WARREN, N.J. — Anadigics Corp. has developed what it calls a "microwave dc/dc converter" that could solve a problem that plagues cellular-handset makers using gallium-arsenide RF transmitters and receivers. When incorporated onto GaAs chips, the new Anadigics part allows the devices to function from single 3- or 5-V supply rails. The microwave dc/dc converter is expected to eliminate the external dc/dc converters, charge pumps and capacitors now used to generate a negative-bias voltage for the GaAs power amps and low-noise amplifier receivers (LNAs). Such converters take up space and add weight to cellular handsets, as well as tap into battery life.

Anadigics' breakthrough is effectively a switching power supply that operates in the range of 4 to 10 GHz. Most dc/dc converters operate in the range of 250 kHz, said Shihab Al-Kuran, manager of design automation for Anadigics (Warren, N.J.). Anadigics' early hybrid ran at about 300 kHz. The problem with operating at these frequencies is that you need a relatively big capacitor-"a large bucket"-to move the charges around. By elevating the switching frequency into the high-GHz range, a much smaller capacitor is required-so small that it could be integrated right on-chip with the dc/dc converter.

Because GaAs FETs have lower gate capacitance than MOS or bipolar devices, they operate more efficiently as RF amplifiers. The problem is that the GaAs MESFETs used in cellular handsets are depletion-mode devices, said Al-Kuran. They are always "on," and it takes a negative voltage on the order of 4.2 V to turn them off or to get them to swing low. This has been true not just for GaAs FET amplifiers, but for GaAs level shifters and current mirrors as well.

It is possible to use enhancement-mode GaAs FETs, but these device types will generally not put out enough power to drive the antenna of cellular handsets, Al-Kuran said. And "combo" devices-GaAs FETs with bipolar hetero- junctions-have yet to emerge in the low-cost, high-volume arena occupied by depletion-mode GaAs FETs.

While the current requirement is very small, the negative-rail generator has required separate dc/dc converters-effectively charge pumps with a number of relatively large (2.58 mm²) ceramic chip capacitors.

This market has thus far been served by charge pumps from analog component suppliers like Maxim Integrated Products and National Semiconductor Corp. Anadigics developed its own hybrid circuit dc/dc converter as a solution to the negative-bias problem, but the need for external capacitors meant up to 6.8 mm² in board space consumed.

The microwave dc/dc may not be marketed in a separate package. Anadigics has applied for a patent on the converter, and-since it takes up only 0.2 mm² of die area-will attempt to integrate it on a new line of GaAs FETs, which can be used for cellular power amps and LNAs. The device will run off single positive supply rails in the range of 1.5 to 7 V. It will generate a full -4-V negative-bias voltage from a 3-V input, said Al-Kuran.

The elevated switching frequency of the microwave dc/dc converter, moreover, keeps oscillations from the converter from interfering with the low-level signals processed by the RF amplifier.

The major drawback of dc/dc switching at high-GHz frequencies is that the efficiency tends to drop. The Anadigics device obtains a respectable 40 percent efficiency; the power dissipation is minimal since the current requirement is so low.

Market analyst Stan Bruederle, who tracks the RF device market for Dataquest Inc. (San Jose, Calif.), said that this development would "certainly extend the application of [GaAs] MESFETs" and make the power-management job easier for cell-phone designers. But he did express some reservations about the ultimate utility of Anadigics' invention: "I'm not convinced the RF power amps are the only thing using a negative supply in the cell phone," Bruederle said.

A wild card is the fact that silicon devices-particular IBM's silicon germanium-now coming on stream promise to do what GaAs does, and these SiGe devices operate from a single supply rail. The coming challenge for both battery-powered GaAs and SiGe makers, Bruederle said, is to come up with the 4 watts demanded by the evolving GSM 900 standard.

The GaAs supplier, located here, is expected to detail the development at the Wireless/Portable-by-Design Conference, to be held Feb. 22-26 at the Santa Clara Convention Center in Santa Clara, Calif.