Rack-mount amplifiers serve a variety of purposes in commercial, industrial, and military systems, where power, gain, efficiency, and linearity are important. In line with these requirements, noted amplifier supplier CTT, Inc. (www. cttinc.com) has expanded its lines of rack-mount power amplifiers to cover the frequency range of 500 MHz to 31 GHz with as much as 50 W output power. The amplifiers can be used for antenna profiling, dielectric heating, plasma generation, as exciters, in simulators, and in test systems. They are also useful for satellite-communications (satcom) systems, digital radio, pointto- point communications links, and telemetry systems.

The latest additions to CTT's rackmount amplifier family (Fig. 1) are suited to narrowband as well as wideband applications. They are based on a variety of different transistor technologies, such as as silicon bipolars, gallium arsenide field-effect transistors (FETs), and gallium nitride (GaN) high-electron-mobilitytransistor (HEMT) devices. The new amplifiers fall into three groups: the RP, RS, and RG series. The RP series was designed to operate in the linear region to maximum third-order intercept point while the RS series is designed to operate in the saturated mode to achieve maximum output power. The RG series employs GaN devices to achieve higher power with wide bandwidth and high efficiency at frequencies through 3 GHz.

Models are available in a variety of frequency ranges and bandwidths. In some cases, a model can be ordered with differences in gain and output power over a single frequency range (see table). For example, model RPM/060-4148 is designed for use from 2 to 6 GHz. Two versions of the amplifier are available, with 44 dB linear gain and +39 dBm output power at 1-dB compression, or 48 dB linear gain and +41 dBm output power at 1-dB compression. Both units feature outstanding (for a power amplifier) noise figure of 5 dB and gain flatness of 2 dB across the 4-GHz bandwidth. As with the other additions to the rack-mount amplifier family, they are housed in an 19 x 5.25 x 17 in. enclosure.

The rack-mount amplifiers leverage the firm's expertise in designing and building compact amplifier modules (Fig. 2). The modules are based on reliable microwave-integrated-circuit (MIC) thin-film circuitry to achieve a good impedance match to a 50-Ohm environment when using low-impedance packaged discrete power transistors. The modules are surrounded by AC-to-DC power circuitry, heat sinks, and a variety of protection circuitry and components.

The rack-mount power amplifiers incorporate large-area heat sinks and forced-air cooling fans as part of a simple but effective thermal-management scheme. At these power levels, even the most efficient transistors transform a large amount of bias energy into heat. The fans and heat sinks help draw heat away from the transistors and maintain the junction temperature of the amplifier's semiconductors within a safe operating region. In addition to thermal management, the rack-mount amplifiers include TTL-based control circuitry, filtering to minimize the effects of electromagnetic interference (EMI) and radio-frequency interference (RFI), voltage regulation and, in some cases, an intelligent sequencing arrangement to protect the output stages during turn-on and turn-off operations.

As options, the rack-mount amplifiers can be equipped with output-power monitoring, waveguide input and/or output flanges, variable or step attenuators to limit input signal levels, alarm circuitry, and even built-in DC-to-DC converters. Options also include thermal shut-down circuitry, coupled output ports, isolator- based protection, and power-supply options for 100/200-VAC or 85-to 264 VAC. The amplifiers are supplied with SMA connectors, with Type N connectors as an option. The amps are available in 17-in. rack-mount housings with 3.5, 5.0, and 7.0 in. heights.

The company has traditionally based its power amplifiers on GaAs devicesthat deliver good linearity and high third-order intercept points, and reach 100 W output power at certain bandwidths. Because of customer demands for wider bandwidths with more output power and higher efficiency, the firm has developed GaAs power amps capable of output levels at 80 and 100 W.

Unfortunately, the efficiency of these amplifier designs is limited by the device technology, and the low voltages and high currents typically required by GaAs FETs. For example, in using a 100-W GaAs FET amplifier with power-added efficiency of only 15 percent, bias power of 666 W would result in the 100 W output power, but also in 566 W of wasted power, typically dissipated as heat (and the need for a large and costly heat sink and/or thermal management scheme). For a GaAs FET device with voltage requirement of +10 VDC, the current requirement for this device is 66 A. Because of this, the DC bias wires must be large enough to handle the high current, and the mechanical design of the amplifier becomes more challenging. Because of the power limitations of GaAs FETs, CTT experimented with the capabilities of other RF power devices, including GaN devices.

The GaN transistors were found to offer the advantage of higher operating voltages (+24 to +50 VDC) compared to GaAs devices, the capability of handling higher junction temperatures, higher efficiency, and wider bandwidths compared to GaAs. By using predistortion or other approaches, the linearity of GaN devices can be made comparable to that of GaAs devices, so that GaN technology can be applied an an effective alternative to GaAs technology for high-frequency, high-power amplification. CTT offers several rack-mount amplifiers based on GaN technology in its RG series, with units covering frequency ranges of 0.5 to 2.0, 1.0 to 2.0, and 1.5 to 3.0 GHz. For example, model RGO/020-4747 makes use of multiple discrete GaN devices to provide 50 W output power (+47 dBm) at 3-dB gain compression from 1 to 2 GHz. It does so with 47-dB gain and 2-dB gain flatness and a moderate noise figure of 6 dB. Additional units in the RG series include the model RGO/030- 4444, with 44-dB gain and +44 dBm output power at 3-dB compression and model RGO/030-4747, with 47-dB gain and +47 dBm output power at 3-B compression.

CTT's rack-mount solid-state amplifiers are designed for low maintenance and high reliability. They are suitable for use as traveling-wave-tube-amplifier (TWTA) replacements, driver and output amplifiers for commercial and military systems. In addition to the rack-mount CW power amplifiers, units are available for pulsed signal applications, operating with TTL control. CTT, Inc., 241 East Java Dr., Sunnyvale, CA 94089; (408) 541-0596, FAX: (408) 541-0794, e-mail: sales@cttinc.com, Internet: www.cttinc.com.