Amplifiers and oscillators can be found at the very heart of many RF/microwave systems and subsystems, and both components are among the first parts of any system design to leverage new technologies. Both components typically rely on active devices, such as the gallium-nitride (GaN) transistors that are currently driving many newer power amplifiers to higher output-power levels at higher frequencies. GaN semiconductor materials are improving and, with them, the active devices that power the latest generation of RF/microwave power amplifiers. At present, these devices largely operate through the microwave frequency range to about 20 GHz; inevitably, this will extend to millimeter-wave frequency bands and in support of the growing line-of-sight communications applications at those higher frequencies.
Oscillators have traditionally produced their output signals as the result of feedback within active semiconductor devices, such as silicon bipolar transistors. But high-frequency oscillators are evolving even further in recent years, taking advantage of advances in microelectromechanical-systems (MEMS) technology to produce high-performance oscillators from nominally passive structures. MEMS technology is often associated with such circuits as RF switches, but more than a few companies have found that the technology does quite well in RF oscillators as well.
MEMS devices may seem novel, but the technology is actually not new. It is essentially a way to apply many of the processes used to form semiconductors, such as the deposition and cutting away of conductive metals to form different gate, drain, and source regions, in the manufacturing of mechanical structures instead. Because of their mechanical nature, many of the earliest MEMS devices were switches, with on and off states capable of high isolation and with the fast switching speeds possible with small semiconductor structures. Companies with strong capabilities in semiconductor processing, such as Freescale Semiconductor, have been producing MEMS devices for a full three decades.
A number of MEMS developers have viewed this technology not just as an alternative approach to existing technologies but as a possible replacement for an established technology. Because MEMS devices are fabricated on silicon wafers—often with the same processing equipment as used for active silicon CMOS circuits—it has been a natural transition for MEMS component supplier Discera to offer MEMS-based clock oscillators as replacements for quartz crystal clock oscillators. The MEMS resonant structures are surrounded by familiar silicon CMOS circuitry, and the oscillators are even marketed as low-jitter and low-power CMOS oscillators. They are available in standard frequencies, such as 10, 20, 25, and 50 MHz, and housed in extremely small packages.
Whether a specifier needs to save power or achieve outstanding stability, these MEMS oscillators can deliver levels of performance that can easily make a circuit designer forget about using a quartz crystal oscillator. Of course, these and other MEMS oscillators lack the long history of proven performance over a wide range of operating conditions, but the accelerated life testing and other measurement programs being conducted by MEMSs manufacturers are showing that this is a technology that will not go away.
Perhaps the beauty of following the evolution of high-frequency amplifiers and oscillators as they grow with technologies such as GaN semiconductors and MEMS circuitry is the aggressive support from model makers and software developers. Creating accurate models can be difficult, but the model makers and software developers in the RF/microwave industry have long risen to the occasion. They have succeeding in building models for emerging devices, such as gallium arsenide (GaAs) metal-epitaxial-semiconductor field-effect transistors (MESFETs) and, more recently, GaN pseudomorphic high-electron-mobility-transistor (pHEMTs) active devices. Similarly, the industry’s model makers are providing designers with effective models for MEMS-based devices and structures that can be used for form familiar components (such as switches) and somewhat-less-familiar components (e.g., clock oscillators).
This quick glance at several of the technologies impacting modern high-frequency amplifier and oscillator design is really just a sampling of the innovative applications of new technologies that regularly take place across the industry. In an industry that is often regarded as “old-school electronics” because of its ties to military applications, the RF/microwave industry is as quick to adopt and adapt the latest applicable technologies.
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