Resistor values and Noise

JTEX

JTEX

Well-known member
Resistors make noise. Thermal (Johnson) noise. The higher the value, the more noise. If you simply go with convenient, common values without paying attention to their noise contribution, you can easily swamp an op amp's self noise with the surounding resistors' noise. The following 3 circuits are functionally equivalent and cost about the same. The only significant difference is the output noise (and input impedance, if it matters to the preceding stage).

A 7dB difference is more than twice the noise! Don't leave performance on the table for no good reason.

Screenshot 2025-12-04 122441.png
 
Pretty sure it does*—with larger wattage resistors having less. In critical circuits you can also spend more $ and get quieter resistors. (Usually along with less noise, you get both tighter specs and less thermal resistance change.) When I was building stereo equipment, trimmed naked foil resistors were the king of the heap if you were looking at a circuit before large amounts of gain were applied. They were also crazy expensive. For what we do, unless you're building mic preamps, it would be way overkill.

*I am too lazy to go off and look at spec sheets, but am thinking my memory is correct here.
 
Thought so. Just being curious now: I think I understand resistor noise will be most apparent on the audio path, but how about power supply vs. audio periphery, say a vref voltage divider vs. a buffer emitter resistor?
 
1. Physical size (or wattage rating) has no impact on intrinsic Johnson (thermodynamic) noise, provided the resistor is not heating excessively. However, size is one of several things that can impact the extrinsic noise, so-called excess noise - which typically has a "1/f - like" frequency dependence. Other things that can affect excess noise include the resistor material, its material composition, etc.

2. Thermal (Johnson) voltage noise is not the only noise to keep in mind, though. There's also current noise, which should not be ignored. E.g., If you look at opamp spec sheets, you'll often find both voltage noise and current noise specified. Both can have flat frequency components (intrinsic) as well as 1/f-like extrinsic noise contributions.

Note: by 1/f-like, I mean the spectral dependence behaves like 1/f^a [i.e., f^(-a)], where often a~1. But values of a can vary or even have different values over different frequency intervals (e.g., when multiple extrinsic noise components are present).
 
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I notice the value of the capacitor increase while the resistor decreases - the value of C1 is 25nF while C21 is 250nF. Does this also introduce noise?

Is out 3 the same amplitude as out 1? Do the resistors affect the signal in an absolute way, or are they relative to each other?
 
The caps do not contribute noise. The 3 circuits have the same output level and frequency response. Not sure I understand the last question.
 
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