fundamentals of rf power amplifiers

PUBLIC USE

TIM DAS

SR. RF APPLICATIONS ENGINEER

FTF-NET-N1998

MAY 18, 2016

FTF-NET-N1998

FUNDAMENTALS OF

RF POWER AMPLIFIERS

PUBLIC USE1 #NXPFTF PUBLIC USE1 #NXPFTF

AGENDA

• Review of RF Power Amplifier Topologies and Their

Application

• Solution Selection Criteria

• Important Considerations for Component Selection

• Common Pitfalls

• Example

PUBLIC USE2 #NXPFTF

Most Common Types of High Power RF Amplifiers

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Linear Amplification (Class A/AB/B, F, F-1, J, …)

• Linear amplification, Class A/AB/B

• Simplest topology

• Compact footprint / small size

• Higher classes of operation (e.g., F, F-1, J, …) possible to enhance efficiency (somewhat)

• Back-off operation required for linearity

− Minimize clipping of peak envelope excursions

− Modest efficiency for high PAR signals due to requirement for back-off

− Typical Applications: h

Pout

Gain

Pout

backoff

PAR

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Doherty Amplification

• Separate Carrier & Peaking amplifiers

• Symmetric, asymmetric, and n-way possible (n≥2)

• Can use Si LDMOS, GaN, GaAs technologies

• Mature, established PA architecture for high power

cellular infrastructure (e.g., metro cell)

• Good efficiency performance possible for

modulated signals with high PAR

• Linearization (e.g., DPD) generally required to meet

linearity requirements

• Typical Applications: Cellular Basestations

PAR

Peaking

Carrier

h

Pout

Gain

Pout

backoff

backoff

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Envelope Tracking (ET)

• Consists of two amplifiers

− Wide bandwidth envelope amplifier supplies

VDD bias (drain current) to RF PA

• Digital Signal Processing is employed to

map optimal PA drain bias with signal

envelope

• High efficiency potential for high PAR

signals & in back-off operation

• Typical Applications: Mobile/portable

communication devices

PA

Env

Ampa(t)

22tQtIta

a(t)+aDC

Pout

h

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Polar Transmitter (EER)

• Consists of two amplifiers

− Envelope amplifier (provides VDD bias / current to RF PA

− RF PA (operates in gain compression)

• Envelope & pm signals must be developed, generally through digital methods

• RF PA is inherently efficient due to operation in gain compression

• Can be difficult to find a suitable envelope amplifier

• Typical Applications: Mobile/portable communication devices (e.g. mobile WiMAX)

tItQ

t

tQtIta

1

22

tan

PA

Env

Ampa(t)

(t)

ttje

a(t)+aDC

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Switch-Mode Power Amplification (SMPA)• SMPA concepts to achieve a linear & efficient PA / transmitter have been the

subject of research (mostly academic) interest for a number of years

• NXP has/is investigating certain SMPA techniques & is following academic

research in this area

+

High h

Good linearity

PUBLIC USE8 #NXPFTF

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

havg

Class A

Class B

n=2

Doherty

Amplifier Architecture

Theoretical Efficiency

• Case study: 8 dB PAR CFR WCDMA

0 1 2 3 4 5 6 7 8 910 -4

10 -3

10 -2

10 -1

10 0

Pk/Avg Pwr Ratio - dB

Pro

ba

bil

ity

0.63

0.08

0.35

0.78

max

max

0

0

)()(

)(

)()(

V

inst

o

V

o

AVG

dVVpV

VP

dVVpVP

h

h

1 Switcher efficiency assumed @ 100%2 Switcher efficiency assumed @ 100%3 Zero power lost due to harmonics of pulse sampling rate, 100% spectral efficiency

A/B

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SOLUTION

SELECTION

CRITERIA

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Parameters That Matter

Output RF Power Peak/Average output power capability is expressed in W or dBm

Frequency band LDMOS (band-specific) 1 MHz to 3.8 GHz

GaN (wideband) 1 MHz to 3.0 GHz

GaAs and SiGe 1 MHz to 6 GHz

Supply Voltage(s) 1.5 V – 13.6 V for Handheld and Mobile applications

26 V – 52 V for High Power LDMOS and GaN

Efficiency High efficiency reduces heat dissipation, which increases reliability,

reduces power supply and cost

Package Physical dimensions and heat dissipation requirements

Linearity Depending on the application, linearity can be indicated via IP3, ACP,

EVM, IP2, etc.

