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22 October 2008


Fujitsu reports record 320W C-band power amplifiers

At last week’s 2008 IEEE Compound Semiconductor IC Symposium (CSICS) in Monterey, CA, USA (12-15 October), Tokyo-based Fujitsu Laboratories Ltd announced the development of two high-efficiency RF amplifiers based on gallium nitride (GaN) high-electron-mobility transistors (HEMTs) with record power output in the C-band: one with 320W output with 57% efficiency (the ratio of DC input power converted to high-frequency output power) and another with 250W output with 60% efficiency. These both significantly outperform the previous record of 220W per package (Figure 1).

Figure 1: Comparison of power and efficiency for C-band power amplifiers exceeding 100W.

According to Fujitsu Laboratories, using GaN HEMT technology increases output power over existing amplifiers using gallium arsenide (GaAs) transistors by more than six-fold (in the case of the 320W amplifier), and should enable the extension of transmission range by 2.4 times. In addition, replacing conventional traveling-wave tube amplifiers (vacuum tubes commonly used for high-output-power microwave applications) with this new technology should enable smaller, lighter, more energy-efficient and longer-lasting transmission systems. Transmission at C-band frequencies (4-8GHz) is relatively unaffected by rainfall and fog, and applications include satellite and fixed wireless communications, wireless access, next-generation mobile phone base-stations, air-traffic control radar, and weather radar.

In particular, high-output-power microwave transmitters are needed to increase the capacity of satellite communication transmissions and extend radar detection range. Due to the fact that traveling-wave tube amplifiers are built around vacuum tubes, they have shorter life spans than solid-state devices and are relatively large and heavy because they require high-voltage power sources. High-output-power solid-state amplifiers based on GaAs transistors can provide only low output power from a single element due to their low breakdown voltage, requiring composite output from many transistors to achieve adequate transmission power. The loss attributable to this composite circuitry results in reduced efficiency. By comparison, amplifiers using GaN HEMTs have higher output power and better heat dissipation than GaAs transistors.

Transistor chips for high-output-power amplifiers are configured with multiple transistors arrayed in parallel. With a single input and output, the signal that moves along the chip center and the signal that moves along the edges traverse different lengths of wired lines, inducing a phase difference. So, transistors are operating out of phase with each other, making it difficult to fully maximize the high-output-power characteristics inherent in the transistors.

Figure 2 (above): Fujitsu's new C-band GaN HEMT-based amplifier.

Fujitsu’s new C-band GaN HEMT-based amplifier (Figure 2) consists of two transistor chips that resolve this while producing high output. The use of multi-divided input pathways prevents the occurrence of phase differences to the input signal between the transistors (Figure 3), enabling GaN HEMTs with high output power densities to operate uniformly and efficiently combine their output power. Since the use of two chips can sometimes create undesirable thermal interference and raise temperatures (reducing transistor output), the two chips are spaced from each other (suppressing mutual interference).

Fujitsu says that these technologies enable the transistors to operate uniformly and should maximize the full high-output-power potential inherent in GaN HEMTs.

Figure 3: Structure of the amplifier circuit.

See related items:

Fujitsu claims first high-output GaN HEMT to cut power in standby mode

Fujitsu develops first impulse radio transmitting at over 10Gb/s in millimeter band

Fujitsu’s GaN HEMT-based mobile WiMAX base-station chosen by UQ

Fujitsu launches GaN HEMT-based mobile WiMAX base-stations

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