Doherty Power Amplifiers: From Fundamentals to Advanced Design Methods

177 Pages·2018·10.36 MB·English

by  Bumman Kim

DOHERTY POWER AMPLIFIERS
DOHERTY POWER AMPLIFIERS
From Fundamentals to Advanced Design
Methods
BUMMAN KIM
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ACKNOWLEDGMENTS
Manypersonsdeservewarmthanksformakingthisbookareality.Themajorpartofthis
book contains the results of more than a decade of research activities performed in my
groupatPOSTECH.Ihavebeenfortunatefromthesupportofnumerousexcellentstu-
dents,whodevotetheirtimeandenergytoworkonDohertypoweramplifiers.Iwould
like to express my sincere gratitude to all those people who have worked on Doherty
power amplifier. The deliberate work is the basis of this book.
IwouldliketoexpressmysincereappreciationtoalltheAcademicPressstaffinvolved
inthisprojectfortheircheerfulprofessionalismandoutstandingefforts.Lastbutnotthe
least,Iwouldliketo thankmyverynearestfamilyfor theirpatienceandunderstanding
during the many days spent working on this book.
ix
CHAPTERONE
Introduction to Doherty Power
Amplifier
1.1 HISTORICAL SURVEY
William H. Doherty was an American electrical engineer, best known for his
invention of the Doherty amplifier. Doherty was born in Cambridge, Massachusetts,
in 1907. He attended Harvard University, where he received his bachelor’s degree in
communication engineering (1927) and his Master’s degree in engineering (1928).
Doherty joined Bell Labs in 1929. At the Bell Labs., he worked on the development
of high-power radio transmitters, which were used for transoceanic radio telephones
and broadcastings.
Dohertyinventedthisuniqueamplifierapproachin1936usingavacuumtubeampli-
fier. This new device greatly improved the efficiency of RF power amplifiers and was
first used in a 50kW transmitter that Western Electric Company designed for WHAS,
aradiostationinLouisville,Kentucky.WesternElectricwentontoincorporateDoherty
amplifiers into, at least, 35 commercial radio stations worldwide by 1940 and many
otherstations,particularlyinEuropeandMiddleEastinthe1950s.WithintheWestern
Electric,thedevicewasoperatedasalinearamplifierwithadriverthatwasmodulated.
In the 50kW implementation, the driver was a complete 5kW transmitter that could,
if necessary, be operated independently of the Doherty amplifier, and the Doherty
amplifier was used to raise the 5kW level to the required 50kW level.
AsasuccessortoWesternElectricCompanyInc.forradiobroadcasttransmitters,the
ContinentalElectronicsManufacturingCompanyatDallas,Texas,considerablyrefined
theDohertyconcept.TheearlyContinentalElectronicsdesigns,byWeldonandothers,
retained most of the characteristics of Doherty’s amplifier but added medium-level
screen-grid modulation of the driver. The ultimate refinement made by the company
wasthehigh-levelscreen-gridmodulationschemeinventedbySainton,whosetransmit-
terconsistedofaclassCcarriertubeinaparallelconnectionwithaclassCpeakingtube.
The tubes’ source (driver) and load (antenna) were split and combined through + and
(cid:1)90° phase-shifting networks as in a Doherty amplifier. The unmodulated radio-
frequency carrier was applied to the control grids of both tubes. Carrier modulation
wasappliedtothescreengridsofbothtubes,butthescreen-gridbiaspointsofthecarrier
andpeakingtubesweredifferentandwereestablishedsuchthatthepeakingtubewascut
DohertyPowerAmplifiers ©2018ElsevierInc.
https://doi.org/10.1016/B978-0-12-809867-7.00001-6 Allrightsreserved. 1
2 DohertyPowerAmplifiers
offwhenmodulationwasabsentandtheamplifierwasproducingratedunmodulatedcar-
rierpower.Andbothtubeswereconductingduringthemodulation.Andeachtubewas
contributing twice the rated carrier power during 100% modulation as four times the
ratedcarrierpowerisrequiredtoachieve100%modulation.Asbothtubeswereoperated
inclassC,asignificantimprovementinefficiencywastherebyachievedinthefinalstage.
In addition, as the tetrode carrier and peaking tubes required very little drive power, a
significantimprovementinefficiencywithinthedriverwasachievedaswell.Thecom-
mercialversionoftheSaintonamplifieremployedacathode-followermodulator,andthe
entire50kWtransmitterwasimplementedusingonlyninetotaltubesoffourtubetypes,
aremarkableachievement,giventhatthetransmitter’smostsignificantcompetitorfrom
RCA was implemented using 32 total tubes of nine tube types.
