Doherty Amplifiers: Design and Analysis

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Doherty Amplifiers Design and Analysis

• Masaya Iwamoto, Aracely Williams, Chengzhou Wang,
  Professor Larry Larson Professor Peter Asbeck
• University of California at San Diego

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Advantages of Doherty Amplifier Architecture

• The Doherty amplifier achieves high efficiency
  over a wide range of output power.
• A key requirement for handset PA applications

Data for representative CDMA cellular handset
For ideal Class A amplifier average efficiency is
1.26
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Principle of Doherty Amplification
Iout
Main PA
l/4
High power operation
Zc2RL
Reff
Pin
Main PA
RL
l/4
Low power operation
Vout
Zo
Voltage swing stays constant gt no saturation
effects
Peaking PA
1. At low input power drive, Main PA only
operates with Reff4RL 2. When Main PA begins
to saturate, Peaking PA turns on and begins to
pull down load Reff. 3. At peak power, both PAs
are presented with Reff 2RL.
Between regions 2 3, Main PA operates at max
efficiency
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Analysis with Ideal Current Sources
l/4
Zc2RL
iL

• VL is independent of ib since the current source
  sees a short circuit looking into the l/4 line.

Va
ia
RL
VL
ib
does not depend on ib !
Va depends on ib through load-pulling ? an
extra knob to control Va
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Design Strategy
For Doherty operation, choose ib so Va never gets
too large gt at low power (ialticritical)
choose gt at high power (iagticritical) choos
e We get
Main (Device A)
Iout
Vout
Va
Va constant as ia increases !
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Analysis with Finite Output Impedance
l/4
Va
iL
Zc
Roa
RL
VL
for ia lt icritical
otherwise
Rob
with g2
Main PA saturated
Output is not a linear function of ia
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Effects of Finite Output Impedance
4RL

• Finite output impedance detrimentally affects
  efficiency when it becomes comparable to the load
  resistance

• Gain varies at low and high drive levels (AM-AM
  distortion)

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Design Methodology for Enhanced Efficiency at
Lower Output Power
for ia lt icritical
otherwise

• By carefully selecting g, high efficiency can be
  achieved at lower power
• However, higher g exacerbates finite output
  impedance effects

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Conclusions Future Work

• Doherty amplifiers achieve high efficiency over a
  wide range of output powers with proper design
  considerations.
• Finite Rout causes design challenges
• AM-AM distortion (drive- level dependent gain)
• Decrease in efficiency as Rout becomes comparable
  to RL
• Experimental demonstration of Doherty amplifiers
  employing HBTs and FETs under investigation.