transitor: the base pin.

[dannyf]

For example, when a diode was first explained to me it was using a 1 way water valve example. Such an example does not show things like voltage drop or recovery times or any other aspect of a diode.

Models work by simplifying things, to get down to the essence of how the real world works. Unfortunately, what "essence" is varies.

Take your diode example. The one directional (check) valve is a good analogy in description the primarily function of a diode -> one direction flow of current.

But that's an imperfect description of a diode. Thus the amendment of a forward drop voltage.

Unfortunately, that amendment itself is an imperfect description too: that forward drop voltage varies with the forward current, and may require further amendments.

...


The point is that what is right or wrong is a relative concept. Just because a certain simplification doesn't work for a particular case doesn't make it invalid for all cases; and vice versa.

[LvW]

nonsense

You must be very sure to declare my position as "nonsense".
(By the way: Also the universities of Berkeley and Stanford produce such "nonsense"; did you ever read the doctoral thesis from W. Shockley?)

Free-electron, I am not so experienced as you in carrier physics (that is not polemic!).
Therefore, as an engineer I prefer to evaluate such questions from measurable properties of a part or a circuit.
I kindly ask you to answer one single question:

How do you explain the rising input resistance of a common-emitter stage with Re feedback (if compared with Re=0)?

From feedback theory we know that current-controlled VOLTAGE feedback increases the input resistance and that current-controlled CURRENT feedback reduces this resistance. Therefore, is it a voltage or a current acting as a feedback signal ? Does the transistor react upon a change of the voltage Vbe or upon a change of the current Ib (of course, Ib changes also)?
As you know, current feedback needs a common node where two currents are superimposed. Where is such a node?   

[free_electron]

perceived resistance you mean...

There is a difference between analyzing a circuit and analyzing the fundamental of a transistor.
your circuits input impedance may alter but that does not alter anything inside the transistor.

Your common emitter resistor gets the sum of two currents ic and ib. this causes a drop over re. You do the math. It involves incoming bias , Vre and the currents.

Simply tying a resistor to the emitter does not change the physical properties of the structure inside the transistor. It also does not alter the carbon content of the resistor.

[LvW]

Simply tying a resistor to the emitter does not change the physical properties of the structure inside the transistor. It also does not alter the carbon content of the resistor.

I am afraid, I have expressed myself not clear enough - my fault. Nevertheless, thank you for your answer.
Let me explain again:
Applying negative feedback to an opamp, of course, also does not change the physical properties of the active unit, but it changes the overall input resistance of the whole amplifier circuit. And that`s what I mean:

You can presume I am aware that Re feedback does not change something inside the BJT.  But the input resistance at the base node is modified due to feedback.

And THAT was my question: Only voltage feedback increases the input resistance at the base node - and exactly this can be observed and measured as a consequence of Re feedback.  If currents are the relevant quantities to control the transistor the input resistance at the base must go down!     
Something wrong?

[AG6QR]

The "voltage control" versus "current control" can be simplified by considering a diode, or even a resistor.  Is a resistor "current controlled" or "voltage controlled"?

Does voltage across a resistor cause current to flow?  Or does current through a resistor cause a voltage to be developed?

Ohms law tells us the relationship between voltage and current, but it doesn't tell us which one is cause and which one is effect.  Often it's convenient to think of a resistor as voltage controlled (think of a resistance heater, where you apply a fixed voltage from a power supply and you can calculate how much current will flow), but other times, it's convenient to think of a resistor as a current controlled device (consider a shunt resistor, where you apply a current and a proportional voltage will be developed which you can measure).  Most of our power sources deliver more-or-less constant voltages, so we usually lean toward thinking that voltage causes current, but in a world dominated by current sources, our thinking might be different.  Would that kind of thinking be demonstrably wrong?

With a diode, the function relating voltage and current is a bit more complex than a resistor's, and not a straight line, but still, the fact that there's a well-defined function relating voltage versus current doesn't tell us which is cause and which is effect.

In my book, it's wrong to insist that the causality can only go one way.  There are two perspectives, and both can be useful.  Neither is wrong, but neither one, by itself, is the Only One True Right Way of looking at things.

For those who would continue to argue it's only one or the other, let me put it another way.  Suppose I gave you two diodes:  One was voltage controlled, and the other was current controlled.  Both had the same I-V curves.  What experiment would you do that would unambiguously distinguish between them?

