Distributed%20Power%20Amplifiers-Output%20Power%20and

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Distributed Power Amplifiers-Output Power and
Efficiency Considerations
Prasad N. Shastry (S. N. Prasad)1 , Senior
Member, IEEE, and Amir S. Ibrahim2
1Department of Electrical and Computer
Engineering, Bradley University, Peoria, IL,
2Mobile Devices, Motorola Inc., Libertyville, IL
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Abstract In this paper the efficiency and output
power limitations of distributed amplifiers (DAs)
and techniques to overcome the limitations are
addressed, and the analysis, design, and measured
results of a tapered drain line distributed power
amplifier are presented in detail for the first
time. The DA has a gain of 10 dB,
power-added-efficiency of 24, and output power
of about 20 dBm, over a 4.5 GHz bandwidth.
Further, the efficiency of the tapered drain line
DA is shown to be comparable to a reactively
matched power amplifier using the same number and
type of transistors. The topology of the tapered
drain line DA is suitable for monolithic
implementation.
Gain Comparison Between DAs, Reactively Matched
Amplifiers, and Tapered Drain Line DAs

• Power and efficiency limiting factors in a
  distributed amplifier
• Transistor pinch-off
• Unequal contribution by transistors to output
  power
• Transistor gate-drain breakdown
• Un-utilized power in the reverse wave
• Non-optimum load impedance presented to
  transistors
• Tapered drain line distributed amplifier
• Each transistor must see its optimal load
  impedance, ROPT.
• Drain line consists of line segments of varying
  characteristic impedance connected between the
  output terminals of the cells.
• The characteristic impedance of the line segment
  between cells m and m1 is given by, Zom
  ROPT/m.
• Gate line is designed as in a conventional
  lowpass distributed amplifier.
• Simulation Results and Analysis
• Transistor NE721 Number of transistors 4
  Frequency range 0-4.5 GHz DC-Bias for each
  transistor Vds 3V Vgs -0.675V Ids 30 mA.
• Conventional lowpass distributed amplifier
  
• Image impedance of gate and drain lines 50 O.
• Reactively matched balanced amplifier
• Optimum load and source impedances at 2 GHz
  ZL 56.32 j14.43 O ZS 62.02
  j59.67 O

Tapered drain line, conventional gate
line-distributed power amplifier.
PAE () Comparison Between DAs, Reactively
Matched Amplifiers, and Tapered Drain Line DAs
P1dB Comparison Between DAs, Reactively Matched
Amplifiers, and Tapered Drain Line DAs
Logical construction of the drain line of the
tapered drain line of a distributed amplifier.
http//cegt201.bradley.edu/rfpage
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• Transistor NE 721 Number of transistors 4
• Amplifier DC-Bias Vds 3V Vgs -0.74V
  Ids (total) 120 mA
• Circuit Boards
• Gate line er 3.1 h 30 mil
• Drain line er 10 h 25 mil
• Heat Sink material Brass

Picture of the amplifier (including the heat sink)
Conventional Distributed amplifier
Conclusions A tapered drain line DA having a gain
of 10dB and power-added efficiency of 24 over a
4.5 GHz band has been realized. The results
showed that the tapered drain line approach
increased the power-added-efficiency from about
11 to 24 across the 4.5 GHz band. This
relatively high value of efficiency for a DA is
comparable to the 27 power-added-efficiency of
the reactively matched power amplifier. Acknowle
dgment The authors would like to thank
Mr. Balamurugan Sundaram and
Mr. Sureshbabu Sundaram, graduate students,
Department of Electrical and Computer
Engineering, Bradley University, for preparing
the illustrations and formatting this paper.
Plot showing simulated and measured magnitudes of
S21.
A four-stage balanced power amplifier.
Measured gain compression at three different
frequencies.
Distribution of power on the drain line for a
tapered drain line DA
Layout of the amplifier with output matching
network
Measured third and fifth order intermodulation
levels versus input power.
http//cegt201.bradley.edu/rfpage