The approach was used by such leading companies as not only Continental but also
Marconiwithfunctionalinstallationsuptothelate1970s.TheIRErecognizedDoherty’s
important contribution to the development of more efficient radio-frequency power
amplifiers with the 1937 Morris N. Liebmann Memorial Award.
Theamplifierhasbeenreinventedrecentlyforuseinmobilecommunicationsystems
using semiconductor devices at higher frequencies. It creates large deviations from the
previous design based on the vacuum tubes. Also, the amplifier is modified to amplify
ahighlymodulatedsignalwithahighpeak-to-averagepowerratio(PAPR).Nowadays,
the Doherty amplifier is the choice of the technique for the power amplification in the
mobilebase-station.Thetechnologycanbeusefulforhandsetpoweramplifier,also.In
this chapter, the basic structure of the Doherty amplifier together with the operational
behavior is introduced.
1.2 BASIC OPERATION PRINCIPLE
The most important property of the Doherty amplifier is the load modulation,
whichcarriesouttheperfectcombiningoftheasymmetricalpowersfromthetwoampli-
fiers.Thereby,onlyoneamplifier(calledcarrieramplifier)operatesatalowpowerlevel,
and the efficiency at the same power level is two times higher than that obtained from
the two times bigger amplifier. The two amplifiers (the second one is called peaking
amplifier) generate powers at a higher power level, and the carrier amplifier operates
atthepeakefficiencymodeinthisregionduetotheniceloadmodulationcharacteristic.
Thispropertyprovidesanefficientamplificationofanamplitude-modulatedsignal.The
load,whichismodulatedbythecurrentratioofthecarrierandpeakingamplifiers,isself-
adjustedforthepeakefficiencyatthetwopowerlevels.Thefirstpeakefficiencyispro-
videdbythecarrieramplifier(CA)atthelevelwhenthepeakingamplifier(PA)isturned
on,andthesecondpeakisatthepowerlevelwhenthetwoamplifiersgeneratetheirfull
powers. Another important characteristic of the Doherty load modulation is that the
overall gain of the amplifier is constant, providing a linear amplification.
IntroductiontoDohertyPowerAmplifier 3
1.2.1 Load Modulation Behavior
1.2.1.1 Load Impedance Modulation
The simplest illustration of the load modulation concept is shown in Fig. 1.1, where a
voltage-controlledvoltagesource(VCVS)isinparallelwithavoltage-controlledcurrent
source(VCCS) and aload resistor R. The impedanceseen by theVCVS,Z , ismodu-
1
lated by the current I , as given by
2
V V
Z ¼ 1¼ 1 (1.1)
1 I I (cid:1)I
1 R 2
VaryingthecurrentI fromzerotoI ¼V /R,Z isvariedfromRto∞.Inthiscircuit,
2 R 1 1
theVCCSmodulatestheloadimpedanceoftheVCVS.InDohertyamplifier,theability
to modulate Z using I is properly employed to track the optimal impedances for the
1 2
amplifier to operate efficiently at the back-off power levels. An important property of
the setup in Fig. 1.1 is that the linearity of the overall system is solely determined by
the linearity of the VCVS because the voltage V across the load is always equal to
out
V .Therefore,linearityisguaranteedregardlessofthevalueofI ,aslongasV islinearly
1 2 1
proportionaltoV .Forthispurpose,theimpedanceZ shouldtrackagivenimpedance
in 1
profileversusV byspecifyingtheI versusV profile.Althoughmathematicallysimple
in 2 in
to define it, realizing a given I versus V profile in practice can be a challenge.
2 in
In the load modulation technique, the VCVS and VCCS have their important
roles. The former ensures the linearity of the amplifier, while the latter acts as the load
modulatingdevice,whoseI versusV profiledeterminestheimpedanceZ seenbythe
2 in 1
VCVS. These two properties are important in derivation of the Doherty circuit
configuration.
The Doherty amplifier uses a different circuit topology for the load modulation. It
consists of two amplifiers (two current sources) and an impedance-inverting network,
which converts the one current source to a voltage source. This converted amplifier
iscalledacarrieramplifier,andtheothercurrentsourceamplifierisapeakingamplifier.
VCVS VCCS
I I
1 2
Z
+ 1 I +
R
Vin V1 + Vout R I2 Vin
–
– –
Fig.1.1 Loadmodulationcircuitdrivenbyvoltageandcurrentsources.