Granted, if you insist that diodes are ALWAYS one way or the other, you might have to say that one of those diodes came from an alternate physics universe, but my point is, what measureable characteristic would distinguish between those two universes?  Or in other words, how do you know that our universe is the one where the control parameter is the one you say it is?

[IanB]

The best approach to this question (in all engineering fields, not just electronics) is what happens when you consider models of how a device operates.

Any given model has operating variables and model parameters that go into the equations relating those variables. The best models are ones where the model parameters are nearly constant over the whole possible range of model variables. If you have such a model, then with values of those parameters you can make design calculations and operating predictions with considerable confidence.

If your model parameters are not constant, then they themselves become variables and your ability to use this model to make design predictions becomes weaker.

For instance, "the forward voltage drop of a diode is 0.6 V". In this model the "0.6 V" part is a parameter. It's a good rule of thumb, except the 0.6 V is not at all constant. With higher currents it could be 0.8 V or 1 V or more. If your circuit design has high currents flowing through  the diode you are going to get a poor design with this model.

There are more precise models of the diode forward voltage that account for varying currents (at the simplest, the graph in the datasheet), and with these models your design can handle large currents.

Same with a transistor. The device model Ic = gain x Ib suffers from the fact that the gain is not a constant parameter. You cannot use this model successfully to design circuits in cases where the gain matters unless you introduce a new model for the gain as a function of other things. And those other things may include voltages.

[dannyf]

Something wrong?

Lots.

I pointed out this to you in your old thread but apparently you still couldn't understood it. Forget about current- or voltage-controlled bjts, just put in a theoretical model of a three terminal device, either voltage or current controlled, and you can work out the math -> you will see that the input impedance of the said circuit is comparable between the two devices. IE, the observed behavior of the circuit reveals nothing about the physical nature of the device in discussion. Thus, any attempt at answering your question is irrelevant to the discussion.

[dannyf]

considering a diode, or even a resistor.

I am not sure if that comparison is valid for this discussion.

Ohm's law certainly doesn't apply to a diode. And in the case of a two terminal device, there isn't anything to control or to be controlled.

I think if you look at the physical mechanism for a transistor, you will have to agree that the recombination of carriers (ie current) controlls the collector current in a bjt. ie., it is current controlled. and many models for bjt devices indeed follow that description.

[free_electron]

Simply tying a resistor to the emitter does not change the physical properties of the structure inside the transistor. It also does not alter the carbon content of the resistor.

I am afraid, I have expressed myself not clear enough - my fault. Nevertheless, thank you for your answer.
Let me explain again:
Applying negative feedback to an opamp, of course, also does not change the physical properties of the active unit, but it changes the overall input resistance of the whole amplifier circuit. And that`s what I mean:

You can presume I am aware that Re feedback does not change something inside the BJT.  But the input resistance at the base node is modified due to feedback.

And THAT was my question: Only voltage feedback increases the input resistance at the base node - and exactly this can be observed and measured as a consequence of Re feedback.  If currents are the relevant quantities to control the transistor the input resistance at the base must go down!     
Something wrong?

still nonsense. you are again describing the behavior of a system. what happens inside the transistor does not change.
sure, your system reacts to voltage , the transistor inside still reacts to the flow of the electrons. the physical mechanism does not change. you need to insert electrons in the emitter to remove the recombination zone. the electrons flow into the emitter and out the base. put an ampere meter there and you will measure that current.
electrons per second is ampere. ergo : bipolar transistors are current driven. pump up the base emitter current and the collector emittor current goes up . the relation between the two is the beta.

it doesn't get more basic than this. stick an ampere meter in the base path. if you get a reading : there is current flowing.

do the same with a mosfet. electrons will flow into the gate and once the gate is fully charged the current flow stops. ergo : gates on a mosfet are capacitors. once fully charged that's it. so in transient a gate of a mosfet is current driven , but statically the mosfet is voltage driven.

you can sing and dance all you want : that is the nature of the beast.

[PlagueDoctor]

Wow! This thread kinda blew up :p

This is not my official reply yet, I just wanted to let you guy's know that I've read all of your comments.
Tandy pretty much spot on replied exactly as I would have replied.

