RF Transceiver Module DesignChapter 6

Power Amplifier

Department of Electronic EngineeringNational Taipei University of Technology

Outline

• Power Capability and Efficiency• Matching Considerations• Classification of Amplifiers• Match to Desired Power• Practical Issues with Power Amplifiers

Transistor saturation Current limits On-chip and off-chip power combing Thermal runaway Breakdown Package Nonlinearity

• Summary

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Introduction

• Power amplifiers (PAs) are used in the transmitter, typically todrive antennas and trade off efficiency and linearity.

• ICs typically have a limited power supply voltage to avoidbreakdown, as well as a metal migration limit for current. Thus,simply achieving the desired output power can be a challenge.

• PAs dissipate power and generate heat, which has to beremoved. Due to the small size of integrated circuits, this is achallenging exercise in design and packaging.

• PAs are among the last circuits to be fully integrated.

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Power Capability

• One of the main goals of PAdesign is to deliver a given powerto a load. This is determined to a large degree by the loadresistor and the power supply.

• Given a particular power supply voltageVCC , such as 3-V, anda load resistanceRL , such as 50Ω, it is possible to determinethe maximumpower to be

This assumes we have a tuned amplifier and an operating point of 3-V, a peaknegative swing down to 0 V, and a peak positive swing up to 6 V.

2 2390 mW 19.5 dBm

2 2 50CC

ac

VP

R= = = ⇒

⋅

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DC-to-RF Efficiency

• Efficiency (dc-to-RF efficiency):How effectively power from the supply is converted into output power

• Assuming voltage and current are in phase, Pout is given by

• Pdc is the power from the supply and is given by

where Idc is the dc component of the current waveform.

out

dc

P

Pη =

21 1 1

1 1

2 2out LP i v i R= =

0 0

1 T TCC

dc CC C C CC dc

VP V i dt i dt V I

T T= = =∫ ∫

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Power-added Efficiency (PAE)

• Power-added efficiency (PAE) takes the gain of the amplifierinto account as follows:

For a high gain amplifier, PAE is the same as dc-to-RF efficiency .

11

outoutout in

dc dc

PPP P GPAEP P G

η−− = = = −

η

< 10%, as G >10dB

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00 2 4 6 8 10 12 14 16 18 20

Gain (dB)

η

20

15

10

5

0

Pou

t(d

Bm

)

Pin (dBm)−5 0 5 10 15

80

60

40

20

0

Efficiency (%

)

1 dB

Pout

PAEPAE decreases as gain compresses

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Matching Considerations

• In order to obtain maximumoutput power, typically the poweramplifier is “power-matched” but not conjugately matched.

• Note that conjugate matching means that and as shown in the figure.

s in∗Γ = Γ L out

∗Γ = Γ

Inputmatching

OutputmatchingAmplifier

sΓ LΓ0Z

0Z

inΓ outΓ

If conjugately matched, ands in∗Γ = Γ L out

∗Γ = Γ

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Matching to versus Matching to ….

• For low input power where the amplifier is linear, maximumoutput power is obtained with . However, this value ofz will not be the optimumload for high input power wherethe amplifier is nonlinear.

• For large-signal operation, tuning is done by determining theoptimal load with doing theload pull.

L out∗Γ = Γ

optΓ

out∗Γ optΓ

Tuning at small signal

Tuning for optimal large signal

sAsB

LBLAoutP

inPBx : 1 dB gain compressionAx : max. uncompressed power

out∗Γ

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Optimum Load Impedance

• An estimate of the optimumimpedanceRopt can be made byadjusting the load so that the transistor current and voltage gothrough their maximumexcursion.

CCV

CR

oVCI

CEV

+

−

o CC C C CEV V I R V= − =

minVdcV maxV

CEV

CI

maxI

minI

max min

max minopt

V VR

I I

−=−

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Class A, B, and C Amplifiers (I)

• PAs are grouped into classes depending on the nature of theirvoltage and current waveforms.

