PNP transistor and argument with my teacher

[Studiot]

I suggest that cooling things with your lecturer would be a good idea.
You will go on to greater things he will be stuck where he is.
You might win the battle but you will certainly loose the war.

 

[Femme_physics]

You're right, I know you are... you're absolutely right in fact, this issue got me worked up though, it's 1:15 A.M. here and I can't go to sleep :( , and I got to wake up at 5:30 to catch a morning bus for my shift. I hope it'll wear down soon so I could get a few hours of sleep. You'll have to excuse me for belaboring the point here. I do have all the evidence I need with the simulators printed and the replies here (also printed). That's enough. Don't worry, I'm only worried about getting justice across, and making sure I get a 100 on the test (Because I was correct about everything according to the class review he did today-- including this!). Many thanks, btw, to the people of this forum. I will do it gracefully. :) I'm a chick after all :P

 

[Studiot]

Sleep well

:zzz:

 

[Antiphon]

I have two degrees in EE, a BS and an MS. I have been designing and building circuits for decades. My circuits are on military ships at sea as we speak.

There is a very simple convention here.

When you are confronted with a voltage of the form Vab, you connect "the red" probe to "a" and the "black" probe to "b" of your hypothetical voltmeter.

For a properly biased NPN, Vbe is positive. For a PNP it's negative.

But don't believe me- believe the manufacturers data sheets. The 2N3904 and 2N3906 are the prototypical Phillips and flathead transistors of the world. Take a look at Vbe for each.

http://www.makershed.com/v/vspfiles/assets/images/2n3906.pdf

http://www.fairchildsemi.com/ds/2N/2N3904.pdf

 

[Femme_physics]

I believe you, Antiphon.

I have a followng question, just for better understanding. If this value Vbe = -0.7 volts was given to an NPN transistor in the same scenario, would then my teacher answer be correct?

And, also, does it even make sense to give Vbe value for an NPN transistor in minus?

 

[I like Serena]

Hmm, forget about a given VBE of -0.7 or +0.7 for a moment. We'll get back to that.
Just to improve understanding, suppose in your circuit the PNP transistor is replaced by an NPN one, leaving everything else the same.

What would VBE become?

 

[Femme_physics]

Let's see, VBE = -0.7

Vbe = Vb - Ve = 0 - 0.7 = -0.7 volts

Veb = Ve - Vb = 0.7 - 0 = +0.7 voltsSame result. Doesn't matter NPN or PNP. If Veb had been defined as -0.7 volts, THEN and only then my teacher answer would be correct

 

[I like Serena]

Uhh... if you replace just the PNP transistor by an NPN one, VBE becomes something entirely different...
Visual queues or something? Does anything *bump*?

 

[Femme_physics]

I'm drawing it in my notebook but I ain't seeing anything different...

Ratch said that of a PNP transistor

the emitter will be more positive than the base if the transistor is in its active region and conducting.

In NPN it appears to be opposite. Yea, you're right ILS, since the base is more positive than the emitter in NPN's, the emitter to the ground will be -0.7 and therefor the voltage that would fall on Re would have been 6.7 volts

 

[Jony130]

If Vbe is equal -0.7V for NPN transistor. This means that NPN transistor in is cut-off.
And we have a similarity situation for PNP. If Vbe = 0.7V for PNP then PNP BJT is in cut-off.

 

[I like Serena]

No... with NPN the transistor won't be in its active region and it won't be conducting... the current *bumps*...

 

[Femme_physics]

At cutoff the transistor does not allow current hence the premise that Ie = 1.2 mA is false to begin with, but the voltage on the resistor Re would still be 6.7 volts

 

[I like Serena]

Yes, the premise that Ie=1.2 mA would be false. It would be 0 mA. Good!

But the voltage across Re would not be 6.7 volts.
What would it be?

 

[Femme_physics]

I still see 6.7 volts...

Soundproof logic #2!

http://img140.imageshack.us/img140/4461/harto.jpg

 

[I like Serena]

Do you agree that Ie=0 mA?

If so, can you apply KVL (and don't use that VBE=0.7 V because it isn't)?

 

[Femme_physics]

Yes, I agree that Ie = 0 mA

If so, can you apply KVL (and don't use that VBE=0.7 V because it isn't)?

No, VBE is -0.7, we agreed.

