What is a TWT Amplifier?

Amplifiers 

What is a Traveling-Wave Tube (TWT) Amplifier? When are they used and how are they different from Solid State Power Amplifiers?

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Editorial Team - everything RF

Jul 22, 2019

A Traveling-Wave Tube or TWT Amplifier is a high power, high-frequency amplifier that is built using traveling wave tubes. A traveling-wave tube is a type of vacuum tube used to amplify high-frequency signals. The RF signal is amplified by absorbing power from a beam of electrons as it goes through the tube.

TWTAs usually operate from 300 MHz up to frequencies over 100 GHz (there are cases where these go as high as 650 GHz). They provide power levels from a few watts to megawatts with a gain of 40 to 60 dB.

A typical TWT Amplifier consists of a traveling wave tube, an RF input section, a power supply & logic interface section, and an RF output section. The TWT is the part that is responsible for the amplification of the signal.

The TWT consists of an electron gun (composed of a cathode, a control or modulating grid, and an accelerator), an RF circuit (delay line), an attenuator and a collector.

The cathode produces an electron beam with the velocity of light at the collector, guided by a magnetic field. The RF circuit propagates a microwave signal in a manner that permits interaction between the beam and the signal. The attenuator isolates the input and output sections to prevent oscillations.

The RF signal also propagates with the velocity of light which is slowed down by the delay line, which is usually a helix. The Helix acts as a slow wave structure. The velocity of the electron beam, traveling through the helix, induces/adds energy to the RF waves on the helix, thus amplifying the signal.