Resistor Noise can be Deafening, and Hard to Reduce

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2021-06-09 14:00:05

It may not be the amplifier causing the trouble; analyze where the noise is actually coming from. A typical operational amplifier circuit contains six uncorrelated noise sources (the smaller ones can usually be disregarded1). The amplifier itself has three separate noise sources: a voltage noise source appears differentially across the inputs; and current noise sources appear in series with both inverting and non-inverting inputs. Remarkably often the problem is not the amplifier, though, but the thermal noise generated by one or more of the three resistors that set the amplifier gain and provide bias current compensation. Analog Devices has over sixty types of op amps whose voltage noise is less than that of a 1 kΩ resistor .

This answer is rarely popular; it is far more satisfactory to blame an imperfect amplifier and replace it with a better one than to admit that there is a fundamental problem with apparently simple components such as resistors. In fact, a remarkably common response to a diagnosis of resistor noise is to seek a source of "good " resistors, with "good " being defined as without thermal noise.

This is impossible. The basic physics of resistance shows2 that the random thermal movement of charge carriers in a conductor always produces electrical noise of value  where k is Boltzmann's Constant (1.38065 x 10-23 J/K), T is the absolute temperature, B is the bandwidth and R the resistance. (We often express this noise in terms of spectral density, making the voltage noise .

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