600-W, C-Band GaAs SSPA Rivals TWTAs

Nov. 26, 2012
Thanks to spatial power combining, these solid state power amplifiers (SSPAs) do not limit the amount of power that could be provided by an individual solid-state transistor—a problem that is often encountered when SSPAs are used in broadcast satellite-uplink terminals.

To provide a C-band uplink signal, commercial satellite broadcasters customarily use traveling-wave-tube amplifiers (TWTAs). Yet some of these uplink terminals may be just as well served by a solid-state power amplifier (SSPA). Because broadcast stations are redundant, there may be a redundant amplifier standing by for every transmitting amplifier. SSPAs require very little warm-up time, which means the redundant station’s standby amplifier can be held in a reduced-power-consumption mode until it is needed. At Wavestream Corp., a C-band amplifier boasting saturated output power beyond 600 W has been designed by Ha Trong Than, George W. Sun, Blythe C. Deckman, Michael P. DeLisio, Chung Younkyu, M.E. Moya, N.T. Schultz, G.S. Cuellar, and Zeng Jiyang.

In this amplifier design, a two-deck approach was implemented to improve heat dissipation and reliability. Because deck-amplifier architectures use spatial power combining, the team combined the output power of 16 internally matched, 45-W gallium-arsenide (GaAs) field-effect transistors (FETs). Each FET had a rated output power of 45 W with small-signal gain of 9 dB.

With spatial power combining, the outputs of many devices are added in air or within single-mode waveguide using a single stage of power combining. Each device or set of combined devices is connected to an antenna or other means to couple energy between the device terminals and the radiating or waveguide mode. Because the energy is distributed and combined in air or within low-loss waveguide, ohmic losses are reduced. The advantage to this approach is that losses in spatially combined systems tend to be fixed and minimal.

Among the other features of this rack-mountable indoor unit, which measures 24 x 19 x 8.75 in., are pre-amplification and driver amplification stages and a level-control variable attenuator. In addition, a predistortion linearizer splits the signal into two paths. One path contains a nonlinear saturating element. By recombining these signals with proper phasing, the team is able to achieve enough gain expansion and phase change to compensate for the PA’s saturation. By performing power combining on two 600-W amplifiers with a magic-T waveguide combiner, the researchers created a solid-state amplifier with 1.4 kW of saturated C-band output power. The unit can be packaged as a fully weatherized outdoor unit with an integrated L-band upconverter as well. See “Design and Performance of a 600-W C-Band Amplifier Using Spatially Combined GaAs FETs for Satellite Communications,” IEEE Journal Of Solid-State Circuits, Oct. 2012, p. 2309.

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