A 900MHz Doherty Amplifier Implemented with Lumped Elements

1

• A 900MHz Doherty Amplifier Implemented with
  Lumped Elements

Y. Zhao, M. Iwamoto, D. Kimball, L. Larson, P.
Asbeck University of California, San Diego
2
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

3
Agenda

• Doherty amplifier architecture
• Doherty with lumped elements
• Design issues
• Simulations
• Measurements
• DSP application in Doherty amplifier
• DSP control mechanism
• Measurement Results
• Conclusions

4
Doherty Architecture

• Low power range
• only Main-PA operates
• High power range
• Main-PA goes into saturation
• Aux-PA turns on

5
Lumped Doherty --- Design issues

• Size of 90 transmission line
• 900M --- 48mm
• 2.4G --- 19mm
• 5.2G --- 8.7mm
• Size of a general PA package 4mm x 4mm

Er 3.48

• Design requirements
• Impedance inversion
• Phase compensation

6
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

7
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
Frequency
50MHz/Div

• CDMA needs 25MHz/836MHz
• 802.11b,g need 83MHz/2.44GHz

8
Lumped Doherty --- Measurement
PAE vs. Pout Lumped Doherty vs. a simulated
Class AB
IS-95 CDMA PDF
Lumped Doherty 14.1 gt 3X Class AB 4.4
9
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

10
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
11
Potential problem --- Linearity
Non-ideal gain and phase in high power region can
cause a linearity problem
For CDMA signal, ACPR may not be good enough
12
DSP Linearization Strategy

• Gain control
• Change Vgg2 according to the instantaneous
  envelope of the input CDMA signal
• Phase control
• Baseband phase predistortion
• Open loop control with lookup table

System block diagram
Doherty Amplifier with DSP Control to Improve
Performance in CDMA Operation, 2003 IEEE MTT-S
Digest, p687-690
13
Measurement --- linearized IS-95 signal
-42dBc
10dB/Div

• High efficiency is maintained
• ACPR improvement of up to 9dB is achieved CDMA
  ACPR specifications are met
• Peak power is limited by the device size

uncorrected
DSP corrected
0.5MHz/Div
14
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