E9 Electronic Noise

Topic

Electronic noise is the random, inevitable fluctuation in voltage or current found in any real-world circuit; it ultimately limits the weakest signal a system can reliably detect. Thermal noise is present in any resistor at temperatures above absolute zero, generated by the spontaneous thermal agitation of charge carriers within the material; it does not depend on current flow or the frequency in question—it is "white" noise, uniform across the spectrum—and it increases with both temperature and resistance value.

Shot noise arises for a different reason: electric current is not a continuous flow but a stream of discrete charge carriers crossing a potential barrier—such as the one described in the first model of this catalog—one by one. It, too, has a white noise spectrum, and its intensity increases with the average current flowing through the circuit. At low frequencies, however, flicker noise dominates; caused by microscopic defects and imperfections in the materials, it becomes more intense as the frequency decreases.

A system's noise figure quantifies the additional noise it introduces relative to the theoretical minimum. When multiple stages are connected in cascade—as in a radio receiver—a counterintuitive phenomenon occurs: the first stage of the chain overwhelmingly dominates the noise figure of the entire system. This is precisely because the noise added by that first stage is amplified by all subsequent stages, just as the useful signal would be.

For this reason, this module—which begins by introducing three noise sources of distinct physical origins—concludes by explaining why the first amplifier in any sensitive receiver is designed with such care to minimize its own noise. It is not because this is the most critical stage for signal amplification, but because it is the only stage where the noise introduced cannot be corrected by any subsequent design.

ID:4055