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 l.jpg

A 900MHz Doherty Amplifier Implemented with Lumped Elements

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

University of California, San Diego


Introduction l.jpg
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 l.jpg
    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg
    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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