Amplifier technology tends to change over time, depending on the active devices available. Although active-device technologies have advanced a great deal over the past 20 years—with semiconductor technologies such as gallium arsenide (GaAs) maturing, and gallium-nitride (GaN) devices offering tremendous promise in terms of high power levels at high frequencies—vacuum tube devices still play major roles in RF/microwave amplification applications. A variety of technologies are employed in high-frequency amplifiers, each with its own set of benefits and features.

Amplifiers for RF/microwave applications are available in a wide range of shapes and sizes, as well as frequency ranges and power levels. This diversity stems from the many different needs for these amplifiers, from low-noise amplification in receivers to boosting signals to high power levels in transmitters. Also factoring in are the many additional medium-power stages in between the receiver and transmitter.

Companies such as Skyworks Solutions, for example, target different applications with different sets of performance levels in their miniature low-noise amplifiers (LNAs). For use in Long-Term-Evolution (LTE) and wideband-CDMA (W-CDMA) cellular communications infrastructure applications, the firm’s model SKY65369-11 surface-mount amplifier features a typical noise figure of just 0.9 dB from 832 to 862 MHz with a 35-dB gain control range. To keep things small, it is supplied in a 16-pin MCM housing measuring just 8 x 8 x 1.3 mm. For broader frequency coverage, the same company’s model SKY67015-396LF LNA achieves almost the same noise figure (at typically 1 dB) but covers a frequency range from 30 to 3000 MHz. Suitable for ISM-band applications, it is supplied in a similarly small housing as the model SKY65369-11 amplifier and includes 15.5 dB fixed gain across its frequency range.

Certainly, no solid-state technology has shaken up the RF/microwave amplifier design world in recent years quite like gallium nitride (GaN) active devices. The number of companies now producing GaN amplifiers is large and growing, due to the high power density of the technology and the interest on the part of such customers as the Defense Advanced Research Projects Agency (DARPA). One of those GaN producers, TriQuint Semiconductor, which has been working on GaN technology since 1999, recently received a $2.7 million contract from DARPA for the nominal purpose of tripling the power-handling capabilities of GaN circuits. This Near Junction Thermal Transit (NJTT) project will build on TriQuint’s GaN on silicon carbide (SiC) technology to achieve higher RF/microwave solid-state power levels than currently available.

According to James L. Klein, TriQuint’s Vice President and General Manager for Infrastructure and Defense Products, “We are very pleased that DARPA selected TriQuint to develop this critical technology. Like other programs we have supported, NJTT will set the stage for substantial MMIC performance enhancements including reduced size, weight, and power consumption while increasing reliability and output power.”

TriQuint hopes to combine its GaN-on-SiC process technology with new thermally conductive materials, thus reducing heat buildup around the active GaN devices and permitting higher output-power levels in their GaN amplifiers and devices (click here for more on emerging thermal-management materials). TriQuint has several partners in the project, including the University of Bristol, Group4Labs, and Lockheed Martin. TriQuint is also heading process and manufacturing projects on GaN devices and amplifiers for the United States Army, Navy, and Air Force laboratories.

Similarly, late last year, RF Micro Devices, Inc. received a $2.1 million contract from DARPA to enhance the thermal efficiency of GaN circuits used in high power radar and other military systems.Also part of DARPA’s NJTT efforts to improve the power-handling capabilities of GaN amplifiers, RFMD believes that a solution will be found as a combination of its GaN-on-SiC device technology and the use of thermally enhanced diamond substrate materials. Jeff Shealy, Vice President and General Manager of RFMD’s Power Broadband business unit, said at the time of the contract’s announcement that “RFMD is excited to work with DARPA to apply new technologies to our existing portfolio of GaN-based high power RF amplifier products. We expect the NJTT program will result in a new generation of higher performing, more compact RF high power amplifiers (HPAs) with lower operating temperature and greater RF power-per-unit area.”

RFMD, which has been involved with GaN technology since 2000, is also working with Group4Labs on the contract, along with the Georgia Institute of Technology, Stanford University, and the Boeing Co. The firm has been a strong supplier of GaN-based power amplifiers for cable-television (CATV) applications.

For those seeking an informal education on GaN technology, Advantech Wireless offers an eight-page white paper on GaN amplifiers, “A new generation of Gallium Nitride (GaN) based Solid State Power Amplifiers for Satellite Communication,” available for free download from the firm’s website. It details how GaN amplifiers fare in satellite-communications (satcom) applications when compared with silicon LDMOS or GaAs-based power amplifiers. The GaN amplifiers are claimed to be about 50% smaller than their technology counterparts, with considerably less power consumption and less generation of heat. Advantech Wireless, which designs and manufactures GaN power amplifiers through Ku-band frequencies for commercial and military use, is currently offering its GaN power amplifiers as replacements for traveling-wave-tube amplifiers (TWTAs) in satcom applications.