With the exception of 'flame wars can be confusing'
I actually find them quite helpful, even though some of it's is confusing. but that's why we have google/books

unfortunately I've got to go to work. and before I give a real reply, I want to test out on some breadboard what I think I have learned from you guy's.
my problem was, up until now I've pretty much used transistors as JFET's, so that's how I though about them. and since all my circuits have been so low power with wide tolerance components, it hasn't been much of a problem. But I'm really glad I asked, since some of the stuff I was thought early on via word of mouth seems to have been way off base.

so yeah, I'll get back to you guys when I'm done work, and after I've googled some of the stuff you guys are saying, and experimented on some breadboard. I think I'll also make a little animation to visualize it, just so you guys can check to see if I have it right

Thank you guys so much!

[Simon]

nonsense

You must be very sure to declare my position as "nonsense".
(By the way: Also the universities of Berkeley and Stanford produce such "nonsense"; did you ever read the doctoral thesis from W. Shockley?)

Free-electron, I am not so experienced as you in carrier physics (that is not polemic!).
Therefore, as an engineer I prefer to evaluate such questions from measurable properties of a part or a circuit.
I kindly ask you to answer one single question:

How do you explain the rising input resistance of a common-emitter stage with Re feedback (if compared with Re=0)?

From feedback theory we know that current-controlled VOLTAGE feedback increases the input resistance and that current-controlled CURRENT feedback reduces this resistance. Therefore, is it a voltage or a current acting as a feedback signal ? Does the transistor react upon a change of the voltage Vbe or upon a change of the current Ib (of course, Ib changes also)?
As you know, current feedback needs a common node where two currents are superimposed. Where is such a node?   

I hate to rain on you but YOU have made this into a more complicated question that need be and we seem to be having a mud slinging match over use of terminology. Granted i have not be practicing for decades and i have no peices of paper to show but i have read enough BJT explanations to become fairly bored and I think we are confusing here the actual functionality of a BJT with how it is MADE to operate in a circuit.

The BE junction is a diode and as a diode and LED has a certain voltage to current relationship. I don't know about this particular diode but LED's drop their voltage requirements ("resistance") when they heat up and if a transistor does the same you have a piss poor controllable device. Sure you can feed less than the junction voltage and control it but show me the millions of applications where this is done, I think we are confusing properties of BJT's with properties of circuits containing them.

I'm sure the OP is extremely confused now as he/she never asked for such level of physics that take us to splitting the aton if that is really what your on about.

[Simon]

my problem was, up until now I've pretty much used transistors as JFET's, so that's how I though about them. and since all my circuits have been so low power with wide tolerance components, it hasn't been much of a problem. But I'm really glad I asked, since some of the stuff I was thought early on via word of mouth seems to have been way off base.

So are you:
1)working precisely with the few mV of range that get this sort of operation (which implies your much more knowledgeable than you make out)
2) been blowing BJT's up, surely not ?
3)putting a resistor in series and not realizing that your using ohms law to convert your so called voltage input into a current input ?

I'll go with 3!

[PlagueDoctor]

So are you:
1)working precisely with the few mV of range that get this sort of operation (which implies your much more knowledgeable than you make out)
2) been blowing BJT's up, surely not ?
3)putting a resistor in series and not realizing that your using ohms law to convert your so called voltage input into a current input ?

I'll go with 3!

none of the above really.
1: although I do like to measure to mV I will not claim to posses suck knowledge. but this is the type of operation I've been getting (I work in frequency ranges of ~ 1hz or slower normally, since I don't have an oscilloscope, and tend to use a diode to test the logic of my circuits, and need to be able to see the data with my naked eye.

2) I have been using transistors, I check the data sheets every time before I use any component to make sure I can supply the required voltage/current without blowing anything up. and looking at the symbol/name is the first thing I check, to make sure I didn't mix up my parts :p

3)no :p when I do that I do it purposefully for dividing voltage and such (I'm quite familiar with ohm's law)

I've essentially been using existing circuits to make different logic gates that I didn't have on hand(like those seen here http://electronics.stackexchange.com/questions/72187/resistor-values-in-transistor-logic-gates) , or to make H bridges to drive small hobby motors, with some small changes here & there where needed. and assembling them like Lego blocks.

so far that has worked for me. though I won't deny I've had my share of problems, and I have no problems admitting that has to do with my lack of understanding, but I've always managed to get my circuits to do what I wanted them to do at the end of the day with enough tinkering and Googleing.

