Crossed-field amplifier

Crossed-field amplifier

The Amplitron is a high power tube in the microwave region and belongs to the group of transit-time tubes, in which the electrons flow interacts with a along a delay line running electromagnetic wave. It is similar in its structure strongly to the magnetron.

For the Amplitron following terms are used in the literature, which sometimes also mark different purposes:

Crossed-field amplifier ( engl. crossed field amplifier, short CFA ) is the characteristic function generic term for enhancing transit-time tubes with transverse to the electron current directed magnetic field. Platinotron (from Greek platino - plateau ) is the original term for CFA. Amplitron CFA as a switchable power amplifier. The designation is a trademark of Amplitron Raytheon Manufacturing Company. Stabilotron: special form of Amplitrons with feedback and resonant circuit as a stable RF generator. The term Magnetronverstärker is a trivial name.

Design and function

The main difference between magnetron and Amplitron is that the Amplitron has an odd number of resonators at the anode and has no closed delay system with only one output, but both an input and an output features.

The space between the cathode and the anode is called the spill cavity. Parallel to the cathode, there is a strong magnetic field of a permanent magnet. In the barrel chamber, the electrostatic field of the anode voltage and the magnetic field of the permanent magnet acting on the electrons. Since the magnetic force lines perpendicular point to the electrostatic lines of force between the anode and cathode, this cross So, does this tube also crossed-field amplifier.

Due to the deflection by the magnetic field, the electrons move not on the shortest path to the anode, but on more circular paths substantially parallel to the delay line. The high frequency to be amplified is now so high that the electrons are too slow in spite of their acceleration by the electrostatic field to reach the anode, within a period of the oscillation.

The voltage applied at the input of high frequency interferes with their voltage of the high voltage. The electrons are now dependent on the instantaneous amplitude of the high frequency time slowed ( negative half- wave of the oscillation ), sometimes additionally accelerated (positive half-wave). Therefore, in the course room now exist electrons with different speeds. The slower electrons are overtaken by the faster and thus form strong electron bunches, which can now emit energy in the high frequency clock to the delay line. At the output the amplified high-frequency signal can now be picked up.

The cathode and cold platinum electrodes are used; they work due to secondary electron emission and thermionic cathodes have the advantage over a much longer lifetime (lower evaporation rate ) and an insensitivity to the bombardment of residual gas ions. It can be achieved lifetimes to about 20,000 h.

, The anode voltage ( high voltage to approximately 60 kV ) the Amplitrons off, the input signal is passed unamplified to the output. Thus may be modulated by switching or varying the high voltage output of the amplifier. The switching of the high voltage can be done with or thyratron switching triode.

Amplitrons have similarly high efficiencies such as the magnetron (22 to about 70%) and, as a high-frequency generator ( oscillator combination - amplifier or via a resonant circuit is fed back ) from a lower to those phase or frequency jitter.

Amplitrons can work on badly matched loads ( see Standing Wave Ratio ): while in other microwave generators of the load reflected waves lead to increased power loss or phase errors, this will pass the Amplitron backwards and leave it on its receipt. Before this can be coupled with a directional coupler, and attenuated / circulator.


Amplitrons be as high frequency amplifier ( about 100 MHz to about 10 GHz) used in pulsed radars and particle accelerators. You can generate pulse powers of several megawatts and permanent services of more than 100 kW and have over klystron amplifiers the advantage of lower frequency and phase variations in changes of the anode voltage, which facilitates their modulation.

Modulated Amplitrons (CFA ) are used for data communication in space.

They are also used in continuous operation for industrial purposes ( heating, plasma excitation ).

Web Links / References

Manufacturer L-3 - data from CFA with comparison table of Raytheon types

  • Electron tube