Why doesn't current flow to ground in some circuits? (followup question)

Question

This is a follow-up question to the following questions:

Why doesn't current flow to ground in some circuits?

When does and when doesn't current flow to ground?

I have no issue accepting the answer that current does not flow into a ground, and that ground can be seen as a zero-reference level that does not interfere with current flows. I also can (unwillingly) accept that multiple grounds in a circuit can be seen as these points being connected as in the second question I linked to.

However, in an electronics course I took in my undergrad we were presented with usage of ground in circuits like the current mirror below:

Here we have two currents \$I_\text{ref}\$ and \$I_\text{o}\$ which are both flowing in the same direction, and applying Kirchhoff's law at the node just above ground implies that the current has nowhere to go but through ground! This contradicts the "no current through a single ground" principle right?

I can understand the justification that ground in this context is used as a shorthand for a closed circuit where the bottom portion of the circuit has zero potential (grounded), but this just grinds my gears to be perfectly honest and was for sure a big source of frustration and confusion when I took the course.

Am I missing something here or do I just have to accept that ground can mean different things depending on context?

The triangle symbol is shorthand for a power supply with its positive terminal on the triangle and it's negative terminal on the ground symbol. If you draw that out you'll see that no current flows to ground (which would violate KCL and thus is impossible), only back to the power supply. Essentially this is just a convention used to keep diagrams simple and focused on the relevant parts rather than drawing an identical power supply over and over. — user1850479, May 23 '23 at 13:54
I may be wrong, but as far as I know the principle means that the sum of currents going into and out of the ground in a typical circuit is zero, since it goes between negative and positive ends of a power source and whatever current goes through transistors, resistors and such, the same amount of current goes back to the power source's other end, and the ground serves only as a voltage reference point. — Edin Fifić, May 23 '23 at 14:03
Try to read here http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf — G36, May 23 '23 at 14:07
Thanks for the link @G36, that looks very pedagogical and is what I'm after. — Hugo, May 23 '23 at 15:20
You're looking at a small 'snippet' of a circuit there, but missing the greater context from which it's taken. In as 'real' circuit there would be one or more 'power supplies' connected both to the 'ground' reference point as well as supplying current to the various sections of the circuit. In the case of your circuit snippet, there's an implied power supply somewhere which supplies current to both Iref and Io - and it's that which completes the current loop. — brhans, May 23 '23 at 19:31

score 3 · Accepted Answer · answered May 23 '23 at 13:56

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In electronic we used GND as a reference point.

How to find Voltage based on reference nodes?

Note that all terminals connected to ground are likewise connected to each other. Thus we can simplify the circuit diagram.

Take a look at this example:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

And this is exactly the same case that you see on your circuit diagram with current mirrors.

schematic

^{simulate this circuit}

answered May 23 '23 at 13:56

G36

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@Hugo You haven't finished your question. The question was? – G36 May 23 '23 at 15:11
Thanks for a clear straightforward answer. I understand this rationale and it makes sense to me as a simplification of a circuit, its just that ground in the right hand side is not just a zero voltage reference, it is also a shorthand for closing the circuit as shown in the diagrams to the left. Thinking of ground as a "free floating wire" does not work in the r.h.s but it does in the l.h.s. Different symbols feels the most appropriate to differentiate the two, but I think I will just have to accept this as standard in the field and learn to differentiate the two from context. – Hugo May 23 '23 at 15:15
When the cookie crumbles for me is when I try to figure out where current is going, and that is when the two "types" of ground become important. In the l.h.s. there is no current through ground, but in the right if we just look at it as any ordinary circuit then there must be a current through ground. – Hugo May 23 '23 at 15:18

score 1 · Answer 2 · answered May 23 '23 at 15:31

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In most electronics, and paricularly in your sample circuit, the Ground symbol simply indicates the point in the circuit that we wish to call "Zero Volts", and use as a reference when measuring voltage elsewhere in the circuit. It does not imply any connection to the "outside world", or to an infinite current sink.

Frequently the Ground symbol also indicates a connection to the negative terminal of the power supply.

answered May 23 '23 at 15:31

Peter Bennett

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However it **does** imply that all ground points are connected to each other, and it **often** implies the negative terminal of the power supply. – user253751 May 23 '23 at 22:47

user253751 · Answer 3 · 2023-06-22T20:00:36.897

When you build the circuit, all ground symbols are to be connected together with wires - always*. And the ground symbol itself is not something you build. It's both a marker for zero volts and an instruction "connect this to zero volts". Of course if there is only one, there is nothing to connect it to, so it only fulfils the first function in this case.

The current mirror you are studying is not a complete circuit but just one part/section/fragment of a circuit.

It's implied from context that somewhere there is a power supply, the power supply is connected to the ground symbol and the upwards pointing arrow, and you are not studying the power supply right now so you didn't draw it. Something is making the current flow through this circuit section from top to bottom, and you are not really interested in why the current is flowing, you are interested in how the current mirror affects it.