• Class A amplifiers can be designedto have more gain than class B orclass C amplifiers. However, theachievable output power is nearlythe same for a class A, class AB, orclass B amplifier. For a class Camplifier, where the transistorconducts for a short part of theperiod, the output power is reduced.

CCV

CL LR

Ci

Cv

Bv

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Class A, B, and C Amplifiers (II)

• The maximumcollector voltage is approximately 0 V to 2VCC .While the vC is assumed to be sinusoidal (tuned circuitfiltering), iC may be sinusoidal, as in class A operation, or maybe nonsinusoidal, as in class B or C operation, which isdetermined mainly by howthe transistor is biased.

Bias voltage

t

t

t

ThresholdBv

Cv2 CCv

CCv

0Ci,maxCi

,nomCi

,minCi

Bias voltageThresholdBv

Cv2 CCv

CCv

0Ci,maxCi

,nomCi

,minCi

t

t

t

Bias voltageThresholdBv

Cv2 CCv

CCv

0Ci,maxCi

,nomCi

,minCi

t

t

t

Class A Class B Class C

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Classification by Conduction Angle

• Conduction angle: The fraction of the full cycle for whichcurrent is flowing in the driver transistor. Class A (Cond 360°): current is always flowing

Class B (Cond 180°): current flows for exactly half of the time

Class AB (180°Cond 360°)

Class C (180°> Cond)

• A higher conduction angle results in better linearity but lowerefficiency.

Class Conduction Angle (deg.)Theoretical Max.

Efficiency (%)Normalized Output

Power

A 360 50 1

AB 360 ~ 180 50 ~ 78.5 ~ 1 (max 1.15@240o)

B 180 78.5 1

C 180 ~ 0 78.5 ~ 100 1@180o, 0@0o

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Improve Efficiency

• Power loss in the transistor must be minimized (current shouldbe minimumwhile voltage is high, and vice versa).

• For class B and C, the current is set to zero for part of thecycle where the voltage is high. There is still an overlap ofnonzero voltage and current to cause some loss.

• Classes D, E, F, and S, are designed such that the voltageacross the transistor is also nonlinear, leading to higherefficiencies, in some cases up to 100%.

• A different way to improve efficiency, while potentiallymaintaining linearity, is to power a linear amplifier fromavariable or switched power supply. This is the basis for class Gand H designs.

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Class A, B, and C Efficiency

• Efficiency for this maximum possible voltage swing is

( ),max

max

2 sin 2

4 sin cosout

dc

P

P

θ θηθ θ θ−= =

−

maxη1

0.9

0.8

0.7

0.6

0.5 θA Cond. angle2 360θ =

B Cond. angle2 180θ =

Efficiency = 78 %

Efficiency = 50 %0 45 90 135 180

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Class A, B, and C Output Power

• At a conduction angle of 180° and 360° (or θ = 90° and 180°),Po,norm,max is 1. In between is a peak with a value of about 1.15at a conduction angle of about 240°. This might appear to bethe optimumconduction angle for maximumoutput power,however, in real life, or in simulations with other models forthe current, this peak does not occur.

,max,normoP

Class A

( )0.5η =Class B

( )0.78η =

1.2

1.0

0.8

0.6

0.4

0 90 180

0.2

0

3601800θ

2θConduction angle

Class C Class AB( )1η =

,max,max,

,

1 2 sin 2

1 coso

o normo norm

PP

P

θ θπ θ

−= =−

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Class D Amplifier

• The two transistors alternately switch the output to ground orto VCC . The output filter (Lo and Co) is tuned to thefundamental frequency resulting in a sine wave at the output.While class D amplifiers can have high efficiency and havebeen demonstrated in the 10-MHz frequency range, they arenot practical in the gigahertz range.

oL oC

CCVoL oC

CCV

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Class E Amplifier (I)

• The capacitorC is the combination of the parasitic transistoroutput capacitorCp and an actual added capacitorCA .

