Transcript Document

A 900MHz Doherty Amplifier
Implemented with Lumped Elements
Y. Zhao, M. Iwamoto, D. Kimball, L. Larson, P. Asbeck
University of California, San Diego
Introduction
 To increase overall efficiency of RF PA
 Use Doherty structure
 To shrink circuit size for integration
 Use lumped elements to replace transmission lines
 To achieve good linearity
 Use DSP to control Doherty Amplifier
7/16/2015
Agenda
 Doherty amplifier architecture
 Doherty with lumped elements
 Design issues
 Simulations
 Measurements
 DSP application in Doherty amplifier
 DSP control mechanism
 Measurement Results
 Conclusions
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Doherty Architecture
 Low power range
 only Main-PA operates
 High power range
 Main-PA goes into
saturation
 Aux-PA turns on
Load-line of main amplifier
Iout
High power
operation
Low power
operation
Vout
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Lumped Doherty --- Design issues
 Size of 90° transmission line
 900M --- 48mm
 2.4G --- 19mm
Er = 3.48
 5.2G --- 8.7mm
 Size of a general PA package
4mm x 4mm
 Design requirements
Zin
Zl
 Impedance inversion
 Phase compensation
Zin  K 2 / Zl
K  L  1 / C
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Lumped Doherty --- Design issues
 C-L-C (9.1pF, 2.7nH)
 Impedance inverter
 90 degrees delay
 Provide VDD to Aux-PA
 Harmonic trap
 L-C-L (1.8nH, 15pF, 3.3nH)
 90-degree phase shifter
 part of the matching
 DC block
 L-match (13pF, 2.2nH)
 Output impedance match
 DC block
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Lumped Doherty --- Measurement
 A GaAs MESFET-based hybrid Doherty
amplifier was built with lumped elements
Drain efficiency, PAE and Gain
Bandwidth
S21
1dB/div
1dB Bandwidth ~ 110MHz
 CDMA needs 25MHz/836MHz
Frequency
50MHz/Div
 802.11b,g need 83MHz/2.44GHz
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Lumped Doherty --- Measurement
PAE vs. Pout
Lumped Doherty vs.
a simulated Class AB
PAE average 
IS-95 CDMA PDF
 Pout    Pin 
 Pdiss 
Lumped Doherty – 14.1% > 3X Class AB – 4.4%
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Comparison --- Lumped and distributed
Similar PAE performance
Slightly narrower bandwidth
for lumped Doherty
 ADS simulation
 Same devices and design
 Check 1dB bandwidth/900MHz
 the lumped Doherty 130MHz
 the distributed Doherty 150MHz
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Comparison --- Lumped and distributed
 Size reduction by using lumped elements is dramatic
 Connection lines and components in hybrid circuit
can be further shrunk
in a IC module
schematic
Circuit area
Quarter-wave lines
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Potential problem --- Linearity
60
0
50
-10
40
-20
30
-30
20
-40
10
-50
0
ACPR (dBc)
For CDMA signal, ACPR may
not be good enough
PAE (%)
Non-ideal gain and phase in high
power region can cause a linearity
problem
-60
5
10
15
20
25
30
Average Power (dBm)
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DSP Linearization Strategy
 Gain control
System block diagram
 Change Vgg2 according to the
instantaneous envelope of the
input CDMA signal
 Phase control
 Baseband phase predistortion
 Open loop control with
lookup table
“Doherty Amplifier with DSP Control to Improve Performance in CDMA Operation,”
2003 IEEE MTT-S Digest, p687-690
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60
20
-20
50
18
-30
16
40
14
30
12
20
10
ACPR (dBc)
uncorrected
Gain (dB)
PAE (%)
Measurement --- linearized IS-95 signal
DSP corrected
-40
-50
-60
-42dBc
-70
10
8
0
6
-80
5
10
15
20
25
30
Average Power (dBm)
 High efficiency is maintained
5
10
15
20
25
10dB/Div
uncorrected
 ACPR improvement of up to
9dB is achieved; CDMA ACPR
specifications are met
 Peak power is limited by the
device size
30
Average Power (dBm)
DSP corrected
0.5MHz/Div
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Conclusions
Conclusions
 Doherty amplifier helps achieve high efficiency over wide
output power range
 Doherty amplifier with lumped elements shrinks circuit
size while maintaining good performance
 Linearity of the Doherty amplifier can be substantially
improved by DSP to meet the CDMA ACPR specification
 A promising approach for CDMA handset and other
wireless applications such as 802.11
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