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 Amplitron(Crossed-Field Amplifier) 

●Other names in the literature.Platinotron AmplitronStabilotron

●It is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters.

●Raytheon engineer William C. Brown's work to adaptmagnetron principles to create a new broadband amplifier isgenerally recognized as the first CFA

●Their current use is in ground stations for TVRO broadcastingand Deep Space telecommunications networks,intermediate or

final stage in high-power radar systems...

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● In contrast to the magnetron, the CFA have an

odd number of resonant cavities coupled witheach other.

● These resonant cavities work to as a slow-wavestructure: an oscillating resonant cavity excitesthe next cavity.

● The actual oscillation will be lead from the inputwaveguide to the output waveguide.

●  A CFA has lower gain and bandwidth than othermicrowave amplifier tubes (such as klystrons ortraveling-wave tubes); but it is more efficient andcapable of much higher output power.

● Peak output powers of many megawatts andaverage power levels of tens of kilowatts can be

achieved, with efficiency ratings in excess of 70percent.

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The electric and magnetic fields in a CFAare perpendicular to each other (“crossedfields”) 

Without an input signal and the influenceof both the electric field (anode voltage) andthe magnetic field (a strong permanentmagnet) all electrons will move uniformlyfrom the cathode to the anode on a cycloidalpath as shown in figure 2.

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● If the input-waveguide introduces anoscillation into the first resonator (as shown infigure 3), the vanes of the resonator gets avoltage difference synchronously to theoscillation.

● Under the influence of this additionally fieldflying past electrons get acceleration (at the

positively charged vane) or they aredecelerated (at the negatively charged vane).

● This causes a difference in speed of theelectrons. The faster electrons catch theslower electrons and the forms electronbunches in the interaction space between the

cathode and the anode. 

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● These bunches of electrons rotates as likeas the “Space-Charge Wheel” known fromthe magnetron operation. But they cannotrotate in full circle, the “Space-ChargeWheel” will be interrupted because theodd number of cavities causes anopposite phase in the last odd cavity.

● To avoid a negative feedback, into thisresonant cavity may exist a bloc

containing graphite to decouple input andoutput.)

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● The oscillation is still very weak into the first cavity.

But the electron bunches will hit the vanes of thefollowing cavities and will dispense their energysynchronously to the oscillation.

● The alternating microwave field causes the electronsto alternately speed up and slow down near the nextcavity. Simultaneously, the anode near the vanes ishit of the first electrons in cycle of oscillation.

● This causes amplification: the oscillation will bestronger from cavity to cavity therefore.

● The bandwidth of the CFA, at any given instant, isapproximately plus or minus 5 percent of the ratedcenter frequency.

● To avoid ineffective modes of operation theconstruction of CFA contains strapping wires like to as

used in magnetrons.

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● Can amplify the input signals without frequencydistortion. Typically design broad-band microwaveamplifiers, the first stage and the second stage are

traveling-wave tubes (TWT) and the final stage is acrossed-field amplifier.

● When used as the final stages of a transmitter; Itallows rf energy to pass through the tube virtuallyunaffected when the tube is not pulsed.

● When no pulse is present, the tube acts as a sectionof waveguide. Therefore, if less than maximum output

power is desired, the final and preceding cross-fieldamplifier stages can be shut off as needed. Thisfeature also allows a transmitter to operate at reducedpower.

References: www.radartutorial.eu ,http://en.wikipedia.org/wiki/Amplitron