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RF Power Amplifiers VI
RF Power Amplifiers VI
Power Amplifier Solutions
for Base StationsG. Ghione, M . Pirola, R. Quaglia
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RF Power Amplifiers VI
Agenda
Base stations
Doherty Power Amplif ier
L inearity Enhancement Techniques
Digital Predistortion
Feed Forward
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RF Power Amplifiers VI
Base stations
Base stations are the front-end of the mobileinfrastructure
I nteract with:
Mobile TerminalDown link: Base station TX
Up link: Base station RX
Backhaul (BSC)Core of the network
Cable, fiber or microwaves
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RF Power Amplifiers VI
Base stations
StandardsGSM : 950 MHz and 1850 MHz, 50-100 W
UMTS: 2100 MHz, 50-100 W
WiMax: 3500 MHz, 4 W
LTE: 2100-2500 MHz, 50-100 W
F igures of merit
Cost
Linearity
Efficiency
Re-configurability
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A basestation power budget
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Linearity/Efficiency as linked issues
Power amplif ication of variable-envelope signals(var iable-power signals):
LINEARITY: AM and also PM distortion take
place if the PA is used at its full-rated RF power
level
EFFICIENCY: Conventional design of high-
efficiency PA leads to good solution only near the
maximum rated power; if the power is backed offefficiency drops sharply
I ssues: l inearity and/or eff iciency enhancement
techniques
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Why variable-power signals
I ndependent of the variable-envelope issue, the
output power of a PA is not constant:
In single-channel PAs, because the output power is
adapted to receiving station conditions (location,
environment)
In multiple-channel PAs (basestations) because the
total multichannel power undergoes statistical
fluctuations related to location, traffic pattern,
environment
This implies back-off vs. optimum PA operating
conditions
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RF Power Amplifiers VI
Defining backoff
Suppose that a reference operating conditionfor a PA occurs with a given (reference) input
power
We say that the PA operates with a given backoff
(e.g. 3 dB, 10 dB...) with respect to the reference
condition when the input power isreducedby 3,
10... dB with respect to the reference input
power
Alternatively: the PA is backed off3, 10... with
respect to the reference condition
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RF Power Amplifiers VI
Two examples of backoff
A class A PA is backed off with respect to the 1dB compression pointin order to increase
linearityreduce IMPs
An amplif ier is backed off with respect to the
optimum eff iciencycondition because the slowly
varying average input power is decreased
Backoff usually improves linearity, decreases
eff iciency; the eff iciency deter ioration is
maximum in highly l inear (class A) amplif iers
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RF Power Amplifiers VI
Efficiency vs. backoff: class A and B
Max.
class A
power
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RF Power Amplifiers VI
The multichannel amplifier
Basestation ampli f iers for
multichannel systems (e.g. CDMA)
require IMP3 levels of the order of
60-80 dBc as compared to the usual
20-30 dBc in single-channel PAs
This requi rement is extremelysevere and can be avoided only by
using a mul tiplexdemultiplex
structure with an array of single-
channel ampl if ier in the place ofthe mul tichannel one
Cons: MUX-DEMUX design, fixed
channel, less f lexible structure
Channelized PA
Multi Channel PA
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RF Power Amplifiers VI
The linearization issue
Usual ly the -30 dBc IMP3 requi rement can be satisf ied by
working in class A at 1 dB compression point, with ~30-50%
efficiency
To obtain70 dBc the class A amplif ier must work with 20 dB
backoff with respect to 1 dB compression point
Suppose a 100 W amplif ier absorbing 200 W DC power is
working at 20 dB backoff the output power wi l l be only 100W/100 = 1 W with a 0.5% eff iciency!
I f we need real 100 W output power the DC power wil l be 20
kW!!!
Therefore, simple backof f does not solve the problem in an
acceptable way we need to improve the amplif ier linearity athigh input power
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RF Power Amplifiers VI
Agenda
L inearity, eff iciency tradeoff , var iable-power signals, backoff
Doherty Power Amplif ier
L inearity Enhancement Techniques
Digital Predistortion
Feed Forward
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RF Power Amplifiers VI
Doherty Power Amplifier
I nvented in 1936 by W.H. DohertyUsed with MW tubes and modulated signals
Multistage Power Amplif ier
H igh eff iciency also in back-off region
Based on 3 concepts
Load Modulation
Active Load Pull
Impedance Inversion
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Load Modulation
Why a Class B is ineff icient in back-off?
Maximum drive level
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Load Modulation
Why a Class B is ineff icient in back-off?
Maximum drive level
RL = RoptPout = PMAX 78%
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Load Modulation
Why a Class B is ineff icient in back-off?
Half dr ive level
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Load Modulation
Why a Class B is ineff icient in back-off?
Half dr ive level
RL = RoptPout = PMAX-6dB
41%
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Load Modulation
Why a Class B is ineff icient in back-off?
Half dr ive level
RL = RoptPout = PMAX-6dB
41%
Voltage does not
reach zero:
efficiency drop!
