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Network realizations of the optimum noise performance (lowest noise figure at high gain) of a two-terminal-pair amplifier are presented. Two amplifier classes are distinguished: 1) networks that can both generate and absorb power and 2) networks that only generate power, or negative resistance networks. Amplifiers of class 1) can be optimized by first making them unilateral, subsequently employing input mismatch. Negative resistance amplifiers can be first brought into "canonic" form. Subsequent imbedding of part of the canonic network in an ideal circulator optimizes the noise performance. Both optimizations result in source impedances having a positive real part. High gain is achieved by subsequent cascading. The optimization using the ideal circulator yields maser noise performance optimization.