Spatial Combining Leads To MM-Wave Power

This series of amplifiers employ transistor arrays and spatial-combining techniques to generate high output-power levels at millimeter-wave frequencies.

Millimeter-wave power usually suggests large vacuum tubes. The founders of Wavestream Corp. (West Covina, CA) have other thoughts on the topic, however, and now offer spatial-combining techniques in their grid-array amplifiers that promise to generate tube-like power at millimeter-wave frequencies from large arrays of high-frequency transistors. Among the company's first products is a single-chip amplifier capable of 4 W output power at 38 GHz with third-order intercept point of +44.5 dBm.

Wavestream's novel spatial-combining technology combines the output signals of individual transistor elements in free space, rather than in a conventional power combiner. By free-space combining, signal losses are almost negligible compared to a passive power combiner. With the spatial-combining approach employed in Wavestream's grid-array amplifiers, the effective number of amplifier elements is limited only by the number of transistors that can be fabricated on a given chip size.

Essentially, a grid-array amplifier is an integrated circuit (IC) with an array of amplifier elements or unit cells. An input antenna at each unit cell receives horizontally polarized input waves incident on the back of the IC. The input waveforms can be distributed across the IC by means of an inexpensive waveguide adapter. These waveforms are then amplified by the unit cells, and radiated from the face of the IC through an output antenna. Input and output sections are matched by means of on-chip passive elements. Output signals are collected and summed (in phase) in a waveguide port.

One of the first amplifiers produced with the technology was a single-chip, 38-GHz amplifier, using a low-voltage GaAs pseudomorphic high-electron-mobility-transistor (pHEMT) process (see figure). The amplifier achieved +36 dBm (4 W) output power at 1-dB compression at 38 GHz. Gain varied from about 6 to 7 dB with an input-power range of +15 to +35 dBm, while AM/FM distortion was a low 2 deg./dB. The single-chip amplifier featured a third-order intercept point of +44.5 dBm.

Since matching structures are effectively moved off-chip, more of the GaAs chip area can be used for active elements (power), resulting in a lower cost per watt than other solid-state millimeter-wave amplifier approaches. The grid-array amplifiers employ free-space combining to reach efficiencies of 15 to 20 percent. Wavestream Corp., 100 N. Barranca St., Suite 910, West Covina, CA 91791; (626) 331-1272, FAX: (626) 966-0193,

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