# Sinking & sourcing current, pnp & npn

Discussion in 'Electronic Basics' started by Jani Miettinen, Sep 14, 2003.

1. ### Jani MiettinenGuest

I'm studying electronics on my own (as a hobby), reading right now
Art of Electronics, browsing the web to fill some of the gaps (yes,
there are quite some) and toying with CircuitMaker (which helps a
lot to understand the stuff).

Sinking and sourcing current have been referenced to several times,
but they have never been explained in any usable detail. So, here
are the questions -- for now, more might follow :

What is the difference between sinking and sourcing current?

Pnp and npn transistors differ somehow in this respect -- but why,
and what does that mean in practice?

2. ### R. Steve WalzGuest

-------------------------
It is like a switch upstream or downstream of a load.

An NPN is best downstream of the load, emitter to ground, which sinks
current to ground to turn on the load, and this is because it needs a
base current provided by a voltage that shouldn't vary any more than it
needs to in order to stabilize the turn-on point.

The base current makes the collector path voltage drop suddenly grow
and stand the base up higher so the base voltage has to grow as well,
and it can chatter on or fail to turn-on evenly.

If it was on top the load, as seen from a +V high, Ground low diagram,
then the voltage needed to turn on the base would vary right when it
was ramping up to turn-on, due to the voltage drop through the C-E path.

The PNP is best upstream of load, upside down, emitter up to Vcc,
because then we must ground the base to turn it on, and it mirrors the
NPN case, except upside down, and prevents that base instability at
turn-on.

-Steve

3. ### Kevin AylwardGuest

It can be a switch. It can also be a linear amp. I don't see any
assumption that the question refers to switching use only.
Well, I will query the use of the word "best" here. One can have a
push-pull or source sink with either a collector output or emitter
your reply, I will it it to you to expand on this.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

4. ### John PopelishGuest

The words sourcing and sinking just refer to the two possible
directions of current. If the switch or control element is on the
positive side of the load that switch or control element is said to
source current. If the switch or control device is on the negative
side of a load, it is said to be a current sink. This is all based on
the convenient fiction (positive current convention) that current
consists of positive charge moving. So a source lets charge reach the
load, and a sink lets charge leave it (and sink down the drain).

Not technical, at all (or even accurate).

5. ### Winfield HillGuest

John Popelish wrote...
Right. For conventional "current-source" circuits, which use the
transistor's collector for their output, PNP or P-channel types
can only source current and NPN or N-channel types can only sink
current. But note, it's common practise in the industry to refer
to both NPN and PNP types as "current sources," even though in fact
the NPN type is a more accurately called a current sink.

Enjoy your studies, Jani, and best wishes.

p.s. Good book choice!

Thanks,
- Win

7. ### Jani MiettinenGuest

This does make sense.

Earlier I experimented with op-amp integrator *) and found out that
bipolar transistor could not be used (at least I couldn't figure
out how) to reset the integrator capacitor due to the instability
of turn-on point. I suspect this is somewhat a similar situation,
but of course a bit different.

*) like this: http://www.maxim-ic.com/tarticle/images/A207Fig01.gif

8. ### Jani MiettinenGuest

It is!

I have to say that reading The Art of Electronics is inpiring and
enjoyable. It's amazing how you have managed to pack so much
information into it without having it filled to overflowing.

Figuring out why the bad circuits are bad circuits is probably one
of the most educational parts of the book. At first, from the
beginner's point of view, it seemed that absolutely nothing was
wrong with them but soon it was clear why they would not work as

9. ### Kevin AylwardGuest

Rule two. Don't simplify so much that it becomes meaningless.

Oh. You think so. If this were true, just why do you proposes that,
essentially, all audio power amps use emitter follower outputs. This
amounts to 10000's of designs, and billions of actual product sold. How
do you explain that non emitter follower output op-amps are few and far
between.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

10. ### BananaPannaPoe-Guest

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Sinking is bringing the ckt to 0v or low. Sourcing is bringing the ckt to
the source voltage or high.
NPN is negative emitter, positive base, negative collector material.
Biasing the base more negative caused conduction. More positive cuts it off.
PNP is positive emitter, negative base, positive collector. Sourcing the
base causes conduction. sinking the base cuts it off.

12. ### Kevin AylwardGuest

Its your reply, I don't want to intrude to much. I'll give you a hint
though. Output impedance.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

13. ### R. Steve WalzGuest

------------
Which my switching examples didn't need to concern themselves with,
of course. My point exactly in asking you to do the gain and stability
analysis.

-Steve

14. ### Kevin AylwardGuest

But I pointed out that switching was not assumed in the question.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.