RF Power Amplifier Design Markus Mayer & Holger Arthaber Department of Electrical Measurements and Circuit Design Vienna University of Technology June 11, 2001
Contents � Basic Amplifier Concepts � Class A, B, C, F, hHCA � Linearity Aspects � Amplifier Example � Enhanced Amplifier Concepts � Feedback, Feedforward, ... � Predistortion � LINC, Doherty, EER, ... 2
Efficiency Definitions P η = OUT � Drain Efficiency: D P DC − − P P 1 � Power Added Efficiency: η = = η ⋅ OUT IN 1 PA D P G DC 3
Ideal FET Input and Output Characteristics I DS V =0 GS I m g m V =V GS P V GS V DS 2V P V P 0 0 V K V DD V DSmax Ohmic Saturation Breakdown − V V κ = DD K V DD 4
Maximum Output Power Match I DS V =0 GS I m g m V =V GS P V GS V DS 2V P V P 0 0 V K V DD V DSmax Ohmic Saturation Breakdown − V V = R DS max K OPT I m 5
Class A I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 6
Class A – Circuit V DD D G R L S η = κ ⋅ 50% D = G G (e.g. 14 dB) A η = κ ⋅ 48% PA 7
Class B I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 8
Class C I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 9
Class B and C – Circuit V DD f 0 D G R L S Class B Class C η = κ ⋅ η → 78 % 100 % D D = → G G - 6dB (8 dB) G 1 A η = κ ⋅ η PA → 0 % 65 % PA 10
Influence of Conduction Angle 11
Class F (HCA ... harmonic controlled amplifier ) I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 12
hHCA (half sinusoidally driven HCA) I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 13
Class F and hHCA – Circuit V DD Z o (n) 0, n=1 I D inf, n=odd Z e (n) V DS R L 0, n=even inf, n=even Class F hHCA η = κ ⋅ η = κ ⋅ 10 0% 10 0% D D = = + G G - 5dB (9 dB) G G 1dB (15 dB) A A η = κ ⋅ η = κ ⋅ 87 % 96 % PA PA 14
hHCA – Third Harmonic Peaking I DS I DS I m I m V GS V DS Q 2V P V P 0 0 V K V DD V DSmax 0 p 2 p V GS V DS p 2 p Q 15
Third Harmonic Peaking – Circuit V DD 3f 0 D G f 0 R L S η = κ ⋅ 91% D = + G G 0.6dB (14.6 dB) A η = κ ⋅ 87 % PA 16
Linearity Aspects 17
Linearity Aspects � Class A � Class AB � Class B � Class C 18
Linearity Aspects � Ideal strongly nonlinear model � Strong-weak nonlinear model 19
Amplifier Design – An Example � Balanced Amplifier Configuration Port 1 Z=50 Ohm Port 2 Z=50 Ohm 20
Amplifier Design – Simulation � Gate & Drain Waveforms Gate waveforms Drain waveforms 1 1000 25 5000 Inner Drain Voltage (L, V) Inner Drain Current (R, mA) Amp Amp 20 4000 0 500 15 3000 -1 0 10 2000 5 1000 -2 -500 0 0 Inner Gate Voltage (L, V) Inner Gate Current (R, mA) Amp Amp -3 -1000 -5 -1000 0 500 1000 1300 0 500 1000 1300 Time (ps) Time (ps) 21
Amplifier Design – Simulation � Dynamic Load Line & Power Sweep Dynamic load line Power Sweep 1 Tone 8000 40 80 IVCurve (mA) Output Power (L, dBm) IV_Curve 70 Amp 6000 Dynamic Load Line (mA) PAE (R) 30 60 Amp Amp 50 4000 20 40 2000 30 10 20 0 10 -2000 0 0 0 3 6 9 12 15 0 5 10 15 20 24 Voltage (V) Power (dBm) 22
Amplifier Design – Measurements � Single Tone & Two Tone 60 60 P A E [% ] 4 0 8 0 P A E [% ] 1 d B C P 3 5 7 0 50 50 3 0 6 0 P out [dBm], IMDD [dBc], Gain [dB] 40 40 P out [dBm], Gain [dB] 2 5 5 0 P out P o u t IM D D G a in 30 30 2 0 4 0 G ain G a m m a In P A E P A E 1 5 3 0 20 20 1 0 2 0 10 10 5 1 0 0 0 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 0 5 10 15 20 25 30 35 P in [d B m ] P in [dB m ] 23
Amplifier Nonlinearity � Gain and Phase depends on Input Signal � 3 rd Order Gain-Nonlinearities: 24
Amplifier Nonlinearity � Higher Output Level (close to Saturation) results in more Distortion/Nonlinearity 25
Nonlinearity leads to? � Generation of Harmonics � Intermodulation Distortion / Spectral Regrowth � SNR (NPR) Degradation � Constellation Deformation 26
Intermodulation and Harmonics 27
Spectral Regrowth 10 ACPR 1 >60dB ACPR 2 >60dB 0 ACPR 1 =16dB ACPR 2 =43dB -10 relative power / dB -20 -30 -40 -50 -60 -15 -10 -5 0 5 10 15 relative frequency / MHz � Energy in adjacent Channels � ACPR (Adjacent Channel Leakage Power Ratio) increases 28
Reduced NPR (Noise Power Ratio) � Input Signal � Output Signal of Nonlinear Amplifier � Degradation of Inband SNR � „Noisy“ Constellation 29