In embracing the growing popularity of GaN amplifier technology, EMPower RF is selling its lines of GaN power amplifiers as replacements for silicon bipolar, MOSFET, LDMOS, and GaAs FET amplifiers. It is offering the newer GaN amplifiers as smaller, lighter, and more reliable units for a given frequency range than any of the other solid-state amplifier types. The firm offers both GaN amplifier modules and complete amplifier systems with power supplies in a rack-mount housing.

As an example of the former, model BBM3K5KKO is a compact Class AB linear GaN power amplifier design capable of 100 W minimum output power and 125 W typical output power from 500 to 2500 MHz. It provides 50-dB minimum power gain with -20 dBc typical harmonic levels and -70 dBc typical spurious levels. At a package size of 7.4 x 3.6 x 1.06 in., it consumes 10 A from an external +28-VDC supply. It is also available as a rack-mount unit with the power supply inside the housing.

Of course, DARPA wouldn’t enjoy its successful track record in research without “hedging its bets” and investing in a number of different technologies for high-frequency amplifiers. The organization still believes that silicon technologies will support high-frequency amplification through millimeter-wave frequencies.DARPA’s Efficient Linearized All-Silicon Transmitter ICs (ELASTx) program is seeking novel approaches for increases in power amplifier efficiency, while at the same time achieving improved linearity by way of integrated linearization architectures. One of the goals of the program is a silicon-based transmitter with 65% power-added efficiency (PAE) with low distortion for 64-state quadrature-amplitude-modulation (64QAM) waveforms. The program is looking at bandwidths of 3.5 GHz at 45 GHz, 5 GHz at 94 GHz, and 8 GHz at 138 GHz for these next-generation silicon amplifiers and transmitter ICs.

According to Sanjay Raman, the DARPA Program Manager for the project, “Millimeter-wave amplifiers have been demonstrated at this power level before, but this is a record with silicon-based technologies. Producing this level of output with silicon may allow integration on a chip with complex analog and digital signal processing. In the 42-to-125-GHz range, this would enable high bandwidth/data-rate transmitters needed for satellite communications at potentially very low cost and size, weight, and power.”  

1. Amplifiers for satcom applications must be housed in miniature, light-weight packages. (Photo courtesy of MITEQ.)

An important design goal for many applications is sufficient amplifier power for a light-weight package, especially in airborne applications or in satcom systems. Amplifiers for the latter, such as the JDMW-Series amplifiers from MITEQ, are low-noise amplifiers (LNAs) designed to operate from 18 to 21 GHz with 30-dB gain in a hermetic package measuring just 1.18 x 0.87 in. and weighing just 23 g (Fig. 1). These LNAs feature a noise temperature of 97 K (a noise figure of only 1.25 dB) with current consumption of only 75 mA at +12 VDC. The amplifier has an operating temperature range of -30 to +65°C and yields +8 dBm output power at 1-dB compression. The amplifiers are available with numerous options, including RF input limiters and waveguide flanges.

For any RF/microwave amplifier technology, delivering consistent performance levels with high reliability is an important goal whether the amplifier is for low-noise or power applications. As an example, the model ZHL-100W-13+ power amplifier from Mini-Circuits is designed to withstand short-circuit and open-circuit operating conditions even when running at full output-power levels, but depends on a heat sink to dissipate excess heat (Fig. 2). The amplifier is also designed to be unconditionally stable under a wide range of operating conditions. The transistor amplifier is rated for 100 W typical saturated output power from 800 to 1000 MHz but is also usable from 750 to 1050 MHz. It provides 50-dB typical gain with gain flatness of typically ±1 dB from 800 to 1000 MHz. Supplied with SMA input connectors and Type-N output connectors, it draws 10 A at a typically supply of +28 VDC. The amplifier, which has a typical noise figure of 7 dB, achieves +49 dBm typical output power at 1-dB compression and +50 dBm typical output power at 3-dB compression.

2. The large heat sink is required to help dissipate heat from the power amplifier’s active devices. (Photo courtesy of Mini-Circuits.)

To achieve the high reliability, users are asked to provide proper heat sinking and heat removal from the amplifier, ensuring that its making base-plate temperature is +60°C to ensure proper performance. Users can establish favorable long-term conditions for the amplifier by supplying a heat sink with thermal resistance of 0.035°C/W or better.

In spite of the excitement about GaN technology, solid-state amplifiers have not yet replaced RF/microwave tubes and amplifiers based on vacuum tubes. As noted in this report, such amplifiers may be considerably larger than solid-state amplifiers for the same frequencies, but they are also capable of much higher continuous-wave (CW) and pulsed output-power levels. It is the hope of organizations such as DARPA that vacuum tubes may one day be replaced at high frequencies by solid-state amplifiers with much higher power densities than possible today. But for now, tubes and transistors coexist in RF/microwave applications—often within the same radar or communications system—and each technology is supported by a large number of reliable suppliers.