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Doherty Power Amplifier Design
David W. Runton, Michael D. LeFevre, Matthew K. Mellor
RFMD, Chandler, AZ, [email protected]
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  2
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  3
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  4
The Traditional Balanced Amplifier
• Both amplifier A1 and A2 contribute equally to Pout
h, Drain Efficiency
• Both have standard Efficiency vs. Pout characteristics
A2
A1
Pout
Page  5
The Doherty Amplifier
• A1 operates most of the time - handles average signal
• A2 operates only when peak power is needed
h, Drain Efficiency
• A1 and A2’s operation is dependent on each other
Ideal - A1 + A2
Carrier Amp
A1
A2
Peaking Amp
Pout
Page  6
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  7
Operational Fundamentals – Class A
Waveforms
2.5
Under basic loadline condition
Voltage
Zero knee Current
2
𝑖𝐷 𝑡 = 𝐼𝑃 ∙ cos(𝜔𝑡)
1.5
𝑣𝐷𝑆 𝑡 = 𝑉𝑃 ∙ cos(𝜔𝑡 + 𝜑)
1
0.5
0
0
90
180
270
360
450
540
630
Vknee = 0
Ibias = 0.5
VDC = 1.0
Isignal = 0.5
Vsignal(fund) = 1.0
*Reference [1]
Page  8
720
Operational Fundamentals – Class B
Waveforms
2.5
Load Resistor – RL
Adjust Input Drive for Max V
Voltage
Zero knee Current
2
The output waveforms must be
expanded into its Fourier series
components
1.5
1
𝑖𝐷 𝑡 = 𝐼0 + 𝐼1 ∙ cos(𝜔𝑡)
+ 𝐼2 ∙ cos(2𝜔𝑡)
+ 𝐼3 ∙ cos 3𝜔𝑡 + ⋯
0.5
0
0
90
180
270
360
450
540
630
Vknee = 0
Ibias = 0
VDC = 1.0
Isignal = 1.0
Vsignal(fund) = 1.0
720
Vds is simplified due to short
circuited harmonics
𝑣𝐷𝑆 𝑡 = 𝑉𝐷𝐶 − 𝑉1 ∙ cos(𝜔𝑡)
*Reference [1]
Page  9
Operational Fundamentals – Class B at half power
Waveforms
2.5
Voltage
Zero knee Current
2
1.5
1
0.5
0
0
90
180
Vknee = 0
VDC = 1
Ibias = 0
270
360
450
540
630
720
Drive Signal → -6dB
Efficiency Drops by 2
*Reference [2]
Page  10
Operational Fundamentals – Class B (Load Modulation)
Waveforms
2.5
Voltage
Zero knee Current
2
1.5
1
0.5
0
0
90
180
Vknee = 0
VDC = 1
Ibias = 0
270
360
450
540
630
720
RL→2xRL
Efficiency Restored
*Reference [2]
Page  11
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  12
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*
*Reference [3]
Page  13
Textbook Load Modulation
Case I
Both amplifiers contributing equally
I1
I2
+
-
V
RL
+
-
Case II
Peaking amp off
I1
+
-
Z1  Z 2  2RL
*Reference [3]
Page  14
V
RL
I2  0
Z1  RL
Doherty Topology – Definitions
Create a splitter
• Wilkinson
• Gysel
• Hybrid
length 
 Peaking
 Carrier
Car

Z Doherty  Z O , length 
4
 Carrier
Pk
Page  15
 Peaking
Z xfmr

4
ZO


, length 
4
2
Practical Circuit Load Modulation
2xZO
Car
I
+
-
 Carrier
Z Doherty  Z O , length 
2xRL
In package/PCB
Match
ZO
2
High Power
Low Power
• The real implementation modulates Zo→2xZo
• At the current source plane we want RL→2xRL
• How do we get this?
Page  16

4
Designing the Doherty – Peaking off state
Car
Pk
 Carrier
 Peaking
Includes: In package
and PCB Match
Z Doherty  Z O , length 
Z 
ZO
2
• At the combiner node, we want Zpk = ∞
• When the peaking amp is off
• An additional phase shift can create this,  Peaking
Page  17

