Current sources vs sinks

[Analog Kid]

We're taught, from an early age, that this is the proper configuration for a current source:



and that this is the proper setup for a current sink:



But what if instead we use this as a sink?



and this as a source?



Question 1: Will these alternate configurations work?
Question 2: Will they work, but with some deficiency?

I'm really not trying to stir up a hornet's nest here: I'm just trying to wrap my head around this basic electronic concept.
It seems to me that really, either way should work, because if we think of the transistor as a device that passes current through from collector to emitter (conventional current flow direction, not electron flow), similar to a valve with some fluid flowing through it, why should it matter whether we tap the current at the top or at the bottom?

Now there is the matter of base current, which flows out of the emitter of the NPN and therefore adds to that current.
However, I_B is so small compared to I_E that it's basically "swamped" and can therefore be ignored, right?

This is basically the same as the high-side switch vs. low-side switch thing.
Just to be clear, I am excluding applications like precision current sources here; I'm proposing a typical current source that's just accurate enough.

[TimFox]

The distinction between source and sink:
A source supplies current to an external load from its internal power source.
A sink is a load for an external source.

Example: the traditional 7400 TTL output uses an NPN collector as a sink for current sourced by the next gate’s input emitter (from its power supply) to pull the output voltage down sufficiently for logic-low output.  For logic high, the totem-pole pull-up is a source to supply enough current (less than the low-state current) to obtain a voltage sufficiently positive to turn off the next gate’s emitter.

The four circuits you show are all sources, of different current direction, from the indicated power source “V+”.

[Analog Kid]

The four circuits you show are all sources, of different current direction, from the indicated power source “V+”.

OK, but that didn't really answer my question, unless the answer was implicit (in which case I missed it):
Will those all work?
Will they all work equally well?

And if they're all sources, what would a current sink look like?

[pqass]

The four circuits you show are all sources, of different current direction, from the indicated power source “V+”.

OK, but that didn't really answer my question, unless the answer was implicit (in which case I missed it):
Will those all work?
Will they all work equally well?

And if they're all sources, what would a current sink look like?

Yes the alternates will work (as a switch) BUT a base current must flow to turn on the transistor (WRT its emitter) and keep it on.
The only way to do that with a PNP collector on GND is to produce a voltage below ground out of the base resistor.
And with an NPN collector on V+ is to produce a voltage above V+ into the base resistor.

[PGPG]

Have you heard about emitter follower?

[TimFox]

Another distinction, specifically for current:  a good current sink exhibits a low impedance to the external current source, while a good current source has a high impedance to the external sink or load.
If you use a collector (PNP or NPN) to drive the load, that is a reasonable current source.
However, an emitter (PNP or NPN) drives the load from a relatively low impedance and is therefore better treated as a voltage source;  it also works as a current sink from an external source.
This assumes that the transistor is in its active region (not saturated).
A saturated transistor (switched hard ON) presents a low impedance at the collector.
Otherwise, calling the circuit a source or sink depends on the configuration and location of power sources, as I discussed above.

[tggzzz]

The four circuits you show are all sources, of different current direction, from the indicated power source “V+”.

OK, but that didn't really answer my question, unless the answer was implicit (in which case I missed it):
Will those all work?
Will they all work equally well?

And if they're all sources, what would a current sink look like?

That is reasonably well described in a book you have said you possess: TAoE, any edition will do.

An answer much deeper than Tim Fox has already given you will probably require considering semiconductor physics. Sorry about that.

Why don't you look at your copy of TAoE; you will then be in a position to ask a a more pointed question.

[Benta]

The distinction between source and sink:
A source supplies current to an external load from its internal power source.
A sink is a load for an external source.

Good definition.
Unfortunately, a lot of people define a "source" as current flow from V+ and "sink" as to V-. Let's call it "polarity thinking".

[Analog Kid]

I think the answer that @pqass gave is the one that's needed here.
I completely disregarded the biasing requirements for a source/sink circuit.
I believe this pretty much trumps any other discussions here, with maybe the exception of source/sink impedance:
The "alternate" circuits I posted simply won't work without outlandish biasing schemes (V below ground for PNP, V above V+ for NPN).

[TimFox]

The distinction between source and sink:
A source supplies current to an external load from its internal power source.
A sink is a load for an external source.

Good definition.
Unfortunately, a lot of people define a "source" as current flow from V+ and "sink" as to V-. Let's call it "polarity thinking".

That usage is similar to "source" and "sink" in generalized field theory, for example showing the velocity field for weather or the magnetic field from a bar magnet.
This was covered fully in the classic P M Morse and H Feshbach Methods of Theoretical Physics, McGraw-Hill 1953 (2 vols.)
I kept this book on my shelf at work:  when someone asked me a complicated question, I referred them to that book, and they didn't bother me any more.

https://www.physicsforums.com/threads/has-anyone-ever-finished-reading-morse-feshbach-and-courant-hilbert.1054844/

[Analog Kid]

Why don't you look at your copy of TAoE; you will then be in a position to ask a a more pointed question.

Again with the totally unhelpful "I'm not going to answer your question: read the book".

OK, wise guy: I have read what The Art of Electronics by Horowitz & Hill has to say on the subject.
Specifically from pp. 59-62 in my edition.
Yes, they have many good things to say about current sources.
However, they have really nothing to say about the difference betwixt current sources and sinks, which of course is the topic of this thread. All they write is this:

The other examples (using npn transistors) should properly be called current sinks, but the usual practice is to call all of them current sources.

Which of course doesn't really address my questions.

And as I wrote above, the answer given by pqass is a show-stopper that would stop my alternative examples from working without extraordinary base biasing schemes. Which I'm assuming is the real reason that the canonical forms of PNP vs NPN configurations are used.