• A capacitor across the output of the transistor is possible toobtain close to 100% efficiency even with parasitics.

oL oC

CCV

RFC

RCoi

ovcv dcI

si ci

1Q

1. Thechoke is large (onlyIdc flows through it).

2. The output circuit (Lo,Co) Q is high enough(all harmonics are removed).

3. The transistorQ1 behaves as a perfect switch.When it is on, the collector voltage is zero,and when it isoff the collector current is zero.

4. The transistor output capacitanceCp , andhenceC, is independent of voltage.

Assumptions:

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Class E Amplifier (II)

• Switch on : vc is 0, and thereforeic

through the capacitorC is 0. In thiscase, switchis = Idc − io.

• Switch off : is = 0. ic = Idc − io . Thisproduces an increase of collectorvoltagevc due to the charging ofC.

oL oC

CCV

RFC

RCoi

ovcv dcI

si ci

1Q

Open

Closedy y

ov

cv

si

32.8φ = −

omV

θ

θ

θ

θ0 π 2π

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Class E Amplifier (III)

• For lossless components (as in the assumptions), the only lossis due to the discharge ofC when the switch turns on. If thecomponents are selected so thatvc just reaches zero as theswitch turns on, no energy is lost and the efficiency is 100%.

• In practice, because the assumptions do not strictly hold andbecause components will not be ideal, the voltage will not beat zero and so energy will be lost. However, with carefuldesign, efficiencies in the 80% range are feasible.

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Class F Amplifier (I)

• Additional harmonic is added tothe fundamental to produce acollector voltage more like asquare wave. This meansvc islower while ic is flowing, buthigher whileic is not flowing, toa higher efficiency.

dcI

oL oC

RFC

R

cC

oiovcv

ci

1Q

0f

03 f

3L

3C

( )cv θ

( )ci θ

( )ov θ

π 2π

2 CCV

CCV

0

dcIcmi

omV

π 2π

θ

θ

θ

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Class F Amplifier (II)

• Lo and Co make sure the output is a sinusoid. The third-harmonic resonator (L3, C3) causes a 3rd harmonic componentin the collector voltage. At the correct amplitude and phase,this 3rd harmonic component produces a flattening ofvc. Thisresults in higher efficiency and higher output power.

1sin3

9θ

θ2ππ0

sinθ

1sin sin3

9θ θ+

dcI

oL oC

RFC

R

cC

oiovcv

ci

1Q

0f

03 f

3L

3C

9

8cm CCV V∴ =

8

9 cm CCV V=

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Class F Amplifier (III)

• As an aside, the Fourier series for the ideal square wave is

• However, choosingVcm3 = 1/3Vom would produce a nonflatwaveform.

• The efficiency can be calculated asPo/Pdc:

1 1sin sin 3 sin 5

3 5θ θ θ+ + +⋯

( )1 91

92 2 82 88.4 %8 4

cmCCo om

o

cmdc dc CCCC

iVi VP

iP I V V

π

π

⋅⋅ = = = ⋅ =

⋅

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Second-Harmonic Peaking

• A second resonator allows the introduction of a 2nd harmonicvoltage into the collector voltage waveform, producing anapproximation of a half sinusoid. It can be shown that theamplitude of the 2nd harmonic voltage should be a quarter ofthe fundamental. It can be shown that the peak output voltageis given by

and the efficiency is given by

Sum

2nd harmonic

Fundamental

4

3om CCV V=

884.9 %

3η π= ≃

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Quarter-wave Transmission Line

• At fo, the tuned circuit (Lo andCo)is open. Theλ/4 transmission lineis equivalent to having aresonator at all odd harmonics,with the result that the collectorvoltage is a square wave and100% efficiency.

