Amplifier systems are usually designed to provide high levels of output power across RF and microwave frequencies, employing multiple stages of solid-state and/or vacuum-tube amplification. Larger amplifier systems require some form of cooling (e.g., forced-air cooling) to disperse the heat produced by the amplifying active devices, and typically incorporate some form of remote control interface for connection to a computer or control system. At the highest power levels, electron devices such as traveling-wave tubes (TWTs) are still commonly used. Nevertheless, some of the newer solid-state device technologies, including gallium-nitride (GaN) transistors, are gaining favor in high-power RF/microwave amplifier systems.

For many years, a high-power RF/microwave amplifier involved the use of TWTs and other vacuum tubes to generate the output-power levels required for high-frequency signals—typically in communications and electronic-warfare (EW) transmitters. A TWT features three main components: an electron gun (to provide energy), a slow-wave structure (to control the exchange of energy for the high-frequency signal passing through the tube), and a collector (to receive the higher-energy RF/microwave signal).

The high-energy electron beam in a TWT’s electron gun is produced within the tube’s cathode, with an applied high voltage generating thermionic emissions of electrons. This cathode voltage is at an extremely high potential. A variety of different types of cathodes have been designed over the years, with many of these designs featuring extremely stable performance and very long operating lifetimes.

Traveling-wave-tube amplifiers (TWTAs) are still widespread throughout high-frequency applications, renowned for their high output levels and long-term reliability. For example, MCL, Inc., a MITEQ company, recently introduced its model MT2100 TWTA-based power amplifier for outdoor, airborne applications requiring at least 100 W broadband output power from 6 to 18 GHz. It is relatively compact for a high-power amplifier system (weighing just 25 lbs.) and is available with a variety of different cooling approaches, including conduction, forced-air, and liquid cooling.

Quarterwave Corp. offers a number of commercial-off-the-shelf (COTS) TWTAs suitable for military as well as commercial and industrial applications. As an example, the firm’s Series 2004 TWTAs operate from 8 to 12 GHz with pulsed output power levels to 10 kW at 8% pulse duty cycle and pulse widths from 0.05 to 50 μs. The amplifiers can produce minimum output levels of +70 dBm when fed with an input signal at 0 dBm. These amplifier systems, which use forced-air cooling, measure 36.5 x 23.5 x 30 in. and weigh 280 lbs. They are designed for operating temperatures from 0 to +50°C and altitudes to 10,000 ft. for airborne applications such as in pulsed radar transmitters.

Communications & Power Industries (CPI) has studied the performance of TWTs under different conditions and has developed its line of SuperLinear® TWTAs, nominally for efficient satellite-communications (satcom) applications. The firm has optimized the backoff operation of the amplifier circuitry around its TWTs to provide required output-power levels for different satcom systems while still meeting strict linearity and intermodulation requirements for those systems. The company claims that its amplifiers can achieve considerable savings in size and weight and as much as 70% reduction in prime power compared to traditional TWTAs used for the same satcom applications. The firm’s TWTAs are available for a number of different satcom operating frequencies, including at C-, X-, Ku-, and Ka-band frequencies.

The list of TWTA suppliers is still long, and includes such names as Amplifier Research (AR) RF/Microwave Instrumentation, Comtech Xicom Technology, CPI MPP, dB Control, DynamicWave, Instruments For Industry (IFI), L-3 Communications, Electron Technologies, Inc., Microwave Dynamics, Tesat-Spacekom, Thales Electron Devices, TMD Technologies Ltd., and Triton Electron Technology Division (ETD).