I'd just do

Sigma V = -0; 6 - 0 x Re - ...oh

Ok, nevermind, I see what you're talking about.

We got 0 voltage drop at Re! Thanks :)

 

[I like Serena]

Yep!

So what will VBE be then?

 

[Femme_physics]

VBE is the rest of it...6.7 Volts!

 

[I like Serena]

Almost.
You haven't applied KVL quite correctly yet...

 

[Femme_physics]

Sigma V = -0; 6 - 0 x Re - Vbe = 0

Vbe = 6 volts

 

[I like Serena]

Again... almost.
One thing left: the sign of Vbe.
Note (from the note master ) that Vbe = Vb - Ve.
What is Vb and what is Ve?
And what is therefore Vbe?

 

[jim hardy]

I am with Femme on this. Her teacher made a mistake.

My high school teacher (John B Davis) taught us boys " When they give you voltage between two leads, as in " Vbe" , your voltmeter red lead (positive) goes to the first letter and the black one (negative) to second letter".

I note that Fairchild datasheet for 2N3906, a common pnp, gives Vbe in operating range as ~ -0.65 volts, note minus sign.
That means a meter connected as above would find base negative wrt emitter, which is forward biased(emitter positive wrt base).

VBE(sat) Base-Emitter Saturation Voltage IC = -10mA, IB = -1.0mA
IC = -50mA, IB = -5.0mA -0.65(min) -0.85(typ) -0.95(max) V

Now here's a confusion factor - Same datasheet gives emitter to base reverse breakdown voltage as also negative,

V(BR)EBO Emitter-Base Breakdown Voltage IE = -10μA, IC = 0 -5.0 V

NOTE letters reversed for this parameter VEB not VBE . The O means is collector open circuited.
So this time a meter connected as Mr Davis told us would find emitter negative wrt base, reverse biased, which is what that parameter specifies - reverse withstand capability of EB junction.

It's one of those little points of technique that come from playing with real parts instead of simulated ones.

The conventions usually work if one is rigorous.
Only way her teacher is right is if that PNP has suffered an internal Collector to Emitter short . IMHO.

datasheet at http://www.google.com/url?sa=t&rct=...K0hKshhDb60zBONXQ&sig2=cupCh8ZbZ7737zChEnpyeQ

 

[Jony130]

Now here's a confusion factor - Same datasheet gives emitter to base reverse breakdown voltage as also negative, NOTE letters reversed for this parameter VEB not VBE . The O means is collector open circuited.
So this time a meter connected as Mr Davis told us would find emitter negative wrt base, reverse biased, which is what that parameter specifies - reverse withstand capability of EB junction.

It's one of those little points of technique that come from playing with real parts instead of simulated ones.

But I think that Vbe would be even more confusing then Veb.

And this is why manufactures use Veb instead of Vbe.

 

[I like Serena]

I have a followng question, just for better understanding. If this value Vbe = -0.7 volts was given to an NPN transistor in the same scenario, would then my teacher answer be correct?

And, also, does it even make sense to give Vbe value for an NPN transistor in minus?

To get back to your question.A transistor is never a voltage source, and it can't make 6.7 volts out of 6 volts.

However, VBE can have the "other" sign, but then it's in blocking mode as you just saw (and the current is zero).

If the transistor is in conducting mode, you will have a voltage drop of 0.7 volts in the direction of the arrow.

 

[jim hardy]

But I think that Vbe would be even more confusing then Veb. ]

I don't know why they do it, seems to me they could have just reversed the sign in datasheet instead of swapping the letters around..

And this is why manufactures use Veb instead of Vbe.

If you look at Fairchild datasheet for 2N3906 at
http://www.fairchildsemi.com/ds/2N/2N3906.pdf
you'll see they use V_EB in the "ABSOLUTE MAXIMUM RATINGS" and "OFF CHARACTERISTRICS" sections

but they use V_BE in the "ON CHARACTERISTRICS" section !
Both are negative.


The test question showed VBE = -0.7V
which by the convention i was taught would mean emitter positive wrt base, hence positive wrt circuit common , forward biased
so teacher should have written KVL as:
0 = 6 - 0.7 - ReIe ; traversing CCW around loop
and not not:
0 = 6 - ReIe + 0.7



Hmmm for sanity check i looked at OnSemi's datasheet -
they use same notations, V_BE and V_EB but both are positive !
http://www.onsemi.com/pub/Collateral/2N3906-D.PDF
Femme - that should REALLY confuse things !
You might print those datasheets and show to teacher - it's something students ought to be made aware of.

old jim

 

[I like Serena]

If I understand you correctly, you are confirming that datasheets are kind of "sloppy" with the sign of VBE?
And that the intention must be deduced from the context?

