A vacuum tube that amplifies microwaves by controlling the speed of electrons flowing in a straight line with the help of a magnetic field is called Klystron. An amplifier that uses the klystron vacuum tubes to amplify microwave frequency signals is called a Klystron Power Amplifier. These amplifiers are used for very high power microwave applications between 300 MHz and 400 GHz. They are ideal for use in various industries like satellite, radar, wideband high-power communication, medical, high-energy physics etc.

A klystron uses the kinetic energy of an electron beam for the amplification of a high-frequency signal. These amplifiers make use of the transit-time effect by varying the velocity of an electron beam. They  include one or more resonant cavities, which control the field around the axis of the tube. The signal of the amplifier is amplified by the energy of the electron beam and the amplified signal can be received from a cavity at the other end of the tube.

These are of two types

• Two-cavity klystron amplifier: The two-cavity klystron amplifier includes two microwave cavity. The first cavity together with the coupling device is termed as a ‘Buncher’. The second cavity with coupling device is called as a ‘Catcher’. The field direction changes with the frequency of the Buncher cavity. This klystron can be used as an amplifier. 
• Reflex klystron amplifier: Reflex Klystron amplifier is a single cavity apparatus that can be used as an oscillator. This single cavity klystron amplifier can be modulation otherwise demodulation.

Advantages of Klystron Amplifiers

• They can be used to generate far higher microwave power that solid state microwave devices like Gunn diodes.
• They can operate over a wide frequency range from hundreds of MHz (UHF) through hundreds of GHz.

Few practical applications of Klystron Amplifiers

• At SLAC, Klystron Amplifiers are normally employed which have outputs in the range of 50 MW & 50 kW (time-averaged) at 2856 MHz.
• There are Radars usually uses klystrons amplifiers that offer a total output power of 1 MW at 2380 Mega Hertz.
• Global Resource Corporation (GRC) is currently using a klystron amplifier to convert the hydrocarbons in daily materials, coal, automotive waste, oil sands, diesel fuel, oil shale and oil sands into natural gas.