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Author Topic: Transistors - what is the collector/emitter voltage drop?  (Read 2857 times)
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In that he said that whilst there was a 0.7V drop between base and emitter, because of the PN junction, there was "almost no" drop between collector and emitter because the PNP junction "cancelled it out".
This observation is only true of a saturated transistor at low values of the collector current, and the explanation is just plain wrong. Transistor action is not intuitive and can't be explained with such simplistic concepts.

Vce(sat), Vce in the "saturation" mode of transistor action, can vary from about 0.1 V to > 10 V, depending on transistor type and collector current. Obviously, Vce in the unsaturated mode can be as large as the power supply voltage -- the transistor is off.
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.
.

doesn't make sense, does it ?

Yes it does. The lower the collector current the smaller the effect of the collector impedance and consequently a lower collector emitter voltage.

There are a couple of very good treatises on the subject here and here (scroll down to section "2.2.2 The h-parameters" in the second one).

Edited to add quote so that the post makes sense!

good links, I agree, but none of them make this
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that you might expect 1.4V or so voltage drop between collector and emitter (2 x 0.7V)

make sense  smiley-wink
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Lacey, Washington, USA
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that you might expect 1.4V or so voltage drop between collector and emitter (2 x 0.7V)

Well, that doesn't make sense because the CB junction is reverse biased, when considered as a simple PN junction, so the forward bias 0.7V of a simple PN junction does not apply to the CB junction, nor does it apply to the BE junction, either. A bipolar transistor is -much- more complex than just two diodes pointing in or out.

I suggest some deep reading on semiconductor physics if you really want to get a better understanding of this. All the graphs of transistor curves aren't going to do anything more than show you that it is what it is.

And no simple schematic symbol or simplified sketch of an NPN junction is going to help, either. You should have some understanding of the standard model (of physics). Some. Do you know what the N and P mean? What are "holes"?

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Right. Well, do you agree, or not, that you might expect 1.4V or so voltage drop between collector and emitter (2 x 0.7V) or no voltage drop because they two voltage drops cancel out? (one positive, one negative)

Disagree.

Which part do you disagree with?
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The saturation voltage is a peculiar spec which is not needed by MOSFETs. Bipolar junction transistors have worked for decades to get a Vsat down to 0.1 volt, but for a MOSFET it is 0.00000 volts.

The npn has a base p material swamped with electrons from collector to emitter, shorting out any diode drop artifacts. The geometries and impurities can get a low Vsat, but at a cost of voltage, gain, and every other trade off.
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I suggest some deep reading on semiconductor physics if you really want to get a better understanding of this. All the graphs of transistor curves aren't going to do anything more than show you that it is what it is.

So when I Googled "transistor circuits" and looked at the 5th match:

http://www.usd.edu/~schieber/psyc770/transistors101.html

And that said:

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If the transistor is on, the Collector voltage is 1.6 volts higher than the Emitter voltage.

Then it was my bad luck that this particular tutorial was wrong? He specifically mentioned the 2N2222, and my datasheet for the PN2222A quotes Vce(sat) as max 0.3V at 15 mA.
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So when I Googled "transistor circuits" and looked at the 5th match:

http://www.usd.edu/~schieber/psyc770/transistors101.html‎

And that said:

Quote
If the transistor is on, the Collector voltage is 1.6 volts higher than the Emitter voltage.

Then it was my bad luck that this particular tutorial was wrong? He specifically mentioned the 2N2222, and my datasheet for the PN2222A quotes Vce(sat) as max 0.3V at 15 mA.

That is incorrect. I mean -you- are correct, the website is incorrect.

I know it is hard to believe,  smiley but there are websites that are wrong out there... one very famous site had a backwards wrong description of how the Joule Thief circuit works. In response to my corrections, and possibly others for all I know, the webmaster took down the description rather than correct it.

I was without a degree for most of my career. I went back to school a few years ago to finally get that all-important (according to HR departments) piece of paper. At least every other chapter in the electronics book had outright errors in the instructor's answer book.

See my reviews of a couple of books on Amazon:

http://www.amazon.com/review/R2VUZ6NC15HZK3/ref=cm_cr_rdp_perm

http://www.amazon.com/review/R2314GGX26MQFF/ref=cm_cr_rdp_perm

And those are supposed to be instructional books. If you look at other books written by the author of Residential Construction Academy, the book previews show he copies and pastes the same mistakes.
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Yes, thanks. I had a bit more confidence in the YouTube video because he actually demonstrated measuring the voltage difference. Although I thought the comment that the PN and NP junctions "cancelled out" might have been wishful thinking a bit.
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polymorph:
And books.
One of my favorite books is "The Elements of Programming Style" by Kernigham and Plauger. They take examples from programming textbooks as bad examples of programming. It is amazing how many of those programs simply don't work for one reason or another.
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KeithRB: Don't get me started about Arduino books. I have half-a-dozen of them, and only -one- mentions millis().

Nick Gammon: That is a subpage of a university professor, sadly. I see other more subtle misleading things on a quick look at his website. The information he has on a JFET is specious, at best.
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Arduino's native libraries is not what i'd call huge, there should not be an excuse to miss out any commonly used functions.
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Although I thought the comment that the PN and NP junctions "cancelled out" might have been wishful thinking a bit.
I think this comes from over thinking the transistor's equivalent circuit of looking like two diodes. Yes it looks like two diodes when you probe it with a resistance meter but it is not two diodes. You can not make a transistor from two diodes it simply would not work. This is because the only reason a transistor works is that the whole structure is build out of a single crystal.
The fact that there is a voltage between the collector and emitter even when the transistor is as hard on as it can get is an inevitable consequence of how the transistor works. Unlike a FET that does not have any saturation voltage but a saturation resistance.

It is all well and good pontificating about "errors" in web sites or books but Vce is of secondary importance and to a first approximation can be omitted from the discussion. This does not make these discussions wrong, just simplified. This happens all the time in every field of science and engineering. We still teach the idea that electrons "orbit" the nuclease in an atom where it has been known for more than half a century that this is not the case. However the idea of electron shells being in different orbits at different distances is so useful that it is still taught. It is the basis of the whole of chemistry but it is wrong.

The bottom line is don't go saying something is wrong just because it is a simplification because that is how people learn. Give them too much small scale second order irrelevant for what they are doing stuff and they will become swamped with "facts" and never learn anything. Ever thought why the miller effect in transistors is hardly ever mentioned on this forum? It is because most of the time it is not relevant but any explanation of a transistor action without mention of it would be wrong according to some.
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I wasn't pontificating, Mike, I was trying to understand something.

Possibly I was wrong (it wouldn't be the first time), or I misunderstood, or the web page was wrong, or maybe misleading. I was just trying to work out which it was, and then move on to learning the next thing.
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I wasn't pontificating, Mike,
I know, that was aimed at some others.  smiley
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Lacey, Washington, USA
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 So I'm a pedant. I don't like things so oversimplified that they aren't even wrong.  smiley-razz
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