anyways, I'm cutting this close, I got to go to work.. like.. now :p

[jlmoon]

It is the the voltage Vbe that causes a current Ie out of resp. into the emiitter  - and this current is split into one large current Ic and a much smaller current Ib (which is a more or less fixed percentage of Ic). Therefore, the current Ib of course goes into (or out of) the emitter node according to KCL: Ie=Ib+Ic.

nonsense

It is an electrical field that causes electrons to flow into the emitter and out of the base ( for an NPN transistor , using electron model , not conventional model ).
in order for electrons to start moving you need to breakdown the recombination zone (often wrongly called the depletion zone. A depletion zone is an area that doesnt have any free electrons. (note : do not confuse this with depletion in depletion mode mosfets. that is classic model , not electron model !)) between the N and P material. The field required for that is in the order of 0.6 to 0.7 volts. once the recombination zone is gone current flows. Vbe is the field across this band. the bipolar transistor is NOT voltage controller. the relation is Ic = Ib x Beta. Pump more current in the base and you let more current flow in the collector.
Simple as that. Bipolar transistors are current controlled.

i gave a detailed electron model explanation including diagrams already on the forum . search for it.

trust me, i've spent 23 years of my life taming electrons to run where i want them to run. i got a pretty good grasp on that stuff.

Very good explanation, I was wondering if any one was going to explain what really goes on with that depletion zone ...Opps recombination zone.    Great job Free!

[jlmoon]

I've always been to embarrassed to ask this question, since I've designed & built several circuits, beambots, etc using transistors, even helped other people out... Any ways.

(Assuming NPN type) when applying a current to the base pin to allow the current to flow from the collector to the emmiter(or vice versa) where does the current from the base pin go?

I've always assumed either one of two things:

The current of the base pin shares the ground with the emmiter pin. (Which doesn't make sense to me)

Or the current doesn't actually flow to the base pin, but rather puts it under an electrical tension. (Which makes a lot more sense to me)

Or am I way off base? Surprisingly, despite having read quite a few articles about them. Even going as far to learn how they are manufactured. I haven't crossed any text that explains this clearly.

Thank you for your time"

Plague,
I have a couple of great books I suggest you reference if you have the time as well. 

1st.  Chapters 1 - 4 , Fundamentals of Electronic Devices, David A. Bell;  ISBN 0-87909-276-9
I don't know the current version in print, but this is one of many basic books that shed some light on the subject.

2nd.  Highly recommend this one....  Mmodeling The Bipolar Tranisistor, Ian Getreu... ISBN unknown
http://stores.lulu.com/iangetreu
If you can survive a little complex math, this is an invaluable source of information well worth the reading.

Hope this helps,

jlmoon

[amyk]

BJTs are so mundane that everyone feels they are qualified enough to talk about how they work.

[mikerj]

How do you explain the rising input resistance of a common-emitter stage with Re feedback (if compared with Re=0)?

From feedback theory we know that current-controlled VOLTAGE feedback increases the input resistance and that current-controlled CURRENT feedback reduces this resistance. Therefore, is it a voltage or a current acting as a feedback signal ? Does the transistor react upon a change of the voltage Vbe or upon a change of the current Ib (of course, Ib changes also)?
As you know, current feedback needs a common node where two currents are superimposed. Where is such a node?   

A bipolar transistor can quite legitimately be viewed as either current controlled or as a transconductance device.  Both models have advantages in different situations, and to suggest that one or the other is somehow incorrect is ridiculous and undoubtedly confusing to beginners on the forum who are trying to learn.

[LvW]

still nonsense.

Thank you. (Shouldn`t we try to treat each other with some respect?).
 

... put an ampere meter there and you will measure that current.
...ergo bipolar transistors are current driven. pump up the base emitter current and the collector emittor current goes up .
it doesn't get more basic than this. stick an ampere meter in the base path. if you get a reading : there is current flowing.

I am afraid you have overlooked that I never didn`t deny the existence of a base current Ib. Voltage control is not equivalent to assuming Ib=0.
By the way, are universities (Stanford, Berkeley,..) - in your view - also producing  "nonsense" regarding this question? And what about the design-oriented "Art of Electronics"?
Perhaps you should expand your horizon a bit (excuse me, but this is my answer to "nonsense")? 
______________________________________

Gentlemen - I am afraid, there is a fundamental misunderstanding leading to a discussion style, that - unfortunately - in some parts is unsatisfying  and disappointing. Shouldn´t it be possible to exchange technical arguments without being polemic? 

