A 900MHz Doherty Amplifier Implemented with Lumped Elements

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

University of California, San Diego

04/18/23

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

Introduction

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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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Low power range only Main-PA operates

High power range Main-PA goes into

saturation Aux-PA turns on

Iout

Vout

High power operation

Low power operation

Load-line of main amplifier

Doherty Architecture

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Size of 90° transmission line

900M --- 48mm

2.4G --- 19mm

5.2G --- 8.7mm

Size of a general PA package 4mm x 4mm

Lumped Doherty --- Design issues

Design requirements

Impedance inversion

Phase compensation lin ZKZ /2

inZ lZ

CLK /1

Er = 3.48

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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

Drain efficiency, PAE and Gain

1dB Bandwidth ~ 110MHz

Frequency

S211dB/div

A GaAs MESFET-based hybrid Doherty amplifier was built with lumped elements

Bandwidth

CDMA needs 25MHz/836MHz

802.11b,g need 83MHz/2.44GHz

50MHz/Div

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PAE vs. Pout

Lumped Doherty vs. a simulated Class AB

IS-95 CDMA PDF

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

Lumped Doherty --- Measurement

Pdiss

PinPoutPAE average

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Similar PAE performance

Comparison --- Lumped and distributed

ADS simulation

Same devices and design

Check 1dB bandwidth/900MHz

the lumped Doherty 130MHz

the distributed Doherty 150MHz

Slightly narrower bandwidthfor lumped Doherty

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schematic

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

Circuit area

Quarter-wave lines

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Potential problem --- Linearity

0

10

20

30

40

50

60

5 10 15 20 25 30

Average Power (dBm)

PA

E (

%)

-60

-50

-40

-30

-20

-10

0

AC

PR

(d

Bc

)

Non-ideal gain and phase in high power region can cause a linearity problem

For CDMA signal, ACPR may not be good enough

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DSP Linearization Strategy

System block diagram 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

“Doherty Amplifier with DSP Control to Improve Performance in CDMA Operation,”

2003 IEEE MTT-S Digest, p687-690

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Measurement --- linearized IS-95 signal

-80

-70

-60

-50

-40

-30

-20

5 10 15 20 25 30

Average Power (dBm)

AC

PR

(d

Bc)

uncorrected

DSP corrected

0

10

20

30

40

50

60

5 10 15 20 25 30

Average Power (dBm)

PA

E (

%)

6

8

10

12

14

16

18

20

Gai

n (

dB

)

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 0.5MHz/Div

10dB/Div

-42dBc

DSP corrected

uncorrected

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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

Conclusions