FUNDAMENTALS OF RF POWER AMPLIFIERS
Transcript of 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
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AGENDA
• Review of RF Power Amplifier Topologies and Their
Application
• Solution Selection Criteria
• Important Considerations for Component Selection
• Common Pitfalls
• Example
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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
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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
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Saturated Output
Power
Outp
ut
Pow
er
(dB
m)
Input Power (dBm)
Compression
region
Linear region
(slope = small-signal gain)
Gain Compression
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Adjacent Channel Power LeakageError Vector Magnitude
3rd-Order
Intermodulation Distortion
Amplifier Linearity
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Parametric, “Product Selector” Tool at NXP.com
Note link for
“Suggested Drivers”
Look for this link on the
“RF Products” Page
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IMPORTANT
CONSIDERATIONS
FOR COMPONENT
SELECTION
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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
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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
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Power Amplifier Line-Up
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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
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Device Selection
* Excerpt from NXP MMG30271B datasheet available at
http://www.nxp.com/files/rf_if/doc/data_sheet/MMG30271B.pdf
NXP
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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
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Sub-system Reference
Doherty Final
Drivers
Pre-Driver Stage
Advanced
Doherty
Alignment
Module
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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
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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
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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)
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COMMON PITFALLS
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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.
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EXAMPLE
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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
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Device Selection Example
• 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
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DPD will be
employed to
linearize this PA to
ACPR ≤ -55 dBc
Device Selection Example
CFR will be employed to
better accommodate
Pout = 49 dBm
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Device Selection Example
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
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Device Selection Example
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…
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Device Selection Example
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
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Device Selection Example
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
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Device Selection Example
Using Frequency and P1dB selection criteria (±3dB window)
Five GPA options are listed, but gain is too low
C
P
Look through GPA Portfolio
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Device Selection Example
Using Frequency and P1dB selection criteria (±3dB window)
Two Linear Amp options, with adequate gain, are listed
C
P
Look through Linear Amplifier Portfolio
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Device Selection Example
We find that MMA20312BV is the best choice
since its gain and linear performance provides
strong ACP performance to Pout = 19 dBm
C
P
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Device Selection Example
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
Tuned to 2110 – 2170 MHz
28 V
15.5 dB Gain
24% Eff @ Pout = 33dBm
A2T21H410-24S
LDMOS Doherty HPA
Tuned to 2110 – 2170 MHz
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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