High-power amplification at RF and microwave frequencies still involves vacuum tubes in many military systems. Such devices as traveling wave tubes (TWTs) in TWT amplifiers (TWTAs) and cross-field amplifiers (CFAs) are capable of hundreds of watts of continuous-wave (CW) power and kilowatts of pulsed (peak) power in ground-based and airborne systems, and they have served as reliable RF/microwave amplifiers even in space-based applications. But in recent years, claims of “vacuum-tube replacements” from solid-state device manufacturers have been often bold and loud, touting newer device technologies such as silicon carbide (SiC) and gallium nitride (GaN) as the solution for producing the high power levels needed at high frequencies in electronic warfare (EW), electronic countermeasures (ECM), radar, and other military and aerospace systems. What is the truth about real RF/micro-wave power? Can transistors deliver the power levels at the same frequencies as their vacuum-tube counterparts? The answers can be found by comparing the technologies and the power levels available from each, along with related issues, such as power consumption, efficiency, and linearity.

Long before power transistors were being considered for military and space transmitter applications, TWTs were boost-ing signals in satellite communications and other systems in which reliability was of the utmost importance (see sidebar). The reliability of these devices has been impressive over the years, but military system designers have long sought amplification solutions that are smaller in size and lighter in weight, especially in space-based and airborne systems. Because of this search for more compact solutions, military research dollars over the last several decades have helped with the development of semiconductor materials that support higher-frequency, higher-power transistors, such as gallium arsenide (GaAs), gallium nitride (GaN), and silicon carbide (SiC). And, while TWT and TWTA suppliers may have grown weary of hearing about the phenomenal potential performance levels of these newer transistors, they have also benefitted from the military need for more compact forms of amplification, in their development of small but powerful microwave power modules (MPMs) based on miniature TWTs.

So what is the truth? Can transistors match the power levels of vacuum-tube devices such as TWTs and CFAs? As a wise man once said, “that depends.” It depends on many factors, including frequency range, instantaneous bandwidth, and how many devices are needed to reach a given power level. And this last factor is one of the chief differences in how transistors and tubes are used within a system because, at some frequencies, a solid-state amplifier can be designed and built with the same output power as a TWTA, although it will generally require multiple transistors to match the output power of a single TWT. While it is possible to sum the contributions of many power transistors to achieve relatively high power levels, it also requires sacrificing some of that power to the insertion loss of power combiners in multiple-transistor amplifiers.

TWTs are elegant in their simplicity and reliable because of the small number of parts. TWTs can be designed with different types of components, but common to all types are some form of electron gun, a slow-wave structure, such as a helix, high-power magnets to focus the emitted electron beam, a collector, and some form of input and output couplers to inject and collect an RF or microwave signal. In essence, the injected RF/microwave signal interacts with the electron beam in the slow-wave structure, with a resulting transfer of energy from the electron beam to the electromagnetic RF/microwave signal. The amount of energy transferred is characterized by several TWT parameters, such as output power, gain, and efficiency. As its name implies, the collector is the end point for the electron beam and is designed to effectively dissipate its remaining energy.

Two of the more popular TWTs in use in military systems are those with a helix slow-wave structure and coupled-cavity TWTs, which use a slow-wave structure formed of a series of cavities coupled by slots. Over the years, improvements in the cathodes used as electron guns have increased the reliability of TWTs and TWTAs, while also supporting higher current densities. Also, smaller, higher-powered magnetic circuits, such as periodic structures, have resulted in smaller tubes without sacrifices in output power. These smaller tubes and tube amplifiers are particularly attractive for weight-sensitive airborne applications requiring high transmit power, including in unmanned aerial vehicles (UAVs). By applying three-dimensional electromagnetic (EM) simulation software, TWT designers have also been able to closely study the EM field interactions of different tube components in order to refine physical structures and improve output power and efficiency. In addition to TWTs and CFAs, high-power vacuum-electronic devices employed in military systems include kly-strons (usually as amplifiers) and magnetrons (usually as oscillators).

On the transistor side, the variety of power devices seems to be growing decade by decade. Early high-power devices were silicon bipolar transistors used mostly in pulsed applications with short duty cycles. But following the development and qualification of enhancement-mode silicon MOSFETs, they were found suitable for both CW and pulsed applications, and generally required much simpler impedance matching networks for broadband operation. Yet, both devices were limited in frequency range, prompting development of higher-frequency substrate materials, including indium phosphide (largely used for lower-power, millimeter-wave frequencies) and gallium arsenide (GaAs) in both discrete device and integrated-circuit (IC) forms. Major investments, including the DoD’s microwave and millimeter-wave monolithic microwave integrated circuits (MIMIC) program of the mid-1980’s and early 1990’s, have made GaAs the material of choice for both low-noise and power microwave transistors. Still device developers have sought higher power densities from transistors using a number of substrate materials, including GaN, SiC, GaN on SiC, or mat-erials with excellent thermal conductivity, such as sapphire or diamond.

