ABSTRACT
This chapter examines the noise calculations and aims to describe a more fundamental physical approach and discusses the nature of several noise mechanisms. Noise can have a great variety of causes, but they all have one feature in common with respect to the signal being measured. There is an increasing need to control signal-to-noise ratios because in communication and telecommunication applications the distance to be bridged is still growing. Noise current increases at lower frequencies for operational amplifiers and bipolar transistors, but increases at higher frequencies for field effect transistors. The noise voltage en¯, or more precisely the equivalent short-circuit input Root mean square noise voltage, is defined as that noise voltage which would appear to originate at the input of the noiseless amplifier if the input terminals were shorted. The chapter also discusses the relationship between the noise voltage source, the noise current source and the noise figure NF.