Ruggedness Various Tests of survivability to abnormal levels of RF energy appearing

at an RF port

Price Many products and package variants provide many options…

Plastic packages priced 20% less than ceramic package devices


Saturated Output

Power

Outp

ut

Pow

er

(dB

m)

Input Power (dBm)

Compression

region

Linear region

(slope = small-signal gain)

Gain Compression


Adjacent Channel Power LeakageError Vector Magnitude

3rd-Order

Intermodulation Distortion

Amplifier Linearity


Parametric, “Product Selector” Tool at NXP.com

Note link for

“Suggested Drivers”

Look for this link on the

“RF Products” Page


IMPORTANT

CONSIDERATIONS

FOR COMPONENT

SELECTION


Competitive Strength of NXP RF Power Solutions

• #1 player in RF Power

• Leader in performance and ruggedness

• Advanced multi-stage IC portfolio

• Leader in cost-effective over-molded plastic packaging

• Product longevity programs

• Undisputed manufacturing reliability and consistency

• Broadest portfolio in the industry:

− Cellular: 160+ transistors in production / 275 reference designs

− Industrial: 40+ transistors in production / 150 reference designs

• Devices tested for Ruggedness

• Tuned and Tested Test Fixtures available to customers to use as a reference design and evaluation vehicle

• Simulation models available via the NXP website: ADS or AWR


Output Power Considerations

• Depending on application, power may be specified as :

− Peak power : specify max instantaneous power

− Average power

− Continuous or Pulsed

• Several stages are necessary to provide high output power from a low power source

− Serial stages increase gain

− Parallel stages increase power

Ppeak

Pavg

Pavg=Ppeak

Pulsed

CW

Final StageDriverPre-Driver

The Driver Amplifier must have sufficient output

power capability to drive Final Stage to its required

output level

The Final Stage may have a range of load

impedance over which it must “survive”

e.g. 8:1 VSWR

The pre-driver provides the

needed gain

GaAs

SiGe

GaAs

LDMOSLDMOS

GaN


Power Amplifier Line-Up


Device Selection

* Excerpt from NXP AFT21S230S datasheet available at

http://www.nxp.com/files/rf_if/doc/data_sheet/AFT21S230S_232S.pdf

Datasheet review reveals

that the transistor

• is designed for Doherty applications

• is designed for the appropriate

frequency band

• has appropriate power capability,

i.e., Psat ~ 47 dBm + 7.0 dB when

operated in class AB into a

“tradeoff” load impedance

http://www.nxp.com/files/rf_if/doc/data_sheet/AFT21S230S_232S.pdf


Device Selection

* Excerpt from NXP MMG30271B datasheet available at

http://www.nxp.com/files/rf_if/doc/data_sheet/MMG30271B.pdf

NXP

http://www.nxp.com/files/rf_if/doc/data_sheet/MMG30271B.pdf


Tuned RF Evaluation Fixtures

• Amplifiers tuned for specific

applications

• Optimized performance

• Impedance matched RF ports

• Each test fixture is available with a

dataset including

− BOM, component layout, PCB layout

file

− Detailed performance data

• Customers are encouraged to copy

the design


Sub-system Reference

Doherty Final

Drivers

Pre-Driver Stage

Advanced

Doherty

Alignment

Module


Radio Front-End Reference Design

Driver Stage Amplifier • Intermediate power

• Class AB or increasingly

Doherty

Predriver Stage

Amplifier• Low power, typically low

voltage, 5 V GaAs

• Class A

Circulator and Coupler• Non-Linear signal split for

input to DPDTo Filter & Antenna

RF Tx Out

To/From Transceiver

RF SRx Out RF Tx In

Final Stage Amplifier• High power

• Doherty

DC, Control, Monitoring• Regulators

• Temperature

• Bias

• Current

• VSWR

• Power Detection

• Circuit Protection

• Others

Receive Path

(not shown)• Switch (TDD)

• LNA


Validation Levels for Simulation Models

• No comparison to product measurements

• No peer review required

• No report demonstrating how representative model is to product

Level-0

• Model simulations compared to product measurements

• S-Parameters

• Power Sweeps

• Single Impedance, Input & Output, set by test fixture

• Multiple frequencies measured at same impedance

Level-1

• Model simulations compared to product measurements

• S-Parameters

• Load-pull Contours around MXE & MXP

• Power Sweeps

• Multiple Impedance, MXE & MXP, states set by load-pull for each frequency

• Multiple frequencies measured

Level-2


150 Other Reference Designs Available

Consult your NXP Product Rep

for

• Datapaks describing

Application Specific Solutions

• Ordering samples

• Access to Reference Design

and Evaluation Fixtures

• Access to Simulation Models

• Characterization Data (e.g.