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RF Power Amplifiers VI
Load Modulation
What happens if I change the load?Half dr ive level
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Load Modulation
What happens if I change the load?Half dr ive level
RL = 2RoptPout = PMAX-3dB
78%
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RF Power Amplifiers VI
Load Modulation
What happens if I change the load?Half dr ive level
RL = 2RoptPout = PMAX-3dB
78%
Voltage
reaches zero!
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RF Power Amplifiers VI
Load Modulation
I f the load varies from 2Roptto Roptwhen inputpasses from half drive to full drive the eff iciency
stays high (near 78%)
How can this be realized?
We need to change dynamical ly the load
Active load pull
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RF Power Amplifiers VI
Active Load Pull
Principle of working
The load presented at device 1 depends on the
cur rent pumped by device 2 into the commonload
We need decreasingof Z1if input increases
1
2
1
21
1
1 1
I
IR
I
IIR
I
VZ
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RF Power Amplifiers VI
Impedance Inversion
Permits the r ight modulation of the load
I2 = 0 Z1= R I2= I0 Z1= R/2Choosing R = 2Ropt gives exact results
20
2
20
0
0
20
2
1
1II
IR
II
IR
I
IIR
RZ
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RF Power Amplifiers VI
Doherty Power Amplifier
Schematic Representation
90 degrees line permi ts the phase adjustmentM: main amplif ier , P: peak amplif ier
Behavior of the peak must be studied
M
P
Z Inverter
90 Degrees LOA
D
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RF Power Amplifiers VI
Peak Amplifier
Peak amplif ier is turned off up to half dr ive
vin
iout
vin
vout
vin
h,Pout
78%
PMAX-6dB
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RF Power Amplifiers VI
Peak Amplifier
Peak amplif ier is turned off up to half dr ive
Then i t turns on with double gmrespect to Main
At the end, the two amplif iers deliver the same
power, that is 6dB higher respect to half drive
vin
iout
vin
vout
vin
h,Pout
78%
PMAX
PMAX-6dB
f
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RF Power Amplifiers VI
Peak Amplifier
Peak amplif ier is commonly realized as
Class C amplifier
Class B with input control
The class C conf iguration is more commonInput control implies more added complexity
I n both cases, a greater gain respect to the main
amplif ier is needed
Due to the slow wake up of class C amplifier,
the ratio is 2.5, not 2
P k A lifi ti
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Peak Amplifier: practice
I n the original implementation, tubes where used
gm was controllable
With solid states device two solutions are adopted
Peak periphery greater respect to Main one Gain, PAE L inear ity, Hybrid circui tsUneven input power splitting
Same devices, linearity Gain, PAEBase station realizations require high linearity
Same devices, small unbalancing of inputs
20% of PAE improvement is commonly obtained
D h t P ti l li ti
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Doherty: Practical realization
I f this is a Doherty:
D h t P ti l li ti
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RF Power Amplifiers VI
Doherty: Practical realization
I f this is a Doherty:
.is this a power amplifier?
D h t P ti l li ti
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RF Power Amplifiers VI
Doherty: Practical realization
The Doherty is an intr insical ly narrow band circui t, due
to impedance inver ters
All devices have parasitic elements that inf luence the
matching
I f an imaginary part is involved in the load of the Main,the impedance inverter does not satisfy the rules of
impedance rotation studied before
Solution:
Insertion of an offset line
The bandwidth is sti l l reduced
D h t P ti l li ti
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Doherty: Practical realization
When the peak is off , i t is necessary that i t does not
inf luence the common load
I t has to appear as an open circui t
Presence of output parasitic change the impedance
Solution:
Insertion of an offset line
S22 of the peak must be 1
The bandwidth is sti l l reduced
Typical bandwidth of a Doherty is near to 5%
D h t P ti l li ti
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RF Power Amplifiers VI
Doherty: Practical realization
Typical bandwidth of a Doherty is near to 5%
M
P
ZInverter
90+a
Deg
L
O
AD
OMN Offset LineIMN
IMN OMN Offset Line
D h t P bl
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RF Power Amplifiers VI
Doherty: Problems
Theoretical eff iciency curve is never reached in practical
realizations, for many reasons
Knee voltage
Slow wake up of Class C
Trade-off with linearity
F ine tuning is necessary in base-station to achieve the
best l inearity, in particular on the gate bias levels
Bandwidth is l imited because the eff iciency, but also and
mainly the linear ity (AM -PM ), drop fast
D h t E l
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RF Power Amplifiers VI
Doherty: Examples
Jaewoo Sim et. al., Proceedings of Asia-Pacif ic
M icrowave Conference 2007
D h t E l
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Colantonio et. al., High efficiency solid state power
amplifiers, John Wiley and Sons
RF Power Amplifiers VI
Doherty: Examples
A d
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RF Power Amplifiers VI
Agenda
L inearity, eff iciency tradeoff , var iable-power signals, backoff
Doherty Power Amplif ier
L inearity Enhancement TechniquesDigital Predistortion
Feed Forward
P di t ti
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RF Power Amplifiers VI
Predistortion
Predistortion: the input signal of the power
amplif ier is conditioned to compensate its non-
l inear effects
PA
Predistortion
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RF Power Amplifiers VI
Predistortion
Predistortion: the input signal of the power
amplif ier is conditioned to compensate its non-
l inear effects
I n actual base-stations, conditioning is acted on
