Environmental Engineering Reference
In-Depth Information
Chapter 8
NOISE
Electronic noise is caused by small spontaneous fluctuations of currents
and voltages associated with circuit components. For this reason noise
cannot be predicted exactly, nor completely eliminated, but only can be
minimised. Under this definition we explicitly exclude all the disturbance
and interference (e.g. electrostatic and electromagnetic couplings) coming
from sources external to the system being studied, most of which are
deterministic and can be completely eliminated by adequate shielding,
filtering, or by changing the system physical location.
Noise in electronic circuits is originated by resistors (which generate
thermal, or white, noise) and by active devices. For instance, bipolar
transistors contain different sources of noise: thermal noise, 1/f noise, and
shot noise [M88].
In this chapter, after recalling some basic definitions, we shall
concentrate our study on the effect of noise in feedback amplifiers. We will
show that the noise properties of closed-loop amplifiers are not influenced
(in any sense) by feedback. However, an added feedback network, when
made up of resistive elements, will add noise.
8.1 BASIC CONCEPTS
We shall now recall some definitions associated with noise. For a broader
coverage of the subject of noise in electronic system design, the interested
reader is referred to [MC93], [GM93], [L94], [LS94], and [F88].