dcI

oL oC

RFC

R

bC

oiov

ci

1Q

4λ

0

( )cv θ

( )ci θ

( )ov θ

π 2π θ

θ

θ

cmi

omV

π 2π

2 CCVFund. + Odd harmonics

Fund. + Even harmonics

Fundamental

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A Class-F-driven Class-E PA

• One useful application of a class F amplifier is as a driver forthe class E amplifier. A class E amplifier is ideally driven by asquare wave.

cvoL oC

RFC

LRcC

0f

ov

cv

1Q

1Q

CCV

CCV

C

biasV

0L 0C

3L 3C03 f

Class-EClass-F

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Class G Amplifiers

• The class G amplifier has been used mainly for audioapplications, although recently variations of this structure havebeen used up to 1 MHz for signals with high peak-to-averageratios (high crest factor), for example, in digital telephonyapplications. This topology uses amplifiers powered fromdifferent supplies. For lowlevel signals, the lower supply isused and the other amplifier is disabled.

1CCV

1EEV

2EEV

2CCV

R

Driv

er

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Class H Amplifiers

• Power supplies track the input signalor the desired output signal. Thus,power dissipated is low, since thedriver transistors are operated with alow-voltage VCE. The efficiency canbe much higher than class A.

• The power supplies use a highlyefficient switching amplifier.

• This technique has mainly been usedfor lower frequencies. However, it canbe modified so that the power supplyfollows the envelope of the signalrather than the signal itself.

CCV ′

outVinV

1Q

2Q

CCV ′Linear

amplifier

Driv

er

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Class S Amplifier

• The class S amplifier has as an input a pulse-width modulated(PWM) signal to turnQ1 andQ2 on or off as switches with aswitching frequency much higher than the signal frequency.

• Lo andCo form a lowpass filter that turns the PWMsignal intoan analog waveform. If only positive outputs are needed, onlyQ1 andD2 are required. For negative signals, onlyD1 andQ2

are necessary.

1Q

2Q

1D

2D

1C oL

oC R

• The switching frequency must besignificantly higher than the signalfrequency, this technique is notviable for amplification of signals inthe gigahertz frequency range.

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Power Contour

• The load pull technique is used to find the power contour.

Inputmatching

OutputmatchingAmplifier

sΓ LΓ0Z

0Z

inΓ outΓoutZ

inZ

Max. output power ,max ,, o L optP Γ

Max. operating power gain, L out∗Γ = Γ

Gp circles

Power circles

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Matching to Achieve Desired Power (I)

• Given a particular power supply voltage and resistance value,the achievable amount of power is limited byPo » V 2 R.Obviously,R must be decreased to achieve higherPo .

LR

L

CLR

CL

RR

R

2 2o CCP V R≈

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Matching to Achieve Desired Power (II)

• It is possible to increase the bandwidth by using a higher orderof matching network.

RLR

intR LRintRR

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Matching to Achieve Desired Power (III)

• Sometimes higherQ is desired for narrowband applications.

• Bond wire inductance can be used for realizing seriesinductance.

RintR

1L 2L

aCbC LR

LR

2LbC

intR

aC1L

R

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Transistor Saturation

• Efficiency increases rapidly with increasing input signal untilsaturation of the input device occurs. After saturation,efficiency is fairly constant, but drops somewhat due to gaincompression (PAE goes down).

• Classes A, B, and C are usually operated just into saturation tomaximize efficiency. Class E (and sometimes F) is operated asa switch, between saturation and cutoff.

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Current Limits

• The PA requires huge transistors with very high currenthandling capability with multiple emitter, base, and collectorstripes (multiple fingers), as well as multiple transistorsdistributed to reduce current density and heat concentration.

• Each finger and each transistor is treated the same as everyother finger and transistor. This is important in order to avoidlocal hot spots, thermal runaway, and mismatch of phase shifts.

• Metal lines have to be made wide to avoid problems withmetal migration. To increase current capability, it is possible touse multiple metal layers

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On-Chip Power Combining

• Combine multiple transistors at the output todistribute the heat and limit the currentdensity in each transistor.

