Klystron is the simplest vacuum tube that can be used for amplification or generation of microwave signal. The operation of klystron depends upon velocity modulation which leads to density modulation.
Principle- high velocity electron beam is formed, focused and sent down along a glass tube to a collector electrode, which is at a high positive potential with respect to the cathode.
The input and output are taken from the tube via resonant cavity. The region between buncher cavity and catcher cavity is called drift space. The first electrode controls the number of electrons in the electron beam and focuses the beam. The velocity of the electrons in the beam is determined by the beam accelerating potential. When electrons travel through the drift space, they are subjected to RF potential at a frequency determinate by the cavity resonant frequency. The amplitude of this RF potential between the grids is determines by the amplitude of the input signal in case of an amplifier or by the amplitude of the feedback signal from the second cavity if used as an oscillator.
