Where does voltage noise come from? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 11, 2023, 02:11:20 am
content: I get current noise, but what's the source for voltage noise?Also, is there voltage noise in tubes? what's the source there?

author: IanB
author_rank: Super Contributor
timestamp: September 11, 2023, 02:33:17 am
content: Quote from: ELS122 on September 11, 2023, 02:11:20 amI don't get it. Why do you "get" current noise? What do you get, exactly?How can you get current noise without voltage noise? How can there be current without voltage? So how can there be current noise without voltage noise?In industrial automation, 4-20 mA current loops have much greater noise immunity than 1-5 V instrument interfaces. If this is so, what exactly is your question?

author: IanB
author_rank: Super Contributor
timestamp: September 11, 2023, 03:09:02 am
content: Voltage can exist without current (e.g. static electricity). Current cannot exist without voltage (except for superconductivity, which does not normally occur).So voltage noise can exist as random fluctuations in potential, without there needing to be any current flowing.You introduced an op amp into the discussion, which you did not mention in your first post. If your question was about op amp noise, it would have been good to say that.Voltage noise is the random fluctuation in voltage that is measured across a resistor even when no current is flowing (perhaps called thermal noise, Johnson noise or Nyquist noise). This is noise introduced internally within circuit elements. This also happens inside semiconductors.There is also noise that can be induced externally from surrounding EM fields and radiation.It might be said that voltage is the birth of noise, while current is the death of noise.

author: Smokey
author_rank: Super Contributor
timestamp: September 11, 2023, 03:12:48 am
content: "where does voltage noise come from?"......Well.... when two electrons love each other very much.....

author: Swainster
author_rank: Frequent Contributor
timestamp: September 11, 2023, 03:34:39 am
content: From the mention of tubes and op-amps, i get the feeling that this question could actually be about the origins of excess noise, i.e., 1/f or pink noise, vs thermal noise, I.e. gaussian or Johnson noise.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 03:35:57 am
content: Simple case: thermal ("Johnson" or "Nyquist") noise: the theory predicts theavailable noise powerfrom a resistor, which is independent of the resistance.The available noise power is given byN=kTx BW, wherekis Boltzman's constant (from thermodynamics),Tis the absolute temperature in K, and BW is the bandwidth where the noise is measured.You can express that noise either as a noise voltage generator in series with the resistor, or a noise current generator in parallel with the resistor, but only one at a time since it's the same noise.For active devices (solid-state or tube), you take the total noise output of the circuit, divide by the voltage gain to that output, and that is represented as an equivalent voltage noise, referred to the input.When you analyze the noise of a transistor in the audio range, you find that it can be represented as an equivalent input current (in parallel with the input) and an equivalent input voltage (in series with the input), as discussed in the noise textbooks, to which I refer you.In the audio range, for a BJT or JFET, those two equivalent sources are almost uncorrelated (statistically independent), but at RF there is some correlation that affects the calculation of total noise (and results in a complex value for optimal source impedance).

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 03:42:04 am
content: If you short the input to an amplifier, then that allows you to measure the equivalent input noise voltage.If you connect a high resistance to the same input, the noise goes up and that gives you the equivalent input noise current.Note that these areequivalentvalues, not physical generators at the input: they result from all the noise generators in the circuit (active and passive) creating noise at the output, with equivalent input values calculated from the measured gain.If you have a pure resistance held at an elevated temperature, the random excitation of the charge carriers in the resistor will generate noise at its terminals, which can be considered either as voltage or current (not both at the same time), according to the Johnson noise theory.

author: jonpaul
author_rank: Super Contributor
timestamp: September 11, 2023, 03:48:39 am
content: Read a few books on noise measuremtn and noise reduction.Most opamp mfg like ADI, TI, had app notes on noise.j

author: magic
author_rank: Super Contributor
timestamp: September 11, 2023, 05:36:20 am
content: In gain devices like transistors or tubes, so-called voltage noise is the input referred equivalent of the output current noise.There is noise in the collector/drain/anode current if the input terminal is held constant.The output noise current can be divided by transconductance of the device.Now you know how much input AC voltage would produce the same noise in a "noiseless" device.That's your input-referred voltage noise.You can pretend your device is noiseless except that much voltage noise in series with the input.There is also input current noise, in the form of base current noise in BJTs.Base current noise flowing through internal base resistance may produce additional voltage noise.

author: EPAIII
author_rank: Super Contributor
timestamp: September 11, 2023, 08:01:47 am
content: Outside of superconductors, there is no such thing as zero impedance. There is always some impedance between any two points. Even a solid silver conductor only a fraction of a micron long and a full meter square in cross section will have some resistance. You can't get away from it.Now, knowing that you have both current and resistance/impedance, have you ever heard of Ohm's Law?E = IRThe current noise, that you say you "get" multiplied by the NON ZERO resistance/impedance will give you the Voltage noise or at least one component of it. And in all of science you will be hard pressed to find a theory that is better established and that has fewer exceptions than Ohm's Law.You simply can't escape the tie between Voltage and current. You can't because R is NEVER zero, except with superconductors as I mentioned above. This even applies at each and every point inside of your OP amp.And by the way, in the real world most noise measurements are made using Voltage, not current.You ask one ridiculous and poorly phrased question after another. Why don't you spend some time reading a good E&M textbook. It does not need to be a late edition, a used one will be quite OK. And when you understand enough of the basic science to formulate sensible questions, then ask them.https://www.google.com/search?q=popular+college+e%26m+textbooks&client=firefox-b-1-d&sca_esv=564268709&sxsrf=AB5stBgl6GJ8mx-YQhT0qTNSiZLdDjsjvQ%3A1694415072098&ei=4Lj-ZPzHBdmoqtsP0Z2u8AM&oq=popular+college+E%26M+text&gs_lp=Egxnd3Mtd2l6LXNlcnAiGHBvcHVsYXIgY29sbGVnZSBFJk0gdGV4dCoCCAAyBRAhGKABMgUQIRigAUjPkgFQjhtYkHZwBHgBkAEAmAGLAaABkBKqAQQxNi44uAEByAEA-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&sclient=gws-wiz-serp#cobssid=sQuote from: ELS122 on September 11, 2023, 03:39:30 am