4 DohertyPowerAmplifiers
R0∠90° I1′ V I2
0
Z Z′ Z
1 1 2
R
I 0 I
1 2 2
Carrier Peaking
amp. amp.
Fig.1.2 OperationaldiagramofDohertyamplifier.
Fig. 1.2 showsanoperationaldiagramtoanalyzetheDohertyamplifiercircuit.Theoutput
load is connected to the carrier amplifier through the impedance inverter (a quarter-wave
transmissionline)anddirectlytothepeakingamplifier.Inthisfigure,theoptimumpower-
matchingimpedanceofthecarrierandpeakingamplifiersatthepeakpowerisR ,andthe
0
load of the carrier amplifier, when the peaking amplifier is off, becomes R /2 due to the
0
parallelconnectionofthetwoamplifiers.Itisassumedthattheoutputcapacitorofthedevice
isresonatedoutandthephasedelayofthequarter-wavelineiscompensatedattheinput.
The impedance inverter has a characteristic impedance of R also. The load impe-
0
0
dances of the carrier amplifier at Z and Z , shown in Fig. 1.2, are given by
1 1
(cid:1) (cid:3)
0
V R I +I
Z0 ¼ 0¼ 0(cid:3) 1 2 (1.2)
1 I0 2 I0
1 1
R 2 2R 2R
Z ¼ 0 ¼(cid:4) 0 (cid:5)¼ 0 (1.3)
1 Z0 I ð1+αÞ
1 1+ 2
0
I
1
where α¼I2=I0. Eq. (1.3) shows that the carrier amplifier represented by a current
1
source I sees the load impedance modulated by the second current source I , rep-
1 2
0
resenting the peaking amplifier. It should be noticed that I is different from I due
1 1
to the impedance change. Also, in the normal Doherty operation, the current level of
thepeakingamplifiervariesfrom0toI ¼I ,themaximumcurrentofthetwoampli-
1 max
fiers,andαchangesfrom0to1.Normally,I andI canhandlethesameamountofcur-
1 2
rent,thatis,thesamesizedevicesforthetwoamplifiers,andZ isR whenI ¼I ¼I
1 0 2 1 max
atthepeakpower,becauseI isequaltoI0 atthepower.Z is2R whenI ¼0andZ is
1 1 1 0 2 1
in between the two values for the I current between 0 and I . This is the Doherty
2 max
load modulation behavior, which is depicted in Fig. 1.3C.
The peaking amplifier provides the open load until it is turned on because the
currentI iszero.Afterturnedon,theimpedanceZ isalsomodulatedsimilarly,which
2 2
is given by
IntroductiontoDohertyPowerAmplifier 5
V
I max
max
e Carrier
urrent Carrier voltag
ntal c Peaking ental Peaking
e m
m
a
a d
d n
n u
u F
F
V /2 V V /2 V
in, max in, max in, max in, max
(A) Input voltage amplitude (B) Input voltage amplitude
m) 5ROPT
h
O 4R
e ( OPT
nc 3R
a OPT
d
e
p 2R
m OPT
oad i 1ROPT
L
V
V /2 V in
in, max in, max
(C) Input voltage amplitude
Fig.1.3 Current,voltage,andloadimpedanceshapesofthecarrierandpeakingamplifiers:(A)current
profiles,(B)voltageprofiles,(C)loadimpedanceprofiles.
(cid:6) (cid:7)
V R I0+I R ð1+αÞ
Z ¼ 0¼ 0(cid:3) 1 2 ¼ 0(cid:3) (1.4)
2 I 2 I 2 α
2 2
The load modulation behavior is also depicted in Fig. 1.3C. The carrier impedance is
modulated from 2R to R and the peaking from infinity to R . In this figure, it is
0 0 0
assumed that each current source is linearly proportional to the input voltage and R
0
is equal to R of the transistor, the optimum power matching resistance. As shown
OPT
inFig.1.3A,I isturnedonatthemidpointduetotheclassCbiasofthepeakingamplifier
2
andisincreasedtothemaximumvalue.I islinearlyincreasedfromthezerogatevoltage
1
due to the class B bias. In this operation, the transconductance of the peaking amplifier
shouldbetwicelargerthanthatofthecarrieramplifierduetoahalfoftheinputvoltage
swing for the maximum current generation. To get the two times larger trans-
conductance,thepeakingamplifiershouldbetwotimeslargerthanthecarrieramplifier.
But in the case, only a half of the peaking current is utilized, wasting the power gener-
ation capability. To solve the problem, uneven driving technique is developed, which
will be introduced in Chapter 2.