Load-Pull data)


COMMON PITFALLS


Common Pitfalls

After first-pass device selection, more

device/solution info is needed…

Request device/solution specific data through

NXP Sales Representative. If it already exists,

you will receive it quickly. If not, NXP Apps.

Eng. can look at collecting that data for you.

Using a device beyond its datasheet specified

operating range

A device will often operate beyond its

specified operating range. Ask for an

assessment of feasibility for your application

via NXP Sales Representative.

“Unconditionally Stable”

vs.

“Conditionally Stable”

A “Conditionally Stable” device should provide

higher levels of RF performance than an

“Unconditionally Stable” device.

Fitting a solution to specific design constraints:

e.g. PCB area, thermal, supplies.

Request Applications Engineering help (via

NXP Sales Representative) when you have

specific design constraints that must be

accommodated.


EXAMPLE


Device Selection Example

• Usually, only a small subset of solution criteria are available/known at the first conversation

• Follow-up conversations are usually needed to narrow options and accommodate design constraints

Cell Tower Transmitter Needs

Parameter Value

Frequency band 2110 – 2170 MHz

Average output power

(POUT)

49 dBm at PA output

Line-Up Gain @ POUT ~59 dB

Line-Up Efficiency @ POUT ≥ 43%

Available supplies for PA Can accommodate 28 V or 48 V

Signal type W-CDMA, 2-4 carriers

Signal PAR 10.0 dB

System Architecture Doherty with DPD and CFR

CFR will reduce PAR to 7.5 dB

System

Architecture,

DPD and CFR is

beyond the

scope of this

presentation

C

P



• Start device selection with RF Power

Transistors

• Start with the Fundamentals

C

P

Parameter Value

Frequency band 2110 – 2170 MHz

Peak Power (P3dB) with CFR 56.5 dBm


DPD will be

employed to

linearize this PA to

ACPR ≤ -55 dBc


CFR will be employed to

better accommodate

Pout = 49 dBm



Parameter Value

Frequency band 2110 to 2170 MHz

Driver P3dB

49 - 15.7 + 7.5 ≥ 40.8 dBm

Driver Linear Pout

49 - 15.7 ≥ 33.3 dBm

C

P

Doherty PA Gaintaken from

A2T21H410-24S

Datasheet

PA Output

Power

After determining expected

capability of Doherty HPA, then

move to Driver selection

PAR

Doherty PA Gaintaken from

A2T21H410-24S

Datasheet

PA Output

Power



C

P

Using Frequency and P1dB selection criteria (±3 dB window)

Since Doherty is operating at 28 V, the driver shall be also.

Eight options are listed. Lets look at narrowing those options…



C

P

Comparing Datasheets…

We find that AFT27S010N is almost perfectly sized,

However, MRF6S20010N is more linear to Pout = 33 dBm

Some DPD systems may have some difficulty linearizing A2T21H410-24S Doherty

driven by AFT27S010N.

We will select MRF6S2001N as the driver for this application



Parameter Value

Frequency band 2110 to 2170 MHz

Pre-driver P3dB

49 – 15.7 – 15.5 + 7.5 ≥ 25.3 dBm

Pre-driver Linear Pout 49 – 15.7 – 15.5 ≥ 17.8 dBm

Pre-driver Gain 59 – 15.7 – 15.5 ≈ 27.8 dB

PA

output

Power

Doherty Gaintaken from

A2T21H410-24S

Datasheet

C

P

After determining expected

capability of Driver, then move

to Pre-driver selection

Driver Gaintaken from

MRF6S20010N

Datasheet

PAR



Using Frequency and P1dB selection criteria (±3dB window)

Five GPA options are listed, but gain is too low

C

P

Look through GPA Portfolio



Using Frequency and P1dB selection criteria (±3dB window)

Two Linear Amp options, with adequate gain, are listed

C

P

Look through Linear Amplifier Portfolio



We find that MMA20312BV is the best choice

since its gain and linear performance provides

strong ACP performance to Pout = 19 dBm

C

P



C

PMMA20312BV

GaAs Linear Amp: Class AB

Tuned to 2110 – 2170 MHz

5.0 V

27 dB Gain

11.5% Eff. @ Pout = 18dBm

MRF6S20010N

LDMOS Transistor: Class AB


28 V

15.5 dB Gain

24% Eff @ Pout = 33dBm

A2T21H410-24S

LDMOS Doherty HPA


28 V

15.7 dB Gain

49% Eff @ Pout = 49dBm

Lineup Parameter

Line-Up Gain 58.2 dB

Output Power 49 dBm

Line-Up Efficiency 46%

PAR 7.5 dB


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