Introduction to noise

Noise in audio circuits hampers the ability to listen to low level details in the signal. In particular, it degrades S/N ratio in low level signal circuits such as found in microphone, moving magnet and moving coil preamplifiers. In this article, Ronald Quan identifies the sources of random noise signals, and presents some basic noise theory and noise models including an analysis of noise in single transistor and cascode stages, including the concept of input-referred noise.