Power Amplifier Efficiency2 day Master Class

Rick Campbell PhDPortland State University

References and Acknowledgements

Steve C. Cripps, RF Power Amplifiers for Wireless Communications, 2nd edition, Artech House 2006

Many ongoing conversations with Frederick Raab, Steve Cripps, and Wes Hayward since 1996

Wes Hayward, Rick Campbell, and Bob Larkin, Experimental Methods in RF Design, ARRL 2003

Herbert Krauss, Charles Bostian, and Frederick Raab, Solid State Radio Engineering, Wiley 2000

textbook:

useful references

acknowledgement

Class Outline:

Definitions and Fundamentals: Class A, B, C, DMorning Day 1

Switches and Waveforms: Class E and Class FAfternoon Day 1

New Developments: Class J, interstage design, drive Morning Day 2

Detailed Study of Current Design ExamplesAfternoon Day 2

Definitions and Fundamentals

Total RF Power OutputEfficiency

Total DC Power Input

Communications EffectivenessEfficiency

Handset Battery Life

Useful Information TransferEfficiency

Impact on Planet Earth

Common Definitions

Sine Wave Power OutputEfficiency

DC Input to PA Collector

Added Sine Wave Power OutputPower Added Efficiency

Total Additional DC Power Input

WattsPower Utilization Factor

dollar

Amplifier Classes A, B, C, D, E, F, ....J

Classic Amplifier Classes A, B, C

Old Terminology that has evolved and muddied

Efficiency numbers are for active device dissipation

Theoretical efficiency may not be a useful concept

For example, rigorously applying PA efficiency concepts to my laptop reveals that it dissipates no energy...but the language and descriptive math models are still useful.

A Little Symbolic Math

Power Supply DCVccIo PDC=

Vp2R

P=L

2RF

P= Device

Sine wave RF power in Resistive load RL

Instaneous device dissipation in ideal class A amplifier with maximum pure sine wave output

Class A Amplifier

Rload

Vcc

-Vcc

Io

0 < I <2Io

constant current source

active device

load

A Little Textbook Math

using:

P= Device

VccR

Io=L

RL = Rload line

VccR L

2P= Device

2VccR L

2P= Device

cos a cos b = cos (a + b)12

cos (a - b)12

+using:

cos 0 = +12

12

2VccR L

2P= Device

VccR L

2P= Device

12

VccR L

2

P= Device12 dt

VccR L

212

PDevice=

End of Math

average device dissipation = half of supply power

Class A Amplifier

Rload

Vcc

-Vcc

Io

0 < I <2Io

constant current source

active device

load

no signal DC device dissipation = Vcc x Io

Class A Amplifier

Rload

Vcc

-Vcc

Io

0 < I <2Io

constant current source

active device

load

no signal DC device dissipation = Vcc x Io

Peak sine wave in load =Vcc 2

2 Rload

Rload

Vcc

-Vcc

Io

0 < I <2Io

no signal DC device dissipation = Vcc x Io

Peak sine wave in load =Vcc 2

2 Rload

Vcc xVcc

Rload=

Vcc 2

Rload=

since DC power supply can’t tell the difference between peak output and no output, at peak output, half of DC power is converted to sine wave in load and half dissipated in device

Rick Campbell23 December 2008

All Transistors MPN5179

100n

L1 6t FT37-43

12 v 50 mA 250 mW output at 12.0 volts375 mW output at 15.0 voltsL1

56 1201nF

1nF

56

150

22

L2 L3 L4

L2 10t T37-6 L3, L4 5t T25-6

50 MHz

Rload

Vcc

-Vcc

Io

0 < I <2Io

From Model to Real Amplifiermodel is only useful if it helps us understand and improve real amplifiers

Class A model Designed, Built, and Measured Amplifier

...more parts, but real parts

...class A model is too simple, but still useful

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

Rload

Vcc

-Vcc

Io

0 < I <2Io

Vcc

Big LBig C

Rload

Vc

Vc L didt

Vcc -=

With fast transistor and appropriate choice of Rload, Vc can be any-thing. Same circuit for PA, switch-ing power supply, ignition system, transistor killer...

From Model to Real Amplifier

Class A model

Inductor stores power supply energy and can supply extra voltage when needed. Capacitor stores power supply energy and can supply extra current when needed.

A Reminder that Active Devices are Interesting

Vgs = Vp

Vgs = 0

Vgs = -.2

Vgs = -.3

Vgs = -.4

Vgs = +.2

Vgs = -.5

Vds = 0 Vds = 8Vds = 2 Vds = 4 Vds = 6

Ids = 0

Idss

Ids = 100mA

Ids = 200mA

Ids = 300mA

Vdd

1 watt ZorchFET

load line

...and resistive loads are laboratory devices

Next: Waveform Analysis

Introduction to PA Waveform Analysis

Device Current and Voltage

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

Vcc

-Vcc

2Vcc

0

Vc

Vo

Vcc

2Vcc

0

Io

2Io

0

device dissipation is product of I and V

Class A Waveform Analysis

Device Current and Voltage

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ - Vcc

2Vcc

0

Io

2Io

0

VccIo x

0 2Iox

2Vcc x 0

32

Vcc 12

x Io

32

Vcc12

xIo

1

0

0

0.75

0.75

Device Power

Class A Waveform Analysis

Device Current and Voltage

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

sketch of instaneous device dissipation

note slightly real waveforms

Class A Waveform Analysis

Device Current and VoltageVcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

sketch of instaneous device dissipation

textbook waveforms

Average

Class A Waveform AnalysisDevice Current and Voltage

Vcc

Big L

Big C

Rload

Vc

Io

Vcc+ -

sketch of instaneous device dissipation

Note: this might still be a perfectly linear class A amplifier--the output signal is a perfect replica of the input signal.

Class A Efficiency Review:

slight deviation from textbook waveform has big impact on device dissipation

textbook waveforms only appear in textbooks

waveform engineering is our primary tool to reduce device dissipation--even at frequencies where we can’t observe waveforms

Next: an alphabetical listing of amplifier classes

“The efficiency of a Class A amplifier” is not a number at the end of several pages of arcane math in a textbook