Wireless
System Considerations
for Power Amplifiers
Agenda
Power Computation
General principles of modulation formats
Impact of modulations on power amplifier
specifications
Average efficiency calculations
Power Computation
Instantaneous power P(t)=V(t)I(
Amplifiers with Improved Efficiency
Doherty Amplifier
Outphasing (LINC or Chireix) Amplifier
Envelope Tracking Amplifier
Envelope Elimination & Restoration Amplifier
Critical problem for power efficiency:
Varying signal power level
Efficiency
1
Average po
ECE 265C
Power Amplifiers
for Wireless Communications
Instructor:
Peter Asbeck
Office EBU1-3607
Phone 534-6713
Fax 534-0556
[email protected]
Objectives
1) Provide foundation in fundamentals of solid-state power amplifiers
as required for graduate work
Impedance Matching
In Power Amplifiers
Impedance Matching
In Power Amplifiers
Impedance Matching 101: Lumped Elements
Procedure for Impedance Matching Using L Sections
Based on Q Concepts
Q Definitions for Reactive Elements
For series and parallel represe
Transistor Technologies
for High Efficiency and Linearity
GaAs HBT Class E
CMOS Differential Cascode with IPD
Requirements for Transistors
in Power Amplifiers
High microwave gain
Iout
Low on-resistance & low knee voltage
FET
High power density
High voltag
Switching-Mode Power Amplifiers
Switching-Mode Amplifiers
Switching-mode minimizes power dissipation in transistor:
when voltage is high, current is zero
when current is high, voltage is minimum
No overlap
Vo
Iout
driver
Vsw
Pdiss
Vh
Isw
Vout
time
time
Ex
Stability of Power Amplifiers
Why Do Microwave Amplifiers Oscillate?
1) Most are intrinsically unstable at low frequencies
(K<1) due to internal feedback in the transistor
Frequency (Hz)
With terminations of an open or a short, transistors will often
osci
Classical Amplifiers
Class C
Class C Amplifier
Conduction angle less than 180 degrees
Iout
Imax
Harmonics
are shorted
Vo
match
match
RL
Vmin
Vo
Vmax
Vce
Vo
IC
Idc
Vout
Iquiescent= 0
Idc < Irf /
Vrf
time
time
Pout (rf) < 1/4 Vrf Irf
Pin (dc)= Vo Idc
max>
Characteristics of
Classical Transistor Amplifiers
Classical Analysis of Power Amplifiers
Underlying assumption:
simple model of transistor
Iout
Imax
vin
iout
Vout
Transistor acts like current source, with Iout a linear replica of vin,
except for limitati
Predistortion
Predistortion
Predistorter
Gain
Phase
PA
PA
Gain
Pin
Pin
Phase
Predistorter
Poutp=Pinf
Poutp=Pinf
Where PA has gain compression, predistorter has gain expansion
(and vice-versa)
Predistortion
Predistorter
Gain
Phase
PA
PA
Gain
Pin
Pin
Phase
Simulation of Power Amplifiers
Power Amplifier Simulation Problem
Vo
Not ideal
match
match
Nonlinear
input
circuit
Lossy
Harmonic response
critical
Transistor response nonlinear
Nonreciprocal
Iout
Iout
Vin
Vin
Power Amplifier Simulation
Small signal linea
Outphasing Amplifiers
Critical problem for power efficiency :
Varying signal power level
Efficiency (%)
Average power
Maximum power
100
OutphasingDoherty
80
EER/ET
60
Class B
40
Class A
20
0
0
0.2
0.4
0.6
0.8
Pout (normalized)
1
Probability
5.0%
Probabili
Envelope Tracking Architecture
Critical problem for power efficiency :
Varying signal power level
Efficiency (%)
Average power
Maximum power
100
OutphasingDoherty
80
EER/ET
60
Class B
40
Class A
20
0
0
0.2
0.4
0.6
0.8
Pout (normalized)
1
Probability
5.0%
Linearization of Power Amplifiers
Linearization Objective
Change design of amplifier to get better linearity
(better gain flatness, better phase flatness vs Pin)
In order to get better ACPR or EVM performance
While maintaining high efficiency
Gain
Efficie
Microwave Power Combiners
Power Combining
Applications of Power Combining
Power Combining Problem for Microwave PAs
Combining power from various unit PAs is one of the central
problems of high power PA design.
CMOS microprocessors have >100M transistors.
Transistor Technologies
for High Efficiency and Linearity
Heterojunction Bipolar Transistors
HBT
Heterojunction Bipolar Transistor
Pros
High microwave gain
High power density =small die size
Straightforward fabrication
(simple lithography
no Vth control p
Analysis of nonlinearity
in power amplifiers
Action of Nonlinear (Limiting) Amplifier
Compresses signal, clipping amplitude peaks
Induces spectral regrowth
specified by ACPR adjacent channel power ratio
& alternate channel power ratio
ACPR1
Causes loss of