Talk:Decoupling capacitor

message: The "Switching subcircuits" section mentions that the sagging supply voltage to an IC is only temporary since the line inductances and so on will eventually "saturate". Is this accurate? The line inductances are parasitic inductances, they are not like inductor components which will generally have some kind of ferromagnetic core. There shouldn't be any (significant) saturation effects. Even if an inductor does saturate, its inductance never goes to zero, it just becomes much smaller. I think whether the parasitic inductances saturate or not is besides the point. The inductances make it hard for the chip to change its supply current quickly when it switches every now and again. By their very nature, these load changes are temporary and transient, so inductor saturation is not a necessary explanation for why the sagging supply is transient, nor does it seem accurate. Correct me if I'm wrong. --Hddharvey (talk) 03:34, 8 August 2018 (UTC)
replies: text: Yes, they do not saturate.Constant314 (talk) 04:05, 8 August 2018 (UTC)
replies: text: The whole idea behind parasitic inductance is that there is more inductance than you would like.  In some cases, though, a series inductor is used as part of a decoupling circuit. They used to be, though rarely are now, in power supply filter circuits, and those were usually iron core inductors. Lossy ferrite cores are now often used for decoupling, especially for RF signals that travel on the outside of cables, or common mode signals on cables that aren't supposed to have common mode.  In that case, they could saturate, and that would reduce the desired effect.  I don't know that this needs to be included here. I suppose there is no article on decoupling inductors. Gah4 (talk) 04:18, 12 August 2018 (UTC)
replies: text:  Yes, I've seen instances where a sub-circuit does have a small series ferrite inductor, however, the inductor is selected such that it does not saturate when the circuit functions correctly as that would defeat its purpose.  But the article was using saturate incorrectly to mean that the inductor current reaches a peak and then returns to normal. Constant314 (talk) 19:09, 12 August 2018 (UTC)
replies: text: The usual case of unwanted saturation is running transformers on too low line frequency (60Hz transformers on 50Hz). Most should have enough margin, but not all. The inductance limits the current, and if it goes down, the current goes up.  LC power supply filters were common in vacuum tube amplifiers, possibly because capacitors weren't as good as now.  More iron makes transformers and inductors more expensive, so they tend not to overdo it. Decoupling gets harder when you have mixed analog and digital circuitry in the same device. Gah4 (talk) 19:56, 12 August 2018 (UTC)
thread_title: Mention of saturating inductances

message: Do you need any more capacitance than the inductance of the supply dictates?--SpectrumAnalyser 18:53, 12 August 2007 (UTC)
thread_title: Misc

message: Should this page be called electrical decoupling to include other forms of decoupling as well as capacitors?--SpectrumAnalyser 19:35, 12 August 2007 (UTC)
replies: text: Please feel free to edit to your satisfaction.--TreeSmiler (talk) 01:31, 3 January 2008 (UTC)

text: That's what I thought too when reading this article, so I added an example schematic Dalva24 (talk) 04:52, 22 September 2017 (UTC)
replies: text: Now reading it again, and if I undertand it correctly, I only added an example for analogue "smoothing" capacitor. Maybe someone can also add an example for digital logic decoupling Dalva24 (talk) 05:08, 22 September 2017 (UTC)

text: Started a subtopic called: Examples. Added two schematics along with their simulated oscilloscopes. Plus added descriptive text to help focus the reader's attention on some key points.Vinyasi (talk) 02:03, 1 January 2018 (UTC)
replies: text: From another discussion on this page, there are sometimes series inductors, forming LC filters, used in decoupling. I don't believe that is enough for a whole page on decoupling inductors, though. For digital circuits, the current pulse due to fast changing signals is (all?) that is important.  For analog circuits, it is more complicated. In that case, you might need to decouple a wide range of frequencies. You need to keep 60Hz (or 50Hz) out of the signal path. Even just keep the two halves of a stereo amplifier apart.  I know of ones that have separate power supplies for each half. We could have a redirect for decoupling inductor if that is useful. Gah4 (talk) 20:01, 12 August 2018 (UTC)
thread_title:  Page title 

