Doherty Power Amplifier Design. David W. Runton, Michael D. LeFevre, Matthew K. Mellor RFMD, Chandler, AZ,

Transcription

1 Doherty Power Amplifier Design David W. Runton, Michael D. LeFevre, Matthew K. Mellor RFMD, Chandler, AZ,

2 Introduction Intentions With high peak to average ratio signals in full use in the commercial world and expanding in the military world, how do we efficiently amplify these signals? 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 Page

3 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 3

4 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 4

5 h, Drain Efficiency The Traditional Balanced Amplifier Both amplifier A1 and A contribute equally to Pout Both have standard Efficiency vs. Pout characteristics A Pout A1 Page 5

6 h, Drain Efficiency The Doherty Amplifier A1 operates most of the time - handles average signal A operates only when peak power is needed A1 and A s operation is dependent on each other Ideal - A1 + A Carrier Amp A1 A Pout Peaking Amp Page 6

7 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 7

8 Operational Fundamentals Class A.5 Waveforms Voltage Under basic loadline condition Zero knee Current 1.5 i D t = I P cos (ωt) 1 v DS t = V P cos (ωt + φ) V knee = 0 I bias = 0.5 V DC = 1.0 I signal = 0.5 V signal(fund) = 1.0 *Reference [1] Page 8

9 Operational Fundamentals Class B.5 Waveforms Voltage Zero knee Current Load Resistor R L Adjust Input Drive for Max V The output waveforms must be expanded into its Fourier series components i D t = I 0 + I 1 cos (ωt) + I cos (ωt) + I 3 cos 3ωt V knee = 0 I bias = 0 V DC = 1.0 I signal = 1.0 V signal(fund) = 1.0 Vds is simplified due to short circuited harmonics v DS t = V DC V 1 cos (ωt) *Reference [1] Page 9

10 Operational Fundamentals Class B at half power.5 Waveforms Voltage Zero knee Current V knee = 0 V DC = 1 I bias = 0 Drive Signal -6dB Efficiency Drops by *Reference [] Page 10

11 Operational Fundamentals Class B (Load Modulation).5 Waveforms Voltage Zero knee Current V knee = 0 V DC = 1 I bias = 0 R L xr L Efficiency Restored *Reference [] Page 11

12 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 1

13 Textbook Load Modulation + - I Z 1 RL 1 I 1 I 1 I V R L + - Doherty achieves Load modulation by using the principle of load pulling using two devices* *Reference [3] Page 13

14 Textbook Load Modulation Case I Both amplifiers contributing equally Case II Peaking amp off I 1 I + - V R L V R L I 1 I 0 Z1 R L Z Z 1 R L *Reference [3] Page 14

15 Doherty Topology Definitions Create a splitter Wilkinson Gysel Hybrid Peaking Car Carrier length 4 Z Doherty Z O, length 4 Carrier Pk Peaking Z xfmr Z O, length 4 Page 15

16 Practical Circuit Load Modulation xz O Car I xr L Carrier Z Doherty Z O, length In package/pcb Match Z O The real implementation modulates Z o xz o High Power Low Power At the current source plane we want R L xr L How do we get this? Page 16

17 Designing the Doherty Peaking off state Car Carrier Z Doherty Z O, length 4 Pk Peaking Includes: In package and PCB Match Z Z O At the combiner node, we want Z pk = When the peaking amp is off An additional phase shift can create this, Peaking Page 17

18 Device Current Device Voltage Doherty The Key to Operation or Why Doesn t it Work? I max V max No Clipping Allowed I max 4 V max 0 V in V in 0 V in V in Input Drive Input Drive *Reference [3] Page 18

19 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 Page 19

20 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 0

21 GaN Device used for Design Example Features Advanced GaN HEMT Technology Peak Modulated Power > 40W Single Circuit for MHz 48V Operation Typical Performance o Pout 47dBm o Gain 0dB o Drain Efficiency 39% o ACP -31.5dBc o Linearizable to -55dBc with DPD Optimized for video bandwidth and minimized memory effects RF IN VGQ Pin 1 (CUT) RF OUT VDQ Pin RF tested for 3GPP performance RF tested for peak power using IS95 GND BASE Large signal models available Page 1

22 Efficiency (%) Efficiency (%) Being Statistically Realistic CHALLENGE: Design a symmetric Doherty Amplifier for adbm average power operation with pdb peak to average ratio 0.8 Doherty Efficiency, CW Case 0.8 Doherty Efficiency, Modulated Case 7.5dB PAR :1 1:1.5 1: 1: :1 1:1.5 1: 1: Backoff (db) Backoff (db) Page

23 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 = apdbm composite power (full peak power) Full contribution of peak power from each amplifier Pout = (ap-6)dbm Carrier amplifier is fully saturated Peaking amplifier is just about to turn on (ap-6)dbm > Pout > (ap)dbm Carrier amplifier maintains saturation without clipping Peaking amplifier is load modulating the carrier amplifier Page 3

24 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 At slightly < adbm composite power If p 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 p is 6dB (for the symmetric case) Page 4

25 Choosing the Load Conditions Composite Power adbm Power from each amp (a-3)dbm Car Pk Page 5

26 Load Contours: (a-3)dbm RFG1M MHz Pout=41.6dBm PAR (prpl 6.6)()() Gt_dB (prpl 0.3)()() Drain_eff (prpl 39.4)()() Data Point (prpl 11.7+j6.9)()() (Z0 ld1 ) (Z0 ld1 ) Page 6

27 Power from Carrier amp: adbm Car Pk Page 7

28 Load Contours: adbm RFG1M MHz Pout=44.1dBm PAR (prpl 4.9)(blk 4.)() Gt_dB (prpl 19.8)(blk 0.0)() Drain_eff (prpl 50.8)(blk 55.4)() Data Point (prpl 11.7+j6.9)(blk 1.6+j10.0)() (Z0 ld1 ) (Z0 ld1 ) Page 8

29 Doherty Design - Outline Concept Introductions Operational Fundamentals The Functional Doherty Design Load Modulation Empirical Doherty Design Example Building the Doherty Amplifier Page 9

30 Static Tuning Reality sets in Pkg/wires PCB Carrier Doherty xfmr Z High Power a-3dbm Z O Pkg/wires PCB Carrier Doherty xfmr Z Low Power adbm Model the circuit Tune under static conditions Assume load modulation xz O Z O Page 30

31 Tuning Tips Carrier Amp Option 1 Peaking Amp in place Option Peaking Amp removed Car Carrier Car Carrier 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 Page 31

32 Tuning Tips Peaking Amp Peaking Car Carrier Carrier Pk Peaking Set the off-state Z of peaking amp with Is this really so important 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? Peaking Page 3

33 Tuning Tips Putting it all together 50% Drain Efficiency (7.5dB 0.01% CCDF) Fully Linearizable with peak power recovery 15% bandwidth Page 33

34 Broadband Performance and Reality Performance is only as good as your load modulation bandwidth Page 34

35 Summary The Doherty Amplifier topology can provide efficiency benefits Implementation is full of pitfalls Variants are many, based on the same concept Page 35

36 Do You Have Any Questions? Page 36

37 References [1] Colantonio, Giannini, Limiti, High Efficiency RF and Microwave Solid State Power Amplifiers, Wiley and Sons, 1999, p 49-8 [] Cripps, S., Doherty RF Power Amplifiers, Theory and Practice, Short Course SC-4, 009 International Microwave Symposium, Boston [3] Cripps, S., RF Power Amplifiers for Wireless Communications, Artech House, 1999, p 5-35 Page 37