How Transistor works - verifying

[cabraham]

Sorry, I guess I wasn't clear enough. I think I'm close to our central disagreement. This is about clearly explaining electrical physics to the general public... and the concept that voltage causes current. But before examining silicon or even conductors, first the simplest basic situation:

E-fields cause charged objects to experience a force, F = q*E, but the reverse is not true: the force on that object is not the cause of the e-field.

Place an electron between the plates of a charged capacitor in vacuum, and the e-field between the plates will produce a force on the electron. But the force on the electron is not the cause of the e-field between the plates. In other words, in the equation F = q*E, the q*E causes the F, but the F does not cause the q*E.

Agreed?

If you find errors or unconventional concepts in the above, or see something I missed, please point it out.

Here is the crux of the issue "and the concept that voltage causes current." You're just assuming that that is the case & as proof you offer this:

"E-fields cause charged objects to experience a force, F = q*E, but the reverse is not true: the force on that object is not the cause of the e-field."

The E field came into existence by separating charges, moving them to do so. That is current, displacement current to be exact. In order for an E field to move a charge creating a current, a current was needed to create the E field. See what I mean by circular reasoning.

Bill if there is one thing I'm trying to convince you of it is this.

Voltage is [b/not the cause[/b] of current, & vice-versa. Charges move because of proximity to other charges. One of the most, if not the most, basic axioms of electrical science is Coulomb's force law:

F = q1*q2 / (4*pi*epsilon*r^2).

From that force law, the E field is derived, then the scalar potential is derived, which we call "voltage".

Regarding the charged plates, I beg to differ. The force exerted on the free electron does influence q*E. The E field decreases due to the force exerted on the charge. Said force times the free electron displacement (dot product of the 2 vectors) equals the energy change of the free electron. This change in energy is equal to that of the E field change in energy.

The force on the electron dot producted with its displacement affects the E field. Why would said force NOT influence q*E? The conservation of energy dictates that F influences q*E. The electron gained energy (or lost it depending on polarity). If F did not affect q*E, how did the change in energy occur?

Picture 2 plates charged to a voltage V, w/ capacitance C. Of course Q = C*V. An electron is placed in between the plates & released, & it moves towards the positive plate. Said electron with its negative charge now adds to the positive charge already on the plate resulting in a decrease in the cap voltage.

I & V are so interactive & inter-related, it is impossible to say that one causes the other. In order for that cap to acquire its charge, current had to exist in order to separate the charges, +ve from -ve. Is this too hard to understand?

Remember that the charged particle being moved by said E field has its own E field as well. Just as the charges on the plates exert a force on the electron, so does the electron exert a force on those plate charges. It is mutual & inclusive. But the mass of the plate & its charges is too great compared to the electron & the plate movement is too small to observe. But the energy change can be measured.

In order for an E field to sustain a current, the E field charge & energy must continuously be replenished. A battery across a resistive heater is a good example. The E field across the battery terminals can move electrons through the wires & resistor. But these electrons reduce the charge on the positive battery terminal, likewise for the negative terminal. The E field starts to decrease immediately when current is drawn. But the chemical reaction in the battery provides energy so that electrons can move against the terminal E field & replenish the spent energy. These electrons moving towards the terminals are a current which creates & replenishes this E field.

This current is not motivated by the E field because the polarity is opposite. Inside the battery electrons are moving against the E field, but outside they move with the E field. It is the chemical conversion of energy which is driving both the current & the voltage.

I cannot understand why this is so hard for some to grasp. The notion that V causes I is so out there that any level of scrutiny can refute it thoroughly. It is utter nonsense.

The cause of electrical phenomena is energy conversion, chemical to electric (battery), mechanical to electric (generator), electric to mechanical (motor), optical to electric (photodetector), electric to optical (LED), etc. In the process I & V participate, but neither is the cause of the other nor the effect.

I will be glad to clarify. I only ask that when presenting theories that are not supported by established science, please present valid reasons why, do not just assume you can dictate how things really are. Again, I only want to show why the canon says what it says. There are darn good reasons why things, bjt or others, are defined a certain way. The fact that one can present an argument as to why definitions should be changed does not do it because another can present a good or better counter-argument.

Claude

 

[wbeaty]

Here is the crux of the issue "and the concept that voltage causes current." You're just assuming that that is the case & as proof you offer this:

"E-fields cause charged objects to experience a force, F = q*E, but the reverse is not true: the force on that object is not the cause of the e-field."

The E field came into existence by separating charges, moving them to do so.

No, e-fields don't inherently require currents. This seems to be our sticking point.

In reality, any charged particle is surrounded by an e-field. A single electron is surrounded by a radial e-field. This is a fundamental element of classical EM physics.

Do you disagree?

 

[cabraham]

No, e-fields don't inherently require currents. This seems to be our sticking point.

In reality, any charged particle is surrounded by an e-field. A single electron is surrounded by a radial e-field. This is a fundamental element of classical EM physics.

Do you disagree?

Bill. you are intentionally making trouble by ignoring my info? When a distribution of charges & their associated E fields, like the charges on the plate of a cap, attract a free charge, exerting force 0n it, some energy is gained by said free charge.

