comm d runt on pa symposium 11

7/30/2019 Comm d Runt on Pa Symposium 11

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Doherty Power Amplifier DesignDavid W. Runton, Michael D. LeFevre, Matthew K. Mellor

RFMD, Chandler, AZ, [email protected]


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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

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Doherty Design - Outline

Concept Introductions

Operational Fundamentals

The Functional Doherty Design Load Modulation

Empirical Doherty Design Example

Building the Doherty Amplifier

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2

3

4

5

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2

3

4

5

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The Traditional Balanced Amplifier

Both amplifier A1 and A2 contribute equally to Pout

Both have standard Efficiency vs. Pout characteristics

A2

A1

Pout

h

,DrainEfficien

cy

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The Doherty Amplifier

A1 operates most of the time - handles average signal

A2 operates only when peak power is needed

A1 and A2s operation is dependent on each other

Pout

h,

DrainEfficiency

A1

A2

Ideal - A1 + A2Carrier Amp

Peaking Amp

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1

2

3

4

5

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Operational Fundamentals Class A

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0 90 180 270 360 450 540 630 7200

0.5

1

1.5

2

2.5Waveforms

Voltage

Zero knee Current

Vknee = 0 Ibias = 0.5

VDC = 1.0 Isignal = 0.5

Vsignal(fund) = 1.0

Under basic loadline condition

*Reference [1]

= cos()

= cos( + )


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Operational Fundamentals Class B

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0 90 180 270 360 450 540 630 7200

0.5

1

1.5

2

2.5Waveforms

Voltage

Zero knee Current Load Resistor RLAdjust Input Drive for Max V

Vknee = 0 Ibias = 0

VDC = 1.0 Isignal = 1.0

Vsignal(fund) = 1.0

= 0 + 1 cos()

+ cos(2)

+ cos 3 +

= 1 cos()

The output waveforms must be

expanded into its Fourier series

components

Vds is simplified due to shortcircuited harmonics

*Reference [1]


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Operational Fundamentals Class B at half power

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Vknee = 0

VDC = 1

Ibias = 0

0 90 180 270 360 450 540 630 7200

0.5

1

1.5

2

2.5Waveforms

Voltage

Zero knee Current

Drive Signal -6dB

Efficiency Drops by 2*Reference [2]


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Operational Fundamentals Class B (Load Modulation)

Page 11

Vknee = 0

VDC = 1

Ibias = 0

0 90 180 270 360 450 540 630 7200

0.5

1

1.5

2

2.5Waveforms

Voltage

Zero knee Current

RL2xRLEfficiency Restored

*Reference [2]


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3

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Textbook Load Modulation

Doherty achieves Load

modulation by using the

principle of load pulling

using two devices*RLV

I1 I2

1

2

11

I

IRZ

L

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*Reference [3]

+

-+

-


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Textbook Load Modulation

LRZZ 221

RL+-

+-V

I1 I2

RL+

- V

I1

02 I

LRZ 1

Case I

Both amplifiers contributing equally

Case II

Peaking amp off

Page 14

*Reference [3]


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Car

Pk

4,

lengthZZ ODoherty

Carrier

Peaking Carrier

Peaking

4

length

4,2

length

Z

ZO

xfmr

Create a splitter

Wilkinson

Gysel

Hybrid

Doherty Topology Definitions

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The real implementation modulates Zo2xZo At the current source plane we want RL2xRL

How do we get this?

2xZO

2

OZ

Car

4,

lengthZZ ODoherty

Carrier

In package/PCB

Match

2xRL

Practical Circuit Load Modulation

+

-

I

High Power Low Power

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At the combiner node, we want Zpk = When the peaking amp is off

An additional phase shift can create this,

2

OZ

Car

4,

lengthZZ ODoherty

Carrier

Peaking

Pk

ZPeaking

Includes: In package

and PCB Match

Designing the Doherty Peaking off state

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DohertyThe Key to Operation or Why Doesnt it Work?

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0 VinVin

2

Imax

2

Imax

4

DeviceCurrent

Input Drive

0

Vmax

Vmax

2

DeviceVoltage

Input Drive

VinVin

2

No Clipping Allowed

*Reference [3]


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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

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GaN Device used for Design Example

RF IN

VGQ

Pin 1 (CUT)

RF OUT

VDQ

Pin 2

GND

BASE

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

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Being Statistically Realistic

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 00.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Doherty Efficiency, CW Case

Backoff (dB)

Efficiency(%)

1:1

1:1.51:2

1:2.5

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 00.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Doherty Efficiency, Modulated Case 7.5dB PAR

Backoff (dB)

Efficiency(%)

1:1

1:1.5

1:2

1:2.5

CHALLENGE: Design a symmetric Doherty Amplifier foradBmaverage power operation with pdB peak to average ratio

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Choosing the Load Conditions


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

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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

Ifp 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 ifpis 6dB (for the symmetric case)

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Car

Pk

Composite PoweradBmPower from each amp (a-3)dBm


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6

6

7

719.2

19.6

19.6

20

20

20

20.4

20.4

20.4

20.8

20.8

20.8

32

36

36

40

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40

44

44

44

48

48

(Z0ld1

)

(Z0ld1

)

4 6 8 10 12 14 16 18 20 22

2

4

6

8

10

12

14

16

PAR (prpl 6.6)()()

Gt_dB (prpl 20.3)()()Drain_eff (prpl 39.4)()()

Data Point (prpl 11.7+j6.9)()()

Load Contours: (a-3)dBm

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Car

Pk

Power from Carrier amp: adBm

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3

4

4

4

55

18.4

18.8

19.2

19.2

19.2

19.2

19.6

19

.6

19.6

19.6

20

20

20

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44

48

48

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52

56

56

56

60

(Z0ld1

)

(Z0ld1

)

6 8 10 12 14 16

4

6

8

10

12

14

16

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)()

Load Contours: adBm

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Static Tuning Reality sets in

Model the circuit

Tune under static conditions

Assume load modulation

ZO

Pkg/wires PCB Doherty xfmrCarrier

Pkg/wires PCB Doherty xfmrCarrier

2

OZ

ZHigh Power

2xZO

ZLow Power

a-3dBm

adBm

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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

Car

Pk

Carrier

Tuning Tips Carrier Amp

Car

Pk

Carrier

Option 1 Peaking Amp in place Option 2 Peaking Amp removed

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Car

Pk

Carrier

Peaking Carrier

Peaking

Tuning Tips Peaking Amp

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

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50% Drain Efficiency

(7.5dB PAR @ 0.01% CCDF)

Fully Linearizable with peak power recovery

15% bandwidth

Tuning Tips Putting it all together

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Broadband Performance and Reality

Performance is only as good as your load modulation bandwidth

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Summary

The Doherty Amplifier topology can provide efficiency

benefits

Implementation is full of pitfalls

Variants are many, based on the same concept

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Do You Have

Any Questions?

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References

[1] Colantonio, Giannini, Limiti, High Eff ic iency RF and Microwave Sol id State

Power Am pl i f iers, Wiley and Sons, 1999, p 49-82

[2] Cripps, S., Doherty RF Power Amplifiers, Theory and Practice, Shor t

Cou rse SC-4, 2009 International Microwave Symposium, Boston

[3] Cripps, S., RF Power Ampl i f iers for Wireless Communicat ions, Artech

House, 1999, p 225-235

comm d runt on pa symposium 11

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