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Realizing Doherty Power Amplifier Designs

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Realizing Doherty  Power Amplifier Designs Powered By Docstoc
					Realizing Doherty
Power Amplifier Designs
David Runton, Michael LeFevre, Christopher Burns
drunton@rfmd.com
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          Carrier
 • Gysel
                                       Car
 • Hybrid
                                                                                     
            length                                      Z Doherty  Z O , length 
                       4                                                              4

                                       Pk
                            Carrier          Peaking               ZO            
                                                          Z xfmr       , length 
                                                                      2            4
 Textbook Load Modulation




                                  I2 
                         Z1  RL 1  
                                  I 
                                     1 

                                                              • Doherty achieves Load
                          I1                I2                  modulation by using the principle
                                                                of “load pulling” using two
                     +                           +              devices*
                     -         V       RL        -




*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
                                                                               I1
                +                           +
                          V      RL                                        +                          I2  0
                -                           -                                               RL
                                                                           -        V
                                                                                                      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
                                Carrier                                       
                                                  Z Doherty  Z O , length 
                        2xRL                                                   4
                  I
              +                                       ZO
              -         In package/PCB
                                                       2
                             Match
                                                   High Power   Low Power

  • The real implementation modulates Zo→2xZo
   • At the current source plane we want RL→2xRL

  • How do we get this?
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, CW Case                                               Doherty Efficiency, Modulated Case 7.5dB PAR
                 0.8                                                                               0.8


                 0.7                                                                               0.7


                 0.6                                                                               0.6
Efficiency (%)




                                                                                  Efficiency (%)
                 0.5                                                                               0.5


                 0.4                                                                               0.4


                 0.3                                                                               0.3                                                   1:1
                                                                      1:1
                                                                                                                                                         1:1.5
                                                                      1:1.5                                                                              1:2
                 0.2                                                  1:2                          0.2                                                   1:2.5
                                                                      1:2.5

                 0.1                                                                               0.1
                   -20   -18   -16   -14   -12   -10   -8   -6   -4   -2      0                      -20    -18   -16   -14   -12   -10   -8   -6   -4   -2      0
                                            Backoff (dB)                                                                       Backoff (dB)
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

                                                                                                                      PAR (prpl 6.6)()()
              16
                                                                                                                      Gt_dB (prpl 20.3)()()
                                                          20 .8
                                                  48                                                                  Drain_eff (prpl 39.4)()()
                                                                                   5
              14
                                                                                                                      Data Point (prpl 11.7+j6.9)()()
                                                                                              48
                                        44


              12
                                        5
                                                                                                .8
                                                                                             20
                            20 .




                                                  20
                                             40      .8                            4
                                                                                   64
                                4
   (Z0ld1)




              10
                                                                                                                            6
                                                                                                                            44
                       20




                                                                                                                      .4
                                    6                                                                              20
              8
                                                                          20 .4         40

              6                                             36
                                                                  20
                                                                                                              20                40
                                                          19 .
                                                              6
              4
                                                   7




                                                                  19                           7      19 .6
                                                                     .2                              36
              2                                                   32

                   4            6                 8                10             12           14             16           18        20       22
                                                                                        (Z0ld1)
Power from Carrier amp - dBm




                        Car




                        Pk
Load Contours dBm

                                                                                          PAR (prpl 4.9)(blk 4.2)()
              16                                                                          Gt_dB (prpl 19.8)(blk 20.0)()
                                                    18 .4       60                        Drain_eff (prpl 50.8)(blk 55.4)()
                                                    18 .8
                                                                       3                  Data Point (prpl 11.7+j6.9)(blk 12.6+j10.0)()
              14                                                                                  19 .2
                                                                                         19 .6
                                     56
                                          .2
                                       19
              12
                                52




                                                                                                                                 4
                                                                                     4           56
                                        .6
   (Z0ld1)




                                                                           20
                                     19




              10




                                                                                                                20
                                                                                                                                     56
                                                                      20
                            4
                                                                                 52
              8
                   44




                                               48
                            19
                               .2




                                                            5                                                               52
                                                                     19                                          5
                                                                        .6
              6
                                                                                                                     19.6
                                                44
                                                                                                      48
                                                                             19 .
                                                                                 2
              4



                        6                      8                              10                           12          14                 16
                                                                                           (Z0ld1)
Static Tuning – Reality sets in


                   Pkg/wires      PCB    Carrier   Doherty xfmr
    ZHigh Power
     dBm                                                        ZO

                   Pkg/wires      PCB    Carrier   Doherty xfmr
    ZLow Power
      dBm                                                         ZO
                                                    2xZO            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




                                        Pk
                             Carrier          Peaking
 • Set the off-state Z of peaking amp with                 Peaking
  • 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?
It Can Work!




• 50% Drain Efficiency (7.5dB PAR @
  0.01% CCDF)
• Fully Linearizable with peak power
  recovery
• 15% bandwidth

				
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posted:10/28/2013
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