High Efficiency Class-E Amplifier Utilizing GaN HEMT

High Efficiency Class-E Amplifier Utilizing GaN HEMT
High Efficiency Class-E Amplifier Utilizing GaN HEMT Technology
William L. Pribble, James M. Milligan, Raymond S. Pengelly
Cree Inc., Durham, NC 27703 USA
Abstract
A class-E power amplifier based on a GaN HEMT cell has been designed and tested. Around 2GHz, the
amplifier provides 10 watts with associated PAE of 85% and gain of 12dB. Class-E amplifiers have
shown high power and PAE at VHF and high efficiency at S-band, but this result is the first demonstration
of class-E efficiency with high associated power, greater than 10 watts, at microwave frequencies. The
unique combination of high current HEMT operation and high breakdown voltage afforded by widebandgap technology, inherent with GaN HEMTs, makes this result possible.
Introduction
The GaN HEMT cell used for this design nominally provides 10 watts at p1dB with associated gain of
17dB at 2 GHz when operated in class-AB mode. The breakdown voltage is typically greater than 100
volts.
Ideal class-E operation produces drain voltages approximately 3 times greater than the bias voltage.
Another limitation that has been derived for ideal operation relates bias voltage, peak current, output
capacitance and maximum operating frequency is shown in the following expression:
fmax:=
Imax
− 12
⋅ cs ⋅ vcc
56.5⋅ 10
Based on this expression, power can only be optimized with high breakdown voltage if the ratio of Imax to
output capacitance remains high. Considering all active devices available for use in microwave
amplifiers, GaN HEMTs offer the best trade for class-E operation.
Design and Results
A non-linear model of the GaN HEMT has been developed and used to design a class-E hybrid amplifier
which operates around 2 GHz. The output match is based on classic class-E theory, integrating the
entire drain-source capacitance, Cds, into the output matching network design. The input match is
designed to provide a reasonable match at approximately 25% Idss, which is the full power operating
point. This should provide peak gain under full RF drive. The amplifier has been constructed using
alumina substrates on which the input and output distributed networks are fabricated. The amplifier, as
constructed, is shown in Figure 1. The amplifier has been measured for saturated output power and PAE
around 2GHz with results shown in Figure 2. The PAE is greater than 82% with output power above 10
watts form 1.9 to 2 GHz. Since the class-E amplifier is modulated through the drain supply, a plot of
power and PAE with drain voltage is shown in Figure 3.
Conclusions
A 10 watt class-E amplifier operating at 2GHz with ~84% PAE has been demonstrated using GaN HEMT
technology. The high current and breakdown voltage, along with high Ft that are possible using the GaN
HEMT make high frequency class-E operation possible.
Copyright © 2006 IEEE. Reprinted from the 2006 IEEE Radio and Wireless Symposium.
This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of
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Output Power (dBm)
Figure 1 – Class-E Amplifier Module
41
86
40.5
84
40
82
39.5
80
39
78
38.5
76
38
1.75
1.8
1.85
1.9
1.95
2
2.05
2.1
power
pae
74
2.15
Frequency (GHz)
41
100
40
90
39
80
38
70
37
60
36
50
35
40
34
30
33
20
32
10
31
pae (%)
Output Power (dBm)
Figure 2 – Class-E Amplifier Measured Performance
0
0
10
20
30
40
Drain Voltage (V)
Figure 3 – Class-E Amplifier Drain Modulation
pow er
pae
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