A microwave amplifier is a device for enhancing the output power signal of a microwave device, usually by increasing the amplitude or height of the wave that directly relates to its power level. This is accomplished by channeling additional input power to the microwave device so that its microwave radiation carries more energy. Such amplifiers are usually operated at low frequencies of the radio wave spectrum, which range around 300 megahertz or higher, and they are used for a variety of purposes from broadband communications to radar systems and electronic warfare by the military. Several different types of microwave amplifier designs exist including the Gyrotron, the Klystron, and the Amplitron, which is also known as the Crossed-Field Amplifier (CFA) or Platinotron.
In broadband communications, a microwave amplifier is based upon transistor technology like the bipolar junction transistor (BJT) and field-effect transistor (FET). Such amplifiers are often multi-stage systems for perpetuating the power of a signal over long distances where the focus of the technology is on the efficiency of the transmitted signal over that of any noise that is introduced along the way. As well, though instability in bias circuits is common to amplifiers, the development of integrated circuits for feedback bias control has acted as a buffering application to make the stability of this type of microwave amplifier much more reliable as of 2011.
The Gyrotron is a microwave amplifier that operates at the higher level of frequencies in the radio spectrum, anywhere from a 20 gigahertz on up to 35 gigahertz power range. It is used for specialized applications such as in uplink towers to orbiting spacecraft and satellites or for planetary radar transmitter systems. Though Gyrotrons were first conceptualized in the late-1950s, early models could only achieve power levels in milliwatts. Russia pioneered research into their development and was achieving power output levels of 22 kilowatts by the late 1970s.
The Crossed-Field Amplifier or Amplitron is another type of broadband microwave amplifier that has properties similar to a magnetron, which makes it a useful device for nuclear accelerators. It can produce maximum power levels in the megawatt range as of 2011, but usually runs at the kilowatt level, which makes it useful in many of the same applications as the Gyrotron. Though the Amplitron can boost efficiency ratings for microwave equipment to around 70%, it is usually used as an intermediate signal amplifier in equipment.
The Klystron is a sort of hybrid among amplification tools like the Gyrotron and Amplitron. It is a microwave amplifier that can produce both low power and high power carrier wave signals, and is used both in telecommunications systems and particle accelerator research. The Klystron predates the conception of the Gyrotron, having been first conceived in 1937 by two brothers, Russell and Sigurd Varian, at Stanford University in the US. The discovery by both men is credited with advancing the development of radar systems in the US and UK before the outbreak of World War II, but the Klystron was incorporated into radar systems used by Germany during the war whereas the Allied powers relied instead on more powerful magnetron equipment first developed in 1940.