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 02:46:50 pm
content: My favorite noise book ishttps://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Motchenbacher_Connelly/Low-noise_Electronic_Design.pdf(1993)The later version is by Motchenbacher and Fitchen:Low-Noise Electronic Design, Wiley, 1973I highly recommend buying a hardcopy of the 1973 edition.

author: CaptDon
author_rank: Super Contributor
timestamp: September 11, 2023, 03:01:38 pm
content: O.K., I have a dumb question here, and I guess the answer is 'No', but is there such a thing as 'low noise resistors'? It appears from Tim's equation that 'a resistor is a resistor' and noise is independent of actual resistance (I would not have guessed that). How are they getting there phenominally low noise floors on modern LNA's used in GPS and space communication? My only guess is chilling of the input stages to get the best possible SNR at the antenna input and first amplifier stage. Reminds me of the old analog T.V. days where folks had fringe area reception and tried to amplify the signal which gave a stronger signal with stronger noise PLUS added amplifier noise and the result was worse not better. The very reason good amplifiers were attached outdoors right onto the antenna. Thinking about space communications, what is considered the ambient noise floor and how is it noted? In R.F. work I think we couldn't see below -140Db or was that Dbm? That was the noise floor of our instruments and signals of interest needed to be about 6 to 10 Db higher. What would be the absolute noise floor for audio at room temperature? What would be the measurement term DBV? Dbm at 600 ohms? I remember the Telcos used the term DBRN. Db referenced to noise, but I don't know what level they assumed for 'noise'. Thank-you in advance to anyone who can refresh my mind on these terms and measurements!!! Cheers!!

author: donlisms
author_rank: Frequent Contributor
timestamp: September 11, 2023, 03:43:20 pm
content: I think Tim's reference to available noise power might complicate things a bit. The references I've seen to resistor noise talk about voltage, and in the real world circumstances, that's what I'm interested in. The equation for that is most definitely not independent of resistance.Since I can never remember Boltzmann's constant when I need it, I use one of the online calculators to find the noise of a given resistance. This would be at the core of the answer to "How quiet is quiet?" and some of the answer to designing circuits to get there.

author: MathWizard
author_rank: Super Contributor
timestamp: September 11, 2023, 03:58:08 pm
content: So what about thermal noise is pure inductors and capacitors ? Is there small changes in inductance and capacitance ? Or do they invoke some resistive parasitic's, and then energy losses ?

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 04:29:41 pm
content: Quote from: CaptDon on September 11, 2023, 03:01:38 pm"Low noise resistors": Johnson's noise theorem refers to ideal resistors, and is very good for resistors without DC current flowing through them, and real resistors also have parasitic inductance and capacitance.For audio and similar purposes, some physical resistors also show "excess noise" due to DC current, which is often called "1/f" noise (due to its non-flat spectrum at low frequencies), "pink noise" (also spectral), or "flicker noise".Old-style carbon composition resistors, where the physical path for current is tortuous, have this as a serious problem, and the best ones are bulk foil and good wirewound resistors, where the path is smoother.This is covered in the Motchenbacher books I cited, and 1/fnoise shows up in semiconductor devices as well.Noise power and voltage: the theorem states that the available noise power is given simply byN=kTx BW. This means that if you connect a resistorRat temperatureTinto a matched loadR, with the load in a cryogenic dewar to reduce its own noise contribution to a negligible level, the power delivered to that load iskTx BW.For most circuit analysis purposes, it is easier to model the resistor noise as a voltage source in series with its resistance. Since only half of that voltage appears across the matched load, simple algebra gives us a valueVnoise2= 4 xkTx BW, which is often given as "volts per root Hz".My mnemonic is that the noise in a 50\$\Omega\$ resistor at room temperature is 0.9 nV/Hz1/2. You can scale that for other resistances by the square root of the resistance.If you use a parallel model instead, the noise current decreases with increasing resistance, while the series model's noise increases with increasing resistance (keeping the noise power constant, independent of resistance).For inductors and capacitors, the reactance does not contribute noise, but the parasitic resistance of the physical part does contribute noise into the series or parallel model of the component.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 11, 2023, 06:52:10 pm
content: There's noise in everywhere. This is cause by the 50/60Hz power in your walls. Broadcasted signals. Cell phones. Stars. Other planets. And even your beating heart.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2023, 07:09:10 pm
content: Technically (and we should all try to use technical terms carefully) "noise" is a random fluctuation in voltage, etc.Other unwanted signals should be called "interference".Your examples are mostly periodic signals.