 1Z=50 OhmPort 2Z=50 Ohm21 Amplifier Design Simulation ~Gate & Drain Waveforms050010001300 Time (ps)Drain waveforms-50510152025-100001000200030004 0005000 Inner Drain Voltage (L, V)AmpInner Drain Current (R, mA)Amp050010001300 Time (ps)Gate waveforms-3-2-101-1000-50005001000 Inner Gate Voltage (L, V)AmpInner Gate Current (R, mA)Amp22 Amplifier Design Simulation ~Dynamic Load Line & Power Sweep03691215 Voltage (V)Dynamic load line-200002000400060008000 IVCurve (mA)IV_CurveDynamic Load Line (mA)Amp0510152024 Power (dBm) Power Sweep 1 Tone01020304001020304050607080 Output Power (L, dBm)AmpPAE (R)Amp23 Amplifier Design Measurements~Single Tone & Two Tone051015202530354005101520253035P in [dBm]P out [dBm], Gain [dB]01020304050607080 PAE [%]P outGainGamma InPAE1dB CP010203040506005101520253035P in [dBm]P out [dBm], IMDD [dBc], Gain [dB]0102030405060 PAE [%]P outIMDDGainPAE24 Amplifier Nonlinearity~Gain and Phase depends on Input signal ~3rdOrder Gain-Nonlinearities.

3 25 Amplifier Nonlinearity~Higher Output Level (close to Saturation) resultsin more Distortion/Nonlinearity26 Nonlinearity leads to?~Generation of Harmonics~IntermodulationDistortion / SpectralRegrowth~SNR (NPR) Degradation~Constellation Deformation27 Intermodulationand Harmonics28 SpectralRegrowth-15-10-5051015-60-50-40- 30-20-10010relative Power / dBrelative frequency / MHzACPR1>60dBACPR2>60dBACPR1=16dBACPR2=4 3dB~Energy in adjacent Channels~ACPR (Adjacent Channel Leakage Power Ratio) increases29 Reduced NPR (Noise Power Ratio)~Output signal of Nonlinear Amplifier ~Input signal ~Degradation ofInbandSNR~ Noisy Constellation30 Constellation Deformation~Input signal ~Output signal of Nonlinear Amplifier (with Gain-and Phase-Distortion)31 Modeling ofNonlinearities~with Memory-EffectszVolterraSeries (= Taylor Series with Memory )~without Memory-EffectszSalehModelzTaylor SerieszBlum andJeruchimModelzAM/AM-and AM/PM-conversion2221)(1)

4 (rrrgrrrfaa +=+= betterperformance32AM/AM-and AM/PM-Conversion~GaAs-PA33AM/AM-and AM/PM-Conversion~LDMOS-PA34 How to preserve Linearity?~Backed-Off Operation of PAzSimplest Way to achieve Linearity~Linearity improving to preserve Efficiency?~Efficiency improving ConceptszDohertyzEnvelope Elimination and ~Linearity improving ConceptszHigher Linearity at constant Efficiency Higher Efficiency at constant Linearity36 Direct (RF) Feedback~Classical Method~Decrease of Gain Low Efficiency~Feedback needs more Bandwidth than signal ~Stability Problems at high Bandwidths37 Distortion Feedback~Feedback ofoutbandProducts only~Higher Gain than RF feedback~Stability Problems due to Reverse Loop38 Feedforward~Overcomes Stability Problem by forward-only Loops~Critical to Gain Error -31dB Phase Error -27dB Cancellation~Well suited for narrowband application39 Cartesian Feedback-30-20-100102030-60-50-40-30-20- 10010relative Power / dBrelative frequency / MHzoriginal signal predistorted signal ~AM/AM-and AM/PM-correction~High Feedback-Bandwidth~Stability ProblemsIQIQIQ modulatordemodulatorOPAsmain inputUMTS example.)