the modulation signal, dur ing digital processing
PAPre-distortion LinearizedOutput
RF Predistortion
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RF Power Amplifiers VI
RF Predistortion
~10 dB improvement of IM3, moderate eff iciency improvement,
better improvements require careful tr imming
Sti l l used at high microwaves and mm-waves
The pre-emphasis character istics can be realized by subtracting a
linear and a non-l inear signal path; sometimes only a cubic pre-
emphasis is generated to decrease IM3s (cubic predistorter)
Implementedwith diodes
Baseband Analysis
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RF Power Amplifiers VI
Baseband Analysis
Baseband signal can be described by a complex
envelope analysis
Signal is down-converted from the carrier
frequency to baseband
I-Q components completely describe the signals
This description depends on the chosen center
frequency
Careful must be adopted: also the absolute
reference of power is lost dur ing conversion
Baseband models
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RF Power Amplifiers VI
Baseband models
Baseband models relates the input and output
complex envelopes of a circui t, in our case of the
Power Amplif ier
Two main categor ies
No memory: output depends only on theinstantaneous input
With memory: output depends also on the past
inputs
Power Amplifier
Baseband Model
x(t) y(t)
Baseband models
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RF Power Amplifiers VI
Baseband models
Extraction of the baseband models can be
performed on time-domain measurement of the
complex envelope
Time is discrete: x[k], y[k]
Training algor ithms depends strongly on the
structure of the model itself
Least square meaning
Iterative algorithms
Remember: a model has a good behavior on the
domain where it was extracted
Baseband models
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RF Power Amplifiers VI
Baseband models
Examples of models
Look Up Table
No memory
Easy to extract
Memory Polynomial
Parallel structure
Odd monomia + F IR
Ef fective with LDMOS
Easy to train
Neural Networks
Base on tanh() function
Versatile
Hard training
Baseband predistortion
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RF Power Amplifiers VI
Baseband predistortion
Baseband predistor tion implements a baseband
model also for the predistortion function
To extract the predistor ter two strategies are
adopted
Direct learning: first a PA model is extracted; then
the DPD model is the inverse model of the PA
Indirect learning: DPD model is extracted directly
from measurements; it is in reality a post-distorterimplemented as a predistorter
Design and testing
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RF Power Amplifiers VI
Design and testing
Both for extraction and validation of the Digital
predistor ter , it is necessary to measure the
complex envelope of the signals at the ports of
the power amplif ier
A baseband setup is needed
We have to
Generate a modulated signal
Measure the output complex envelope
Extract the predistorter
Verify its effectiveness (measuring ACPR, )
Design and testing
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RF Power Amplifiers VI
Design and testing
Typical baseband setup
VSA: programmable
receiver that permits
envelope measurement
ADS: microwave CAD
with instrument
interfaces
I n real i ty, al l chain
between digital signal
and output is predistorted
Implementation
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RF Power Amplifiers VI
Implementation
A predistorter can be implemented on a FPGA
Constrains
Area
Consumption
Elaboration Speed
These must be considered since the design of the
Power Amplif ier : predistortability
Best solutions includes dynamic adaptation
A feedback from receiver is necessary
Design and testing
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The setup includes FPGA
RF Power Amplifiers VI
Design and testing
Examples
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RF Power Amplifiers VI
Examples
Meenakshi Rawat et. al., MTT Transactions, 2010
Examples
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RF Power Amplifiers VI
Examples
R. Quaglia et. al., EUMC 2009
Conclusions
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RF Power Amplifiers VI
Conclusions
Digital Predistortion is widely adopted in Basestations
and permits signicative amelioration in terms of
l inearity vs eff iciency tradeoff
Main constrain of its implementation is the need to act
on a power amplif ier with acceptable AM -AM and AM-PM distortion, otherwise a too complex predistorter is
needed
Costly
Power consuming
Not adaptive
Agenda
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RF Power Amplifiers VI
Agenda
L inearity, eff iciency tradeoff , var iable-power signals, backoff
Doherty Power Amplif ier
L inearity Enhancement TechniquesDigital Predistortion
Feed Forward
Feedforward
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RF Power Amplifiers VI
Feedforward
Very old idea, invented by Black together with the negative
feedback in the 20s
Basic pr inciple: to sum to the output of the power ampli f ier an
error signal compensating for the NL part of the response
Ideal Feedforward Analysis I
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Ideal Feedforward Analysis I
Ideal Feedforward Analysis II
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Ideal Feedforward Analysis II
Ideal Feedforward Analysis III
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Ideal Feedforward Analysis III
Ideal vs real feedforward
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Ideal vs. real feedforward
Amplitude and phase control
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Amplitude and phase control
Feedforward performances I
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Feedforward performances I
Feedforward performances II
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Feedforward performances II
High linearity PA example
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High linearity PA example