AMPLIFER NOISE

In almost every area of measurement the ultimate limit of detectability of weak signals is set by noise  - unwanted signals that obscure the desired signal. Even if the quantity being measured is not weak, the presence of noise degrades the accuracy of the measurement. Some forms of noise are unavoidable (e.g. real fluctuations in the quantity being measured), and they can be overcome only with the techniques of signal averaging  and bandwidth narrowing, which we will discuss in chapter 15. 

Other forms of noise (e.g. radiofrequency interference and "ground loops") can be reduced or eliminated by a variety of tricks, including filtering and careful attention to wiring configuration and parts location. Finally, there is noise that arises in the amplification itself, and it can be reduced through the techniques of low-noise amplifier design. Although the techniques of signal averaging can often be used to rescue a signal buried in noise, it pays to begin with a system that is free of preventable interference and that possesses the lowest amplifier noise predictable.

We will begin by talking about the origins and characteristics of the different kinds of noise that afflict electronic circuits. Then we will launch into a discussion of transistor  and FET noise, including methods for low-noise design with a given signal source, and will present some design examples. After a short discussion of noise in differential [amplifiers] and feedback amplifiers, we will conclude with a section on proper grounding and shielding and the elimination of interference and pickup. 

7.11 Origins and kinds of noise

Since the term noise can be applied to anything that obscures a desired signal, noise can itself be another signal ("interference"); most often, however, we use the term to describe "random" noise of a physical (often thermal) origin. Noise can be characterized by its frequency spectrum, its amplitude distribution, and the physical mechanism responsible for its generation. 

(Acknowledgment: P. Horowitz & W. Hill. 1989. The Art of Electronics. Cambridge: CUP)