# Realizing Doherty Power Amplifier Designs

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```					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

 I2 
Z1  RL 1  
 I 
    1 

I1                I2                  modulation by using the principle
+                           +              devices*
-         V       RL        -

*For more information see: Steve Cripps, “RF Power Amplifiers for Wireless Communications” and “Advanced Techniques in RF
Power Amplifier Design”

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”

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)

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

CHALLENGE: Design a symmetric Doherty Amplifier for adBm average
power operation with pdB peak to average ratio
• Break the challenge into two static cases
• 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

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

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

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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 views: 8 posted: 10/28/2013 language: English pages: 19