Realizing Doherty Power Amplifier Designs David Runton, Michael LeFevre, Christopher Burns [email protected] Introduction • What can be said that hasn’t been said before? • Doherty is old news! • PA suppliers are getting very nearly equal results • “Optimizations”/“tweaks” are simply exploiting tradeoffs • How do we put it all together? • And most importantly, do it quickly… Outline • Textbook Doherty Design Principles • Definition of Terms • The “Classic” Concept • Empirical Doherty Design • Selection of tuning points • Building the Doherty Amplifier • Tuning Tips Doherty Topology – Definitions Create a splitter • Wilkinson Peaking • Gysel Carrier Car • Hybrid length Z Doherty Z O , length 4 Carrier Pk Peaking Z xfmr 4 ZO , length 4 2 Textbook Load Modulation I2 Z1 RL 1 I1 I1 + - I2 V RL + - • Doherty achieves Load modulation by using the principle of “load pulling” using two devices* *For more information see: Steve Cripps, “RF Power Amplifiers for Wireless Communications” and “Advanced Techniques in RF Power Amplifier Design” Textbook Load Modulation Case I Case II Both amplifiers contributing equally Peaking amp off I1 + - I2 V RL I1 + - + - V RL I2 0 Z1 RL Z1 Z 2 2 RL *For more information see: Steve Cripps, “RF Power Amplifiers for Wireless Communications” and “Advanced Techniques in RF Power Amplifier Design” Practical Circuit Load Modulation 2xZO Car I + - Carrier 2xRL Z Doherty Z O , length ZO 2 In package/PCB Match High Power • The real implementation modulates Zo→2xZo • At the current source plane we want RL→2xRL • How do we get this? Low Power 4 Doherty Topologies • There is no differentiation between standard and inverted Doherty topologies • The Point of a Doherty amplifier is load modulation • how you achieve target impedances is irrelevant LET THE FLAMING BEGIN!! Being Statistically Realistic CHALLENGE: Design a symmetric Doherty Amplifier for adBm average power operation with dB peak to average ratio Doherty Efficiency, Modulated Case 7.5dB PAR 0.8 0.7 0.7 0.6 0.6 Efficiency (%) Efficiency (%) Doherty Efficiency, CW Case 0.8 0.5 0.4 0.3 0.1 -20 -18 -16 -14 -12 -10 -8 Backoff (dB) -6 -4 -2 0.4 1:1 1:1.5 1:2 1:2.5 0.3 1:1 1:1.5 1:2 1:2.5 0.2 0.5 0.2 0 0.1 -20 -18 -16 -14 -12 -10 -8 Backoff (dB) -6 -4 -2 0 Choosing the Load Conditions CHALLENGE: Design a symmetric Doherty Amplifier for adBm average power operation with pdB peak to average ratio • To achieve the best efficiency, we need: • Pout = dBm composite power (full peak power) • Full contribution of peak power from each amplifier • Pout = (dBm • Carrier amplifier is fully saturated and acting as a pure current source • Peaking amplifier is just about to turn on • (dBm > Pout > (dBm • Carrier amplifier maintains saturation without clipping • Peaking amplifier is “load modulating” the carrier amplifier Choosing the Load Conditions CHALLENGE: Design a symmetric Doherty Amplifier for adBm average power operation with pdB peak to average ratio • Break the challenge into two static cases • At adBm composite power • Each amplifier is functioning at (a-3)dBm • Full addition of power from carrier and peaking amp recreating all peaks • Amplifier must not clip (with linearization?) • At slightly < adBm composite power • If is 6dB • Carrier amplifier is functioning < adBm and is fully saturated (high efficiency) • If the peaking amplifier is off, this represents the best case efficiency • Be careful if is ≠6dB (for the symmetric case) Composite Power dBm Power from each amp ()dBm Car Pk Load Contours ()dBm 16 PAR (prpl 6.6)()() Gt_dB (prpl 20.3)()() Drain_eff (prpl 39.4)()() Data Point (prpl 11.7+j6.9)()() 20 .8 48 5 14 48 44 5 4 20 . 10 40 .8 20 20 .8 644 6 44 20 .4 20 6 8 20 .4 6 36 4 7 (Z0ld1) 12 40 20 19 . 6 7 19 .2 2 40 20 19 .6 36 32 4 6 8 10 12 14 (Z0ld1) 16 18 20 22 Power from Carrier amp - dBm Car Pk Load Contours dBm PAR (prpl 4.9)(blk 4.2)() Gt_dB (prpl 19.8)(blk 20.0)() Drain_eff (prpl 50.8)(blk 55.4)() Data Point (prpl 11.7+j6.9)(blk 12.6+j10.0)() 16 18 .4 60 18 .8 14 3 56 19 .6 19 .2 .2 19 12 19 .6 10 56 20 20 (Z0ld1) 52 4 4 20 4 56 52 .2 19 44 8 48 5 6 19 .6 19.6 44 48 19 . 2 4 6 8 52 5 10 12 (Z0ld1) 14 16 Static Tuning – Reality sets in Pkg/wires PCB Carrier Doherty xfmr ZHigh Power dBm ZO Pkg/wires PCB Carrier Doherty xfmr ZLow Power dBm 2xZO ZO 2 • Model the circuit • Tune under static conditions • Assume load modulation 16 Tuning Tips – Carrier Amp Option 1 – Peaking Amp in place Option 2 – Peaking Amp removed Carrier Carrier Car Car Pk Pk • The Carrier Amp is where it all happens! • We want no Clipping at full power with Zo impedance • Saturation with peaking amplifier off • Must make assumptions about peaking amp and its ability to load modulate 17 Tuning Tips – Peaking Amp Peaking Carrier Car Carrier Pk Peaking • Set the off-state Z of peaking amp with • Is this really so important Peaking • Can we find some advantage not to set the off-state to ideal? • Conventional wisdom says equal phase in each branch • Class-C peaking amp has large AM-PM component • Where do we want phase alignment? It Can Work! • 50% Drain Efficiency (7.5dB PAR @ 0.01% CCDF) • Fully Linearizable with peak power recovery • 15% bandwidth