message: (comment moved from top of page by User:JulesH)
The title is misleading, as it describes the use of a capacitor as a "smoothing" device - a crude regulator of the output voltage of a power supply.  "Decoupling" capacitors are connected between two circuits to allow a.c. signals to pass from one to the other, while at the same time "decoupling" any d.c. levels.
The source might have a d.c. level of 2.5V with an a.c. signal superimposed on it (say 50mV rms).  If this is input to an amplifier circuit whose input is biased to 1.5V and designed to accept a 50mV rms signal, the d.c. difference will upset the biasing of the amplifier and lead to distortion or no signal transfer.
A "decoupling" capacitor will block the d.c level, and allow the 50mV rms signal to pass, without upsetting the biasing.  — Preceding unsigned comment added by 80.254.147.236 (talk) 10:28, 29 November 2011 (UTC)
replies: text: At the very least this page needs a hatnote.  While power supply bypass capacitors are commonly called "decoupling" capacitors in digital design, the phrase is used with the very different meaning the anonymous commenter above describes in analogue design, particularly audio amplification.  I came here looking for an article on the latter meaning, unfortunately I have not yet found one. JulesH (talk) 15:39, 5 February 2012 (UTC)

text: Found it. JulesH (talk) 15:50, 5 February 2012 (UTC)
replies: text: The latter should not be decoupling capacitors, though they might be coupling capacitors. Gah4 (talk) 23:03, 1 January 2018 (UTC)
replies: text: I just realized that I wrote the above, and am about to disagree with it. The capacitors across bias resistors in common emitter BJT amplifiers, or cathode bias resistors in vacuum tube amplifiers are also bypass capacitors (redirects here) or often enough decoupling capacitors. The difference is that they aren't part of the main power supply, but just the bias power supply. But the page mentions power supply too much. But interstage coupling capacitors in AC coupled amplifiers are different. Gah4 (talk) 07:17, 11 April 2019 (UTC)
thread_title:  Decoupling for digital logic vs in analogue circuits 

message: This phrase in discussion doesn't seem correct to me, "so small and large capacitors are usually placed together in parallel to fully cover circuit bandwidth".  This is because putting 2 caps in parallel merely sums the individual capacitances.  So, you effectively have one capacitor.  On top of that I've never seen a circuit or a schematic showing 2 capacitors in parallel.  This info about capacitors is so basic  it is taught in physics before electrical engineering courses are started.  — Preceding unsigned comment added by 74.77.148.238 (talk) 06:28, 22 February 2017 (UTC)
replies: text: Yes it is common to have more than one capacitor, as larger capacitors turn into inductors at higher frequencies. There are usually a few large capactors for a board, and smaller ones near each IC. Physics mostly ignores this effect. Gah4 (talk) 23:01, 1 January 2018 (UTC)
replies: text: Here is a ref if someone wants to add it:  Constant314 (talk) 14:23, 26 June 2022 (UTC)
thread_title: comment

message: In the discussion I am used to, the capacitors in the power supply are filtering capacitors, to filter out the power line frequency, or for switching power supplies, the switching frequency. These are relatively low (50Hz to 20kHz) and mostly use electrolytic capacitors. Decoupling capacitors, and the ground plane of PC boards, are for the higher frequency signals generated by fast switching digital signals. (That is, high frequency Fourier components.)  These have to be close to the fast switching signals, as PC trace and lead inductance are significant at these frequencies. Larger capacitors, especially electrolytics, have enough inductance that they don't do much at all at higher frequencies. Gah4 (talk) 07:26, 13 April 2018 (UTC)
thread_title:  decoupling, filtering, smoothing, etc. 

message: Regarding the positioning of decoupling or bypass capacitors. I think I agree with the recent change, that they don't need to be positioned between the power supply (or source of whatever is being decoupled) and the rest of the circuit, mostly because of WP:NOTHOWTO. They can't be too far away, as the inductance between them and the rest of the circuit reduces the effectiveness. (In addition to the lead inductance of the capacitor.) There are feed-through capacitors to best reduce any inductance effect. But we don't need all the detail here. As some point, it requires knowing the exact detail of board layout. But also, it is easy to get wrong. I suspect positioning is more art than science. Gah4 (talk) 00:10, 7 July 2021 (UTC)
thread_title:  shunting, art, and science 

message: The article says: distorts the capacitor's behavior at higher frequencies.  This seems to me a strange way to say it. At higher frequencies many capacitors look like inductors. If no other reason, the leads themselves. It can be modeled as a series inductor, and so results in a series RLC circuit with a resonant frequency. But distort usually applies to other, usually non-linear, effects. Gah4 (talk) 08:16, 4 June 2022 (UTC)
replies: text: I reworded it. Feel free to improve it if there is a better wording. Hddharvey (talk) 00:16, 6 June 2022 (UTC)
thread_title:  ''distorts the capacitor's behavior at higher frequencies''