 

[sophiecentaur]

This seems a bit of storm in a teacup. Surely, anyone who is involved in circuit design or analysis at this level can recognise when they are dealing with NPN or PNP and get the sign of Vbe right? The Physics of transistors is pretty consistent.
Life is full of inadequate information sources. We need to use out intelligence about these things. That's what Engineers do. You can't blame a data sheet if you make a howler about how to bias a transistor.

 

[jim hardy]

Indeed , Sophie

perhaps i should have said OnSemi shows the value unsigned not positive.

If i can find my 1963 GE Transistor Manual i will post whatever they said about the conventions back then (my time).

Bottom line is what you said, one must use common sense and be rigorous when writing Kirchoff voltage equations. I was taught "Walk around the loop and write the first sign you encounter for each voltage as its polarity", and it's mighty easy to flip a sign if one gets the least bit sloppy.

If that transistor has a milliamp of emitter current its EB either is forward biased or is shorted internally.

If I understand you correctly, you are confirming that datasheets are kind of "sloppy" with the sign of VBE?
And that the intention must be deduced from the context?

If not "Sloppy" then "Inconsistent". Probably consistent within each manufacturer's house though.

Always sanity-check .
Standards evolve .
As Lavoisier wrote ' Science is but language well arranged... we make progress when we apply to our language the same precision as we do to our observations.' (approximate recollection of a translation)

Now - a little 'Adieu' to this tempest, and i hope it was a Little Ado about Something !

old jim

 

[Femme_physics]

Again... almost.
One thing left: the sign of Vbe.
Note (from the note master ) that Vbe = Vb - Ve.
What is Vb and what is Ve?
And what is therefore Vbe?

Vbe = -6 volts

Veb = 6 volts

:) Thanks for the rest of the endorsements. I already sent my teacher an email (unreplied yet) and I will have a class with him this sunday so I could approach him on that

Much obliged :)

-FP

 

[I like Serena]

So?
How did it go?

-ILS

 

[Femme_physics]

So?
How did it go?

-ILS

FYI - I wrote the full message 6 hours but my laptop crashed! So I got frusterated and didn't rewrite it.

At any rate, this time I'm backing up everything I write.

So... before yesterday electronics class I printed everything and came eager and ready to confront my teacher. I came early to class and waited...I first off showed it to some classmates who didn't even try to argue with me just nodded along. One classmate of whom I argued with before about this exercise (he's a classmate that always gets a 100 in every test and he did 6+0.7! That's why I was originally surprised that both him and my teacher did the same mistake)...well he was overwhelmed by the amount of work I did he just raised his hands in surrender and had no choice but to agree :P heh. 1-0!

As soon as my teacher arrive it was my cue and I approached him with my "pile of proofs" that included your replies, 2 circuit simulators, 2 university PDF files that confirm that in PNP emitter must be of higher voltage than base. Even before I opened it he said "I might have mistaken"... What! I did all that and you just go ahead and admit defeat!? No way! I opened my case anyway and I showed him that the electronic simulator got the same result as I did, and also the logic behind -VBE = VEB and how I solved the exercise. He then said that yes, we should do 6 minus 0.7... but then I told him that in class he wrote 6+0.7 and argued with me for 20 minutes! He was wondering about it and said that we'll look into that later. Fine. I went back to my sit eager to when he calls me back. He said he's letting us have a glimpse of our tests today before they're scanned into the system. Great. Exactly what I needed.
I once again readied my case of proofs and waited till he called me up to see my test. I was going crazy when he kept calling every other name on the list but mine. I was amongst the last he called. There we go! As I approached my test I saw the grade "90" (out of a 100). Without wasting time I flipped to the page of the infamous question. HAH! I saw a big "X" there and "-10 points"! I looked at my classmate who helped me run the simular and gesticulated the fact he X me!

The hell!

OK! Finally, my golden moment of glory has arrived! With great determination, zeal and intensity I opened my case of proof planning to babystep my teacher through this entire exercise!
But then...something really upsetting happened. He just said "It's ok." changed my "X" to a "V" and added me back those 10 points, and said "you got a 100" and changed my grade to a perfect 100!

