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An investigation of noise from modem junction transistors in audio frequency amplifiers is presented. Different circuit configurations are examined and the effects on noise factor of the input termination and operating point are discussed. At least three distinct sources of noise corresponding to different physical phenomena contributing to total noise are identified. In modem junction transistors shot noise and thermal noise set the ultimate limit of noise in transistor amplifiers, and are frequency-independent over the audio spectrum. Semiconductor noise which follows approximately the 1/f law has been found to be just one contributor to total transistor noise. The concept of equivalent input noise resistance is applied to transistor circuits and a comparison is drawn between noise of transistor and vacuum-tube amplifiers.