Constellation Deformation � Input Signal � Output Signal of Nonlinear Amplifier (with Gain- and Phase-Distortion) 30
Modeling of Nonlinearities � with Memory-Effects � Volterra Series (=„Taylor Series with Memory“) � without Memory-Effects α α 2 r r performance = = Θ � Saleh Model a f ( r ) g ( r ) + β + β 2 2 1 r 1 r better Θ a � Taylor Series � Blum and Jeruchim Model � AM/AM- and AM/PM-conversion 31
AM/AM- and AM/PM-Conversion � GaAs-PA 32
AM/AM- and AM/PM-Conversion � LDMOS-PA 33
How to preserve Linearity? � Backed-Off Operation of PA � Simplest Way to achieve Linearity � Linearity improving Concepts � Predistortion � Feedforward � ... 34
How to preserve Efficiency? � Efficiency improving Concepts � Doherty � Envelope Elimination and Restoration � ... � Linearity improving Concepts � Higher Linearity at constant Efficiency � Higher Efficiency at constant Linearity 35
Direct (RF) Feedback � Classical Method � Decrease of Gain � Low Efficiency � Feedback needs more Bandwidth than Signal � Stability Problems at high Bandwidths 36
Distortion Feedback � Feedback of outband Products only � Higher Gain than RF feedback � Stability Problems due to Reverse Loop 37
Feedforward � Overcomes Stability Problem by forward-only Loops � Critical to Gain/Phase-Imbalances 0.5dB Gain Error � -31dB Cancellation 2.5° Phase Error � -27dB Cancellation � Well suited for narrowband application 38
Cartesian Feedback baseband input I modulator main amp. I RF-output OPAs Q Q local UMTS example : oscillator 10 original signal I predistorted signal 0 Q demodulator -10 relative power / dB -20 � AM/AM- and -30 AM/PM-correction -40 � High Feedback-Bandwidth -50 � Stability Problems -60 -30 -20 -10 0 10 20 30 relative frequency / MHz 39
Digital Predistortion � Digital Implementation of „Cartesian Feedback“ � Additional ADCs, DSP Power, Oversampling needed � Loop can be opened � no Stability Problems 40
Analog Predistortion � Predistorter has inverse Function of Amplifier � Leads to infinite Bandwidth (!) � Hard to realize (accuracy) 41
Analog Predistortion � Possible Realizations: 42
LINC (Linear Amplification by Nonlinear Components) s (t) Ks (t) 1 1 K K(s (t)+ s (t)) 1 2 signal s(t) =Ks(t) separation s (t) Ks (t) 2 2 K UMTS example : 10 � AM/AM- and ACPR 1 >60dB s(t) ACPR 2 >60dB AM/PM-correction 0 s 1 (t) ACPR 1 =18dB ACPR 2 =29dB � Digital separation required -10 relative power / dB (accuracy!) -20 � High Bandwidth, -30 oversampling necessary -40 � Stability guaranteed -50 -60 -30 -20 -10 0 10 20 30 relative frequency / MHz 43
Doherty Amplifier � Auxiliary amplifier supports main amplifier during saturation � PAE can be kept high over a 6dB range 44
Doherty Amplifier � Gain vs. Input Power � Efficiency vs. Input Power P OUT doherty configuration (A1+A2) main amp. (A1) aux. amp. (A2) P IN � No improvement of AM/AM- and AM/PM-distortion � Behavior of auxiliary amplifier very hard (impossible) to realize � Stability guaranteed 45
EER (Envelope Elimination and Restoration) � Separating phase and magnitude information � Elimination of AM/AM-distortion � Application of high-efficient amplifiers (independent of amplitude distortion) � Stability guaranteed amplitude information RF input signal separation phase information RF output high efficiency power amplifier 46
EER (Envelope Elimination and Restoration) supply voltage � Analog realization peak detector amplifier � Limiter hard to build � Accuracy problems limiter � Feedback necessary RF input RF output high efficiency peak detector power amplifier � Digital realization � Oversampling + high D/A- amplitude information conversion rates required supply voltage amplifier D digital baseband input � High power consumption A I of DSP and D/A-converters digital D modulator signal � Possible feedback A Q I RF output processor Q elimination D high efficiency � Compensation of AM/PM- A power amplifier phase information distortion possible local oscillator 47
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