4
Doherty – The Key to Operation or Why Doesn’t it Work?
No Clipping Allowed
Imax
Vmax
Device Voltage
Device Current
2
Imax
4
0
Vin
Vin
Vmax
2
0
Vin
2
2
Input Drive
Input Drive
*Reference [3]
Page  18
Vin
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
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  20
GaN Device used for Design Example
Features
 Advanced GaN HEMT Technology

Peak Modulated Power > 240W

Single Circuit for 865 – 960MHz

48V Operation Typical Performance
o
Pout 47dBm
o
Gain 20dB
o
Drain Efficiency 39%
o
ACP -31.5dBc
o
Linearizable to -55dBc with DPD

Optimized for video bandwidth and minimized
memory effects

RF tested for 3GPP performance

RF tested for peak power using IS95

Large signal models available
RF IN
VGQ
Pin 1 (CUT)
RF OUT
VDQ
Pin 2
GND
BASE
Page  21
Being Statistically Realistic
CHALLENGE: Design a symmetric Doherty Amplifier for adBm
average power operation with pdB 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
-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
Backoff (dB)
-18
-16
-14
-12
-10
-8
Backoff (dB)
Page  22
-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 = apdBm composite power (full peak power)
• Full contribution of peak power from each amplifier
• Pout = (ap-6)dBm
• Carrier amplifier is fully saturated
• Peaking amplifier is just about to turn on
• (ap-6)dBm > Pout > (ap)dBm
• Carrier amplifier maintains saturation without clipping
• Peaking amplifier is “load modulating” the carrier amplifier
Page  23
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  24
Choosing the Load Conditions
Composite Power adBm
Power from each amp (a-3)dBm
Car
Pk
Page  25
Load Contours: (a-3)dBm
RFG1M09050 880MHz Pout=41.6dBm
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
.8
20
40
20
.8
644
6
44
20
.4
20
6
8
20 .4
6
36
7
 (Z0ld1)
12
4
40
20
19 .
6
7
19
.2
2
40
20
19 .6
36
32
4
6
8
10
12
14
(Z0ld1)
Page  26
16
18
20
22
Power from Carrier amp: adBm
Car
Pk
Page  27
Load Contours: adBm
RFG1M09050 880MHz Pout=44.1dBm
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
3
18 .8
14
19 .2
56
19 .6
.2
19
12
19
.6
56
20
10
20
(Z0ld1)
52
4
4
56
20
4
52
.2
19
44
8
48
5
6
19
.6
19.6
44
48
19 .
2
4
6
8
52
5
10
12
(Z0ld1)
Page  28
14
16
Doherty Design - Outline
1
Concept Introductions
2
Operational Fundamentals
3
The Functional Doherty Design – Load Modulation
4
Empirical Doherty Design Example
5
Building the Doherty Amplifier
Page  29
Static Tuning – Reality sets in
Pkg/wires
PCB
 Carrier
Doherty xfmr
ZHigh Power
a-3dBm
ZO
Pkg/wires
PCB
 Carrier
Doherty xfmr
ZLow Power
adBm
2xZO
• Model the circuit
• Tune under static conditions
• Assume load modulation
Page  30
ZO
2
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
Page  31
Tuning Tips – Peaking Amp
 Peaking
 Carrier
Car
 Carrier
Pk
 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?
Page  32
Tuning Tips – Putting it all together
• 50% Drain Efficiency
• (7.5dB PAR @ 0.01% CCDF)
• Fully Linearizable with peak power recovery
• 15% bandwidth
Page  33
Broadband Performance and Reality
• Performance is only as good as your load modulation “bandwidth”
Page  34
Summary
• The Doherty Amplifier topology can provide efficiency
benefits
• Implementation is full of pitfalls
• Variants are many, based on the same concept
Page  35
Do You Have
Any Questions?
Page  36
References
[1] Colantonio, Giannini, Limiti, High Efficiency RF and Microwave Solid State
Power Amplifiers, Wiley and Sons, 1999, p 49-82
[2] Cripps, S., “Doherty RF Power Amplifiers, Theory and Practice”, Short
Course SC-4, 2009 International Microwave Symposium, Boston
[3] Cripps, S., RF Power Amplifiers for Wireless Communications, Artech
House, 1999, p 225-235
Page  37