• The base drive can be phase delayedcompared to the shortest path, so it isimportant to keep the line lengths equal.

• With RF or microwave circuits, sharp bendsare to be avoided.

• Phase shift can be determined by consideringthat the wavelength of a 1-GHz sine wave infree space is 30 cm (1.2°/mm). For SiO2with εr of about 4, the phase shift is about12°/mm at 5 GHz. Thus, for a distance of 5mm, we have the phase shift is about 60°.

• Use multiple output pads for parallel bondwires, or a long pad, to connect more bondwires if desired.

First stage

Output driversOutput pads

Base Collectors

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Off-Chip Power Combining (I)

• Power combining can also be done off-chip, using techniquesincluding backward wave couplers (stripline overlay,microstrip Lange) for octave bandwidth, or the branch-line,coupled amplifier.

4λ

Stripline coupler Branch-line coupler Coupled amplifiers

0Z 0Z

0 2Z

4λ

0 2Z

4λ02Z

4λ

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Off-Chip Power Combining (II)

• The ‘‘rat race’’ produces two outputs 180° out of phase (or cancombine two inputs that are 180° out of phase).

• The push-pull arrangement is the same as for a class B push-pull amplifier.

270

90180

Rat-race combiner

0

180

0

180

Ai

Bi

A Bi i−

Combiner for push-pull operation

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Thermal Runaway – Ballasting

• Under high power, the temperature willincrease. With constant base-emittervoltage, current increases with temperature.Thus, ifVBE is held constant, if temperatureincreases, current increases, and as a result,more power is dissipated and temperaturewill increase even more. This phenomenonis known asthermal runaway. Typically,ballast resistors are added in the emitters asa feedback to prevent such thermalrunaway.

inV BEV

ER RV

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Breakdown Voltage

• A measure of breakdown isVCEO,max, which is the maximumallowable value ofVCE with the base open circuited to causeavalanche breakdown . A typical value in a 3-V process mightbe 5 V. In processes where this is not possible, it may benecessary to drop the supply voltage, use cascode devices, andpossibly to make use of more complex biasing, which isadaptive or at least variable.

,max

9

8cm CC CEOV V V= ≤

Linear amplifier: ,max2cm CC CEOV V V= ≤

Class E amplifier:

Class F amplifier:

,max3.56cm CC CEOV V V= ⋅ ≤

2.5 VCCV ≤

• For VCEO,max= 5 V :

1.4 VCCV ≤

4.44 VCCV ≤

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Packaging

• How does one remove heat froma power amplifier?

One possible mechanismis thermal conduction through directcontact, for example, when the die is mounted directly on ametal backing. Another mechanismis through metalconnections to the bond pads, for example, with wires to thepackage or directly to the printed circuit board. In flip-chipimplementation, thermal conduction is through the solderbumps to the printed circuit board.

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Effects and Implications of Nonlinearity

• Linearity of the PA is important with certain modulationschemes. For example, quite linear power amplifiers arerequired to avoid spectral regrowth for filtered BPSK, QPSK,OQPSK, and QAMsignal.

• FM, FSK, and MSK modulation are typically constantenvelope and so allowthe use of nonlinear high-efficiencypower amplifiers.

ff

outP outP

Spectrum regrowth

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Summary

• In this chapter, the various kinds of power amplifiers wereintroduced, including the Class A, AB, B, and C linearamplifiers as well as Class D, E, and F nonlinear amplifiers.

• The linear amplifiers have good linearity but poor efficiency,on the contrary, the nonlinear amplifiers have very goodefficiency but poor linearity.

• For power amplifiers, the matching usually has to be powermatched than conjugately matched.

• Since the transistor in high power operation is usually withhigh current and voltage, there are many practicalconsiderations should be taken into account, such as currentlimits, breakdown, and heat problems.

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