But but..but...but my proofs! But the acceptance of your great defeat and my awesome victory! That's not right! I like to dance over the corpses of my enemies! You can't just give me a 100 like that!That's so...so...so... Ah..screw it, I got a 100! :)

When my classmate of whom I argued with got a glimpse at his test I ran over to see what the teacher graded him on the exercise. My teacher gave him a "V" and all the points for the exercise despite the fact he added 6+0.7.
I was like "You're lucky. If you gotten a 100 I'd have screwed u up now" :P He got 95, making some stupid error in another exercise I guess.

I am the only one in class who got a 100.The people who added 0.7 to 6 should lose the points for the exercise IMO... but I'm not a killjoy so whatever. My teacher needs to amend it though. But I won't go to my head department for that...even though it kinda irks me seeing how I got so far to prove him wrong...but I guess I should be nice to my classmates. But whatever. I got my perfect score.

Thanks a lot everyone!

 

[sophiecentaur]

It just goes to show the huge gulf between teachers and practitioners of electronics. No one who is used to actually designing with transistors would need to have given your problem a moment's thought. PNP always meant 'upside down' to me, when I was making my own circuits up - because there were so many more NPN transistors available. So a PNP transistor would flash red white and blue lights at me to be careful. But it's the diodes in there that should give the clue and not the signs on the numbers given in the question.
But, Jeez - you're so combative with your teacher. There really is no need to take all this so personally, you know. You'll worry yourself into an early grave if stuff like that can wind you up so much!
How are you going to cope with some dumb employee who just won't / can't do what you tell him / her to do? You can't hate 'em all!

 

[Studiot]

The school system is there teach one thing.

That life is unfair.
There are good people and there are bad people and plenty of indifferent people. You will meet all types.

Be glad you are not in the English school system that has has centuries to perfect this.

Well maybe that's a bit harsh but you have to get over this and move on. I can certainly sympathise having seen more examples of unfairness than I want to remember.

 

[I like Serena]

I guess the teacher already knows you... keeping you in suspense.

And I agree that it's bad that you didn't get to dance over the corpses of your enemies. :yuck:

But... congrats! A perfect score! You've got a 100 AND you were the only one!

And more... you had the self confidence that you were right and ready to proof it!

 

[sophiecentaur]

The school system is there teach one thing.

That life is unfair.
There are good people and there are bad people and plenty of indifferent people. You will meet all types.

Be glad you are not in the English school system that has has centuries to perfect this.

I think the reverse is the case, actually. The unfairness comes as a total surprise to most of school leavers.
Unfortunately, the School system fails to teach you one thing, these days. That is that you can FAIL, when you get out School.
Because it has been so politicised, School cannot be allowed to let you 'fail'; you just get a grade which is never classified as a pass or as a fail. This has been achieved by a devaluation process, year by year - certainly in Science and Maths. The UK school system says it aims to be 'comprehensive'. It is the last place that you go which even pretends to be so and it is a real shock to kids who leave it and who don't actually make it into a nice life. Not once in the school life of an average kid, are they allowed to fail and to be aware of it. They are taught to be incredibly complacent for all of their school lives and, surprise surprise, the ones who lack self motivation or a good role model outside school, end up in poor circumstances.
Many of the contribtors to PF, who are educated in the US are clearly aware of just how important their "grades" are. This is such a good thing for them. The average UK student, otoh, is more interested in having money from parents or from a pointless part time job (for buying clothes etc) than in their educational progress. No one wants to be a 'boffin'.
In an economic situation like the present, young people leaving education can't bring themselves to do 'just any job' or even 'immigrant jobs' because they haven't been given the work ethic through their schooling. I cannot blame them; they are victims of a misdirected fashion in education that has been with us for nearly thirty years. The fashion was set by a large number of (bright, late developing) Secondary Modern School leavers who, unfortunately, failed to make it to University (Sec Mod was abysmal, of course) but who did get into Teacher Training Colleges. Many of them had enormous chips on their shoulders (quite excusably) and set about changing education - but in a mis-guided buit understandable way. We are left with the result of their valliant efforts - modified by a reactionary and privileged government system which is the worst of all worlds.
No wonder Gove says he wants O Levels back. I do hope the cycle doesn't repeat itself.