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There's an old symbol for a current source that's just two overlapping circles, reminiscent of a Venn diagram. There is more than one question on here with people asking what the symbol is, but I'm interested to know where it comes from; is it just an abstract symbol with no meaning behind it, or is it meant as a simplified drawing of something?

enter image description here Symbol d in the above image (taken from wikimedia commons) is the one I'm asking about. Symbol a is the one that I am most familiar with, and I've never seen symbols b, c, or e before.

I'm aware that the precise reasoning behind any given symbol doesn't really matter, but I simply have an interest in... "etymology", I guess you could call it? ...now I've gotten to thinking about circuit diagrams as a language, and studying them from a linguist's point of view....

That's for another website, though.

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    \$\begingroup\$ @SolveEtCoagula07 I am aware that this is a constant current source. I'm asking about the origin of the symbol, not what it means. \$\endgroup\$
    – Hearth
    Commented May 10, 2018 at 15:54
  • \$\begingroup\$ Probably not relevant to the question, but I had never seen symbol D used as a current source--only as a transformer in a single-line diagram. Just something to keep in mind for encountering the symbol in future. \$\endgroup\$
    – LMS
    Commented May 10, 2018 at 16:01
  • \$\begingroup\$ More than likely different standards for electrical symbols. For instance, IEC standards have a different symbol for resistors than the American standards. \$\endgroup\$
    – user103380
    Commented May 10, 2018 at 16:10
  • \$\begingroup\$ @LMS I've seen figure D used as a current source mostly in IC internal functional diagrams \$\endgroup\$
    – DerStrom8
    Commented May 10, 2018 at 16:54
  • \$\begingroup\$ ''now I've gotten to thinking about circuit diagrams as a language, and studying them from a linguist's point of view.... That's for another website, though.'' I don't agree with the last statement. It is our language and our history right there. I think that it is kind of nice to be able to talk about all those schematics, different styles and subculture. A Russian ASIC designer might use different schematics than a North-American designer and so on and so forth. Really interesting question this morning and I love the discussions and answers that it sparked. \$\endgroup\$
    – Simon Marcoux
    Commented May 10, 2018 at 17:03

3 Answers 3

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That's technically not just a current source. It's the symbol for a current mirror. If a current source is a circle, then if you want one current to match another current then the second circle represents this mirroring action. The image below from here shows how the symbol is used in a simple differential pair amplifier. In this case, it represents the fact that the current source is actually being generated from some other biasing circuit not shown in this specific image.

I'd recommend looking into how various current mirrors are implemented to understand why one why one may want to represent all of that with this simple diagram. Basically, it's a shortcut to eliminate showing an entire half of the actual circuit here. Especially in op-amp designs where many current sources derived from a single current source are used, this short-hand notation saves a lot of effort and simplifies the look of the schematic while still capturing all relevant information.

enter image description here

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    \$\begingroup\$ I've seen it used as a pure current source. For example, in a noise model of a photodiode, in a textbook from a major academic publisher. \$\endgroup\$
    – The Photon
    Commented May 10, 2018 at 16:28
  • \$\begingroup\$ @ThePhoton I would argue, this question itself is an example of how people may have misunderstood the origins of the symbol since it's mostly used showing the mirrored current side (vs the original mirroring current). There's no reason to assume a textbook or it's authors are flawless. In college, we were given bonus points to point out flaws in the textbook. \$\endgroup\$
    – horta
    Commented May 10, 2018 at 16:41
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    \$\begingroup\$ @Felthry Because the original halfway expanded version is the actual current source image on one vertical path with the current source reflection being on another vertical path with a dashed line connecting them. I don't know if I'm answering your question though. Two circles overlapping eachother is a small concise way to represent two linked current sources/sinks. \$\endgroup\$
    – horta
    Commented May 10, 2018 at 16:53
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    \$\begingroup\$ If there's a spec from IEC or somebody that says "use this symbol for a current mirror and that symbol for other kinds of current source" that would at least be evidence that somebody is trying to hold the line on the original meaning. \$\endgroup\$
    – The Photon
    Commented May 10, 2018 at 16:55
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    \$\begingroup\$ @horta surely that would be sideways compared to this, though? \$\endgroup\$
    – Hearth
    Commented May 10, 2018 at 20:24
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I managed to run a reverse Google image search to see where this picture came from. It found it on the Russian translation of Wikipedia. For some odd reason, the English version of this page has somewhat of a different format than the English version.

Here's the Russian Wikipedia page where this picture comes from. You're going to have to use some translator.

But here's what it says about the picture you have provided:

(a) and (b) are common notations for the current symbol. (c) were established by GOST and IEC. GOST is some type of Russian standards for electrical protocols. (d) and (e) are found in foreign literature, though it never says where exactly so it's possible that it's entirely made up for somewhere really obscure and possibly not credible, except for (d). I have seen (d) from plenty of sources as well. I'm not entirely sure why this Wikipedia page says, "Foreign literature" but I have yet to find the origins of (d).

The picture itself is a creation of one's own work meaning that whoever created this picture did not cite where they got this information.

Further investigation utilizing an old handbook it does indeed contain (c) as an ideal current source.

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    \$\begingroup\$ I've seen symbol d in a number of mostly old resources. The image is simply one found using google. \$\endgroup\$
    – Hearth
    Commented May 10, 2018 at 16:33
  • \$\begingroup\$ @Feithry Analog integrated circuit design, which is where this symbol is very commonly used is a dying industry due to the digital revolution and the better scaling of transistors in the digital domain. From that data, it's not surprising you'd see it mostly in older sources. \$\endgroup\$
    – horta
    Commented May 10, 2018 at 16:46
  • \$\begingroup\$ Thus all cameras simply digitize at the pixels in the imager? No amplification? certainly possible, but the ADC will be an analog beast. \$\endgroup\$
    – analogsystemsrf
    Commented May 10, 2018 at 17:16
  • \$\begingroup\$ @analogsystemsrf I'd call that mixed signal design. There's certainly analog front-ends to most digital chips. But fully analog chips are getting few and far between. \$\endgroup\$
    – horta
    Commented May 10, 2018 at 18:18
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The image I mentioned in a comment above is used by the Danish Wikipedia page on current sources. A Google translate of the page has the following to say.

A power generator - English: "Current Source" (commonly referred to as a Norton generator) is an electronic circuit that can deliver a given power regardless of the load.

Power generators are often used in theoretical analyzes of electronics circuits and are perceived as ideal power generators, ie circuits with infinitely high idle voltage and internal resistance.

Unless something else comes to light I will postulate that it is a symbol for a Wimhurst machine which was a low current high voltage generator from the early years of electrical exploration and would have behaved as an ideal current source under load with a constant speed rotation.

In the same way that our default voltage source was represented on schematics by the battery symbol that mimicked the Voltaic piles of the early electrochemical researches. This is in contrast to most of modern battery technology that has cylindrical or coiled/wrapped cells while still using the flat cell symbol.

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