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The cathode follower is a circuit of increasing importance for television and ultra-high-frequency applications, and has therefore received much attention in technical literature. Most of the publications deal with the cathode-output amplifier, a three-terminal device. In this paper, such systems are treated briefly along with applications of the cathode follower for voltage amplification and signal mixing. This is preceded by a graphical diagram of the cathode-follower characteristic, which may be used for the determination of the direct-current components of such circuits. Cathode followers with a complex load are subsequently considered. These circuits show characteristic feedback phenomena from the cathode into the grid system in which the interelectrode capacitance acts as a coupling element. Most circuits of this nature are two-terminal devices, with action observed in the input circuit, and no external output derived from the cathode. The cathode-follower oscillator is perhaps the most important application of this group. Cathode-follower circuits with one or more resonant circuits in the cathode arm may be used as detectors or filters in connection with frequency modulation. In presenting a variety of cathode-follower applications in the light of a general theory, this paper indicates the wide scope of possible applications with no intention of being exhaustive in every detail. In order to facilitate reading, most of the mathematical derivations have been put into the Appendix.