Transistor Power
How do the amplifiers based on these advanced transistors compare with TWTA designs? Perhaps a sampling of available devices and amplifiers might better tell the story. ECM and EW systems are among the most demanding of military applications, both for their power requirements and their multi-octave bandwidths. Although solid-state devices are capable of tube-like power in pulsed operation, most if not all of the devices are targeted at narrower bandwidths. For example, Microsemi supplies the old and the new, offering both silicon bipolar transistors and newer devices based on SiC. The firm’s model 3134-100M is a common-base silicon bipolar transistor that operates from 3100 to 3400 MHz with 100 W output power when driving pulsed signals with 100-microsecond pulse width and 10-percent duty cycle. When operating from +36 VDC, the device achieves 40-percent collector efficiency and 9.3 dB gain, requiring an input signal at 16 W to reach the rated output power level.

The firm also offers a series of SiC static-induction-transistor (SIT) devices and power amplifier modules, including the model 0405SC-1000M transistor, rated for 1000 W pulsed output power from 406 to 450 MHz when operating from a +125-VDC supply. The output power is achieved at 450 MHz with 50-percent minimum drain efficiency when using 300-microsecond pulses at 10-per-cent duty cycle. In addition, Microsemi supplies a series of solid-state devices and modules for S-band radar systems. Models 3134-65M and 3134-100M are power transistors with 65 and 100 W pulsed output power from 3100 to 3400 MHz while models 3134-180P and 3134-200P are power amplifier modules rated for 180 and 200 W from 3100 to 3400 MHz. Designed for use with 100-microsecond pulses at 10-percent duty cycle, the transistors prom-ise better than 40-percent col-lector efficiency.

In that same 3.1-to-3.4-GHz S-band radar range, Cree supplies its model CGH31240F high electron mobility transistor (HEMT) based on GaN. When operating with 300-microsecond pulses at a 10-percent duty cycle, the device achieves 240 W peak power with 16.6 dB gain and 50-percent efficiency at 2.8 GHz. The firm’s model CGH40120F GaN HEMT is an unmatched +28-VDC device rated for 120 W saturated output power. It has been used in a reference amplifier with 1200-to-1400-MHz instantaneous bandwidth, 100 W CW typical output power, 16-dB typical small-signal gain, and 75-percent typical power-added efficiency.

Based on silicon MOSFET technology, the model HVV0912-150 transistor from HVVI Semiconductors is designed for use from 960 to 1215 MHz in L-band avionics applications such as TCAS, IFF, and DME systems. It provides 150 W output power with 20-dB gain when driving 10-microsecond pulses at a 10-percent duty cycle. It can operate on supplies from +24 to +50 VDC and delivers 43-percent efficiency. Its unique vertical device structure allows it to operate into mismatches as severe as a 20.0:1 VSWR without damage.

Among the higher-power silicon bipolar transistors, the IB1011S1500 from Integra Technologies is designed for L-band radars at 1030 and 1090 MHz. When fed with a 150-W pulsed (10-microsecond, 1-percent duty cycle) input signal at 1030 MHz, it yields 1432 W peak output power with 48.8 percent efficiency. For more broadband use, the firm’s model IB0912M600 bipolar handles L-band TACAN chores from 960 to 1215 MHz. It offers 845 W peak output power and better than 56-percent efficiency at 960 MHz when driving a 90-W pulsed input signal. Both transistors are housed in beryllium-oxide (BeO) packages for good thermal dissipation.

In terms of continuous power and bandwidth, the model NPT1007 GaN-on-silicon transistor from Nitronex can provide 90 W CW power from 500 to 1000 MHz, and 200 W CW output power at 3-dB compression (saturation). The device is usable from DC to 1200 MHz. Usable with supplies from +14 to +28 VDC, the transistor boasts 63-percent typical drain efficiency at 3-dB compression. Additional high-power transistor suppliers include Freescale Semiconductor, with its model MRF6V1430H silicon LDMOS device delivering 330 W peak output power with pulsed (300-microsecond, 12-percent duty cycle) signals from 1.2 to 1.4 GHz, TriQunt Semiconductor, with Powerband GaAs PHEMT devices capable of 50-W pulsed output power from 0.5 to 2.0 GHz, and IXYS RF, with 150-V MOSFETs capable of as much as 550 W CW output power at 175 MHz, and P1dB (www.P1db.com), with silicon bipolars offering as much as 200 W output power in DME and TACAN applications from 960 to 1215 MHz.

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