When you are studying a toilet you do not care where the flush water comes from as long as it comes from somewhere and you don't care where the poop goes as long as it goes somewhere. The city engineer cares that the rain falls and fills up a reservoir and the poop goes to the sewage plant and gets filtered out; the environmental scientist cares that the sewage water gets evaporated by the sun and becomes rain again; the toilet engineer does not. Even if the same person can be all three, the environmental scientist isn't thinking about the water cycle when he fixes his toilet. When he fixes his toilet he sees the water appearing by magic and the poop disappearing by magic because he is only looking at the toilet and not the entire water cycle.

This is what you're doing here with the current mirror. You see current appearing magically at the top of the schematic, going through these components, and disappearing magically in the ground symbol. But there's not really magic - there's a battery, or a generator, or something. But that's outside of this current mirror and when you study the current mirror you don't care about it, you just know there is current flowing through for some reason.

It would be good to remember that the purpose of a diagram is to show you something. This diagram is to be read by a human, not a robot. When you do use a computer to read your circuit diagram, you had better draw the power supply because the computer will not add one for you and your circuit won't work. But this one is for humans to learn about, so it takes liberties to make the diagram more concise and one of those liberties is to not draw the power supply.

By the way you don't have to have the negative side of your current mirror connected to ground. It's just how they chose to draw the one you are studying. It can be -15 volts if you want it to be. Or +15 volts. It does have to be a lower voltage than the voltage of the output, though, or else there's no force pushing the current through the circuit. You'll often find the negative side is connected to the lowest voltage in the circuit, which can be a negative one.

* ^{sometimes you will see circuit diagrams with more than one different ground symbol, and you connect together all ground symbols of the same type, but not ground symbols of different types.}

score 1 · Answer 4 · answered May 24 '23 at 08:49

Not all ground symbols are created equally, and are often not used precisely.

These three symbols are different. Earth Ground means there is a connection to a big metal pole somewhere that is buried in the ground. Chassis ground means there is a connection to the metal casing of the device. Signal ground means the reference voltage that signals in the circuit are measured against. Meaning if we were to place a multimeter on this circuit, we place the black cable on this point when making our measurements. This can be placed in an any location, but is often the negative connection of the power supply to the circuit.

The confusion is that engineers are often lazy, imprecise, and assume that the reader will be able to discern what they mean, instead of doing it right the first time.

The only time you should be drawing the Earth Ground symbol is when there is a low resistance connection to an earthing system (big metal pole in the ground). It is almost always used as a protective feature for high voltage systems, and rarely in other situations for the purposes of this discussion.

In many cases, the earth ground symbol is used when they really mean to use the signal ground. The circuit you provided is an example of this.

I think this has led you to misunderstand the '"no current through a single ground" principle'. What that answer was trying to explain is that no current will flow into a ground itself, as it is simply a label for 1 node of the circuit. It can only ever be one node. This means that if you see two or more locations in the circuit where a ground is used, it means that from this location, there is a connection to the node labelled as ground. It is functionally identical to if you created a label for the positive of the power supply, called it V+, and then replaced every wire connecting to the power supply with a connection to the label instead.

For example, this circuit:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Could have the two nodes labelled as ground and V+:

schematic

^{simulate this circuit}

Note: I have coloured all wires that belong to the same node the same colour.

This diagram looks a little messy, so how about we get rid of the wires, and use the labels to represent the node connection?

schematic

^{simulate this circuit}

This is still the exact same circuit, we've just made it a little easier to read. Note that the current seems to flow into the V+ at the battery, and then out the V+ at the resistors. Similarly it seems to flow into the ground at the resistors and out of the ground at the battery.

For any circuit to work, there needs to be a power supply or battery providing a voltage between two points. Therefore drawing our battery on the circuit is a bit pointless, as we know it has to be there! So as a last simplification, we remove the battery from the drawing entirely:

schematic

^{simulate this circuit}

Note that we have written what V+ means, it is a node that is at a voltage of 9v (due to a power source that hasn't been drawn). Also, since we have not been told what the 9v is in reference to, we can assume it is in reference to the node labelled ground. (remember the labels we had on the battery before we removed it from the drawing).

If we now look at our final drawing, it appears that there is current flowing into a single ground! But we now can see that this is because of the label of the node, which is connected to our power source which has been omitted from the drawing.

I would suggest that the 'one ground means that there is no current flow into it' is misleading you. You need to consider what the ground label means. Is it only showing you what node is used for the black wire of the multimeter? Or does it mean the negative side of the power source in use? What it means will decide whether there is current flow into it or not.

schematic

^{simulate this circuit}

Looking back at this circuit, what does the ground mean here? Is it showing us the negative side of our power source? Well our battery has been drawn into the circuit, and if there was another power source that hasn't been drawn, we would see another label that would tell us about the positive side of the power source. So it is safe for us to assume that this is only a reference point label, and therefore we are considering that the connection between the resistors is the location in the circuit which is '0v', which we will measure all our voltages with respect to. Since it is a reference label, there will be no current flow into it!

Why doesn't current flow to ground in some circuits? (followup question)

4 Answers4