[tggzzz]

Why don't you look at your copy of TAoE; you will then be in a position to ask a a more pointed question.

Again with the totally unhelpful "I'm not going to answer your question: read the book".

OK, wise guy: I have read what The Art of Electronics by Horowitz & Hill has to say on the subject.
Specifically from pp. 59-62 in my edition.
Yes, they have many good things to say about current sources.
However, they have really nothing to say about the difference betwixt current sources and sinks, which of course is the topic of this thread. All they write is this:

The other examples (using npn transistors) should properly be called current sinks, but the usual practice is to call all of them current sources.

Which of course doesn't really address my questions.

And as I wrote above, the answer given by pqass is a show-stopper that would stop my alternative examples from working without extraordinary base biasing schemes. Which I'm assuming is the real reason that the canonical forms of PNP vs NPN configurations are used.

You didn't bother to mention that you had read (some of) the relevant bits in TAoE.

Biassing transistors is always required, so I'm not sure why you regard that as "extraordinary".

TAoE uses the phrase "current sink" at least 70 times; I lost count. Most of those indicate/discuss biassing.

IMHO the best answer to your original question was given by Tim Fox. You were dismissive of his answers. Your subsequent responses indicate that you had asked the wrong question to get the answers you seem to like.

It is, of course, generally true that answering a question right (i.e correctly) is easier than asking the right question.

[Analog Kid]

Biassing transistors is always required, so I'm not sure why you regard that as "extraordinary".

I would regard requiring a PNP base bias voltage of below ground, or an NPN bias voltage of above V+ as "extraordinary" because those wouldn't be possible with a single supply voltage to the circuit.

[TimFox]

Single-supply circuits are now fashionable, but many circuits use split or dual supplies.
There are other ways to bias devices, even with a single supply.

[Analog Kid]

Single-supply circuits are now fashionable, but many circuits use split or dual supplies.
There are other ways to bias devices, even with a single supply.

Well, sure: I suppose one could use a voltage divider, f'rinstance. Or even a charge pump.

[pqass]

NPNs/N-FETs are just overall better than their P versions (due to physics). 
They're faster, beefier, have lower on resistance, more common, and therefore cheaper.
So it may be worthwhile to produce a bias voltage above V+ using a charge pump or DC-DC converter.
Some gate driver ICs provide a bias voltage so the same FETs/IGBTs can be used on both high and low side. 

[tggzzz]

Biassing transistors is always required, so I'm not sure why you regard that as "extraordinary".

I would regard requiring a PNP base bias voltage of below ground, or an NPN bias voltage of above V+ as "extraordinary" because those wouldn't be possible with a single supply voltage to the circuit.

You attach too much importance to a node being marked as "0V" or "ground".

For example, I have a piece of test equipment where, IIRC, some TTL outputs are high/1 if >-18V, and low/0 if <-19.6V. Others in the same equipment are >+2V and <+0.4V respectively. Need to be very careful when placing a scope probe shield on an IC pin

[Analog Kid]

Biassing transistors is always required, so I'm not sure why you regard that as "extraordinary".

I would regard requiring a PNP base bias voltage of below ground, or an NPN bias voltage of above V+ as "extraordinary" because those wouldn't be possible with a single supply voltage to the circuit.

You attach too much importance to a node being marked as "0V" or "ground".

Well, I'll admit that that's a shortcoming in my understanding of electronics.
I'm learning here ...

[TimFox]

Single-supply circuits are now fashionable, but many circuits use split or dual supplies.
There are other ways to bias devices, even with a single supply.

Well, sure: I suppose one could use a voltage divider, f'rinstance. Or even a charge pump.

A resistor can source or sink current,  depending on what you connect it to.

[Analog Kid]

OK, so would you accept this as an accurate illustration of that distinction?



It depends on whether the resistor is above or below the load, correct?

[Benta]

OK, so would you accept this as an accurate illustration of that distinction?

(Attachment Link)

It depends on whether the resistor is above or below the load, correct?

No. Your circuit is still supplying current to a load. Whether the current is positive or negative is irrelevant. It's a "source".

[Analog Kid]

OK, so would you accept this as an accurate illustration of that distinction?

(Attachment Link)

It depends on whether the resistor is above or below the load, correct?

No. Your circuit is still supplying current to a load. Whether the current is positive or negative is irrelevant. It's a "source".

OK, so can someone please show me what a sink would look like? Preferably in schematic form? (Crayon on cocktail napkin sketch perfectly acceptable.)

[SteveThackery]

No. Your circuit is still supplying current to a load. Whether the current is positive or negative is irrelevant. It's a "source".

Hey! This is new to me. So you're saying that the load has to be external?  In your post (#2) you give examples of a current source and a current sink, and you say that in both cases the load is external (ie in the next chip along).

Well, this requirement is news to me. But more importantly, it must mean that your assertion that all four of Analog Kid's examples are current sources is wrong, because none of the loads he draws are external.

So if the load is not "external" (whatever that now means), what are the circuits Analog Kid has drawn called? Obviously not current sources or current sinks.

[Analog Kid]

What they said.
What exactly does "external" mean?
I think it would be very helpful if someone could post examples showing a current source vs. a sink, and showing what an external load looks like.

[PGPG]

No. Your circuit is still supplying current to a load. Whether the current is positive or negative is irrelevant. It's a "source".

I'm not very good in English - it is my second language.
The terms sink and source I met first time in digital ICs datasheets.
How you explain that these outputs are characterized with their source and sink capability even between connecting to output a LED+R to GND or to VCC has absolutely no difference regarding the sense of supplying the current to load - both connections supply the current to LED and in both cases a power source is on the same PCB as IC and load (LED) is also there.