What is a TWT Amplifier?

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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- everything RF

Oct 12, 2020

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, which is slower than the speed of light, and is focused into a tight beam using magnetic field along the axis of the tube. The speed of electron beam in a TWT amplifier is generally 10 to 50 % of speed of light and it depends on the cathode voltage.

The RF signal, however, propagates at the speed of light and there must be a mechanism to slow down its speed to permit interaction between the beam and the signal. This slowdown is achieved by using a slow wave structure, which is usually a helix, and the slowed down RF signal is called the slow wave. The attenuator isolates the input and output sections to prevent oscillations. The velocity of the electron beam, traveling through the helix, induces/adds energy to the RF waves on the helix, thus amplifying the signal.

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