Erbium-doped fiber amplifiers have received much interest for CATV because of their high output power, low distortion, and low noise capability. These devices have only recently become commercially available and probably are not well understood by those who have not been involved with them from a research or applications viewpoint. This paper is intended to present a simple coverage of the principles of operation and characteristics of these amplifiers. Signal gain, saturation output power, and noise properties are discussed. Equations for optical amplifier noise figure are developed and typical data for CATV are given.