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A 900MHz Doherty Amplifier Implemented with Lumped Elements

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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

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slide1
A 900MHz Doherty Amplifier Implemented with Lumped Elements

Y. Zhao, M. Iwamoto, D. Kimball, L. Larson, P. Asbeck

University of California, San Diego

introduction
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
agenda
Agenda
  • Doherty amplifier architecture
  • Doherty with lumped elements
    • Design issues
    • Simulations
    • Measurements
  • DSP application in Doherty amplifier
    • DSP control mechanism
    • Measurement Results
  • Conclusions
doherty architecture
Iout

Load-line of main amplifier

High power operation

Vout

Low power

operation

Doherty Architecture
  • Low power range
    • only Main-PA operates
  • High power range
    • Main-PA goes into saturation
    • Aux-PA turns on
slide5
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
slide6
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
slide7
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
slide8
Lumped Doherty --- Measurement

PAE vs. Pout

Lumped Doherty vs. a simulated Class AB

IS-95 CDMA PDF

Lumped Doherty – 14.1% > 3X Class AB – 4.4%

slide9
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
slide10
Circuit area

Quarter-wave lines

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

slide11
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

slide12
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

slide13
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

conclusions
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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