5 40 Digital Predistortion~Digital Implementation of Cartesian Feedback ~Additional ADCs, DSP Power ,Oversamplingneeded~Loop can be opened no Stability Problems41 Analog Predistortion~Predistorterhas inverse Function of Amplifier ~Leads to infinite Bandwidth (!)~Hard to realize (accuracy)42 Analog Predistortion~Possible Realizations:43 LINC (Linear Amplification by Nonlinear Components)~AM/AM-and AM/PM-correction~Digital separation required(accuracy!)~High Bandwidth,oversamplingnecessary~Stabilit y guaranteedsignalseparations(t)s(t)1 KKs(t)2K(s(t)+1s(t))=Ks(t)2Ks(t)1Ks(t)2- 30-20-100102030-60-50-40-30-20-10010rela tive Power / dBrelative frequency / MHzACPR1>60dBACPR2>60dBACPR1=18dBACPR2=2 9dBs(t) s1(t)UMTS example:44 Doherty Amplifier ~Auxiliary Amplifier supports main Amplifier during saturation~PAE can be kept high over a 6dB range45 Doherty Amplifier ~Gain vs.

6 Input Power ~No improvement of AM/AM-and AM/PM-distortion~Behavior of auxiliary Amplifier very hard (impossible) to realize~Stability guaranteed~Efficiency vs. Input Powermain amp. (A1)aux. amp. (A2)PINPOUT doherty configuration (A1+A2)46 EER (Envelope Elimination and Restoration)~Separating phase and magnitude information~Elimination of AM/AM-distortion~Applicationof high-efficient amplifiers(independent of amplitude distortion)~Stability guaranteedsignalseparationamplitude informationphase informationRF inputRF outputhigh efficiencypower amplifier47 EER (Envelope Elimination and Restoration)~Analog realizationzLimiter hard to buildzAccuracy problemszFeedback necessary~Digital realizationzOversampling+ high D/A-conversion rates requiredzHigh Power consumptionof DSP and D/A-converterszPossible feedbackeliminationzCompensation of AM/PM-distortion possiblepeak detectorsupply voltageamplifierlimiterhigh efficiencypower amplifierRF outputpeak detectorRF inputDADADA amplitude informationphase informationmodulatorRF outputhigh efficiencypower amplifierdigitalsignalprocessorlocal oscillatorsupply voltage amplifierIQIQ digital baseband input48 EER (Envelope Elimination and Restoration)~Five times (!)

7 Oversamplingnecessary to achieve standard requirements~Bandwidth of Magnitude-and phase- signal have higher than transmit signal -30-20-100102030-60-50-40-30-20-10 010relative Power / dBrelative frequency / MHzMagnitudePhase -30-20-100102030-60-50-40-30-20-10010rel ative Power / dBrelative frequency / MHzACPR1>60dBACPR2>60dBACPR1=33dBACPR2=4 0dBACPR1=51dBACPR2=36dBACPR1=53dBACPR2=4 9dBfull bandwidth 3 B0 bandwidth5 B0 bandwidth7 B0 bandwidthUMTS example:UMTS example:49 Adaptive Bias~Varying/Switching of Bias-Voltage depending on Input Power Level~Selection of Operating Point with high PAE~Applicably for nearly each type of AmplifierRF inputpeak detectorbiascontrolRF outputhigh efficiencypower amplifier50 Adaptive Bias3233343536373839402030405060708090ou tput Power / dBmpower added efficiency / %VD= ~Single tone PAE for switched VDD with VGkept constant~Simply to implement Concept~Stability guaranteed~Possible problems.

8 ZDC-DC converter with high efficiency necessaryzPossible Linearity Change (can increase and decrease)especially forHCAs51 Summary~Digital Realization required to achieve Accuracy~Problem of Stability for high Bandwidth Application~Higher Bandwidths (Oversampling) necessary,depending on Order of IMD cancellation~Predistortion gives best Results while keeping Efficiency high (valid for high Output Levels > 40dBm)52 Figure References~F. Zavosh et al, Digital Predistortion Techniques for RF Power Amplifiers with CDMA Applications ,Microwave Journal, Oct. 1999~Peter B. Kenington, High-Linearity RF Amplifier Design ,ArtechHouse, 2000~Steve C. Cripps, RF Power Amplifiers for Wireless Communications ,Artech House, 199953 Contact InformationDI Markus Mayer +43-1-58801-35425 Holger Arthaber +43-1-58801-35420