Let me explain:

I think, some of you didn`t discriminate in their answers between the two activities: (a) Designing a BJT stage and (b) Understanding the working principle of the BJT.

For designing an amplifier stage (i.e. common emitter) we all use the same design principles and the same equations.
There is absolutely no difference - whether we rely on current-control or on voltage control.

And - of course - for designing the resistive voltage divider for base biasing I take the base current into account (Ib=Ic/beta).
But this has nothing to do with the principle how the BJT`s collector current is controlled.

And - as far as the "current-control party" is concerned - don`t they use the quantity Vbe=0.65...0.7 volts in their calculation?
And, of course, they again are using other Vbe values for clas-A/B or class-B operation.  Why? Why do they not start with Ib?
Do you know the meaning of the Temp-Co of the base-emitter voltage (-2mV/K) to keep the Ic value constant?
Is there any similar figure which describes how beta=Ic/Ib depends on temperature?

In short: Everybody is using the same equations - the whole design process is completely independent on the question "current or voltage-controlled".
However, this does NOT mean that we have two models which explain the working principle of the BJT. Of course, as for all other electronic parts, there can be only one single explanation.
And - in response - to one reply: This is not a chicken-egg (voltage-current) problem. Of course, it is always a potential difference (voltage) which drives a current.
No current without voltage.
Of course, for designing circuits we make use of the current source concept (it is a concept! only) but we never should forget if we are using a "concept" or a "model", that it not necessarily reflects the physical reality. That`s good engineering practice.

I hope, this helps to clarify things.
Thank you.
LvW

[dannyf]

there is a fundamental misunderstanding

Yes, exclusively on your part.

Why do they not start with Ib?

It has been explained to you then, multiple times, and it has been explained to you here, multiple times, whether a bjt is current or voltage controlled has no bearing how a circuit around such a device is designed.

Do you know the meaning of the Temp-Co of the base-emitter voltage (-2mV/K) to keep the Ic value constant?
Is there any similar figure which describes how beta=Ic/Ib depends on temperature?

None of that has any bearing on this discussion.

You may want to remember next time that quoting irrelevant facts or terminologies serve no purpose in a discussion other than to undermine your own credibility, especially when such quotes showed your complete misunderstanding of the quoted items, or your "facts" are erroneous.

Hope it helps.

[wazzokk]

Nobody knows at the deepest level how physics really works, all we have is various models of how materials behave.
What matters is which model is appropriate to our requirements.

Dave

[atferrari]

The question in the OP:

(Assuming NPN type) when applying a current to the base pin to allow the current to flow from the collector to the emmiter(or vice versa) where does the current from the base pin go?

was answered in post #2.

The base current flows from the base and out of the emitter.

Basically, the OP (also known in some forums as the TS - thread starter), got the answer.

Done. 

[Zero999]

This is not a chicken-egg (voltage-current) problem. Of course, it is always a potential difference (voltage) which drives a current.

Not true. If the resistance is zero, as in a super conductor, current flows without any potential difference.

[Simon]

And - as far as the "current-control party" is concerned - don`t they use the quantity Vbe=0.65...0.7 volts in their calculation?
And, of course, they again are using other Vbe values for clas-A/B or class-B operation.  Why? Why do they not start with Ib?
Do you know the meaning of the Temp-Co of the base-emitter voltage (-2mV/K) to keep the Ic value constant?
Is there any similar figure which describes how beta=Ic/Ib depends on temperature?

I hope, this helps to clarify things.
Thank you.
LvW

It clarifies nothing other than your stubbornness! in an ideal world Vbe would be 0V but it's not an ideal world so we have to take into account that there needs to be 0.7V added just to make the thing work. If you want to discuss particle physics go and join a forum that specializes in that!

[c4757p]

in an ideal world Vbe would be 0V but it's not an ideal world so we have to take into account that there needs to be 0.7V added just to make the thing work. If you want to discuss particle physics go and join a forum that specializes in that!

wtf...

[T3sl4co1l]

My response to the subject:

Transistor: The Base Pin
Spaceballs: The Transistor Base