The charge moves towrds the plate having a polarity opposite to its own. The E field on said plate is decreased due to the opposite charge now added. The field & associated voltage decrease. In order to maintain a fixed charge/voltage, the cap plates are connected across a battery for instance.

Every time a plate captures a charge, the battery replaces said charge w/ one of the same polarity as the plate, opposite that of the newly acquired charge. Hence when E fields act upon free charges moving them, they give up energy requiring replenishment current to maintain a fixed value of E. Otherweise COE is violated.

Bill, how can you remotely believe that I don't know that "a single charged particle has an E field & that an electron has a radial E field"? Do you think you're the only person that went to college & the rest of us didn't? If voltage was the cause of current, & the proof of this concept was as simple as undergrad field theory, then it would be universally known & taught everywhere. The fact that it isn't should tell us that in order to know more about "what caused current or voltage or E/H fields", we need more info.

Should we discover a new sub-atomic particle that can be proven to give rise to E/H fields, then the next question follows "what gives this new particle its properties?" Causes are a never ending riddle. If A causes B, then what causes A? Oh that's easy, C causes A. Then what causes C?!

Claude

 

[wbeaty]

Bill. you are intentionally making trouble by ignoring my info?

No, I'm drilling down to the cause of our disagreement. Not transistors. Not diodes. Not capacitors. Not current causing voltage.

 

[cabraham]

No, I'm drilling down to the cause of our disagreement. Not transistors. Not diodes. Not capacitors. Not current causing voltage.

I've stated the reason for our disagreement in plain scientific terms. Here it is. Current & voltage are inter-related, sometimes one can give rise to the other & vice-versa. There is no pecking order, in general neither is the cause nor the effect of the other. My examples support that.

Charges move when in the proximity of other charges. F = q*E is a 2 way street. The particle feeling the force from the E field, also imparts its own force of attraction/repulsion on said E field. In addition for every joule of energy gained by said particle, that same amount is subtracted from the source E field. The E field exerts a force & the particle receiving the force exerts its force & alters the source E field.

There is no I before V nor is there V before I. They both interact & participate. A prime example is a switching power converter, take a buck regulator. THe inductor stores energy, then when the switch turns off it releases energy in the form of a current per W = L*I^2/2. If a current sense resistor is in series with the inductor & load, its voltage is determined by I of the inductor, per V = I*R, per Ohm.

The I is independent, V is dependent on I & R. But the load resistor has an output filter cap in parallel. If there is 10 mV of ac ripple voltage on the cap, then the ac ripple current in the load is 10 mV divided by the load resistance. Here, I depends on V & R.

Examples abound. Ohm's law is bidirectional. A current can determine a voltage or vice-versa. Elaboration can be provided if needed. BR.

Claude

 

[wbeaty]

In general neither is the cause nor the effect of the other.

Agreed.

There is no pecking order

I disagree.

Yes, in general current does not require a voltage, and voltage does not require a current. For example, persistent currents in superconductors demonstrate currents at zero voltage. And the e-field surrounding an electron? That's existence proof of a voltage at zero current.

But here's a simpler case, offered in the goal of drilling down to the root of the disagreement.

Take a positive and a negative charged particle, and a distance separating them.

In Classical physics, in EM for beginners, we say that the force on one particle was caused by the e-field from the other. Newtonian "distant action" is an obsolete concept, instead we follow Maxwell and state that EM force upon a charge is caused by the local field experienced by that charge. It's not bi-directional, since the force upon that charge is not creating the pattern of local e-field it experiences.

With two opposite distant electric charges, the e-field causes the force, but the force doesn't cause the e-fields.
​

Yes, obviously there are more complicated situations where things aren't nearly as clear.

I hope we can agree on all of the above, since it's classical EM, straight out of beginner's textbooks.

Emphasis on Beginners. Photon exchange and gauge theory are inappropriate; we're not after ultimate truth, we're crafting a "beginner's explanation" intended for consumption by the general public and 11yr-old children familiar with electrostatic attraction, magnetic fields, etc. (And aimed at same audience reading this thread in the future.)

 

[Studiot]

In Classical physics, in EM for beginners, we say that the force on one particle was caused by the e-field from the other. Newtonian "distant action" is an obsolete concept, instead we follow Maxwell and state that EM force upon a charge is caused by the local field experienced by that charge. It's not bi-directional, since the force upon that charge is not creating the pattern of local e-field it experiences.

I must disagree with you here.

Even the Yankee textbooks I have say that it is convenient to act as though the 'test' charge experinces a field due to the other, but in reality both charges are required to establish the effect.

It takes two to tango.

 

[sophiecentaur]

So, how can we explain BJTs to children? To design an explanation, first describe the basic BJT operation verbally:

1. Base current controls the BE junction voltage
2. BE junction voltage determines height of BE potential-barrier
3. That potential-barrier sets the rate of charges crossing the BE junction
4. Most carriers from the emitter make it all the way to the collector
5. Ic approximately equals Ie

No heresies so far? :)

"children"??
If you really mean children then, apart from saying that a transistor acts a bit like a variable resistor with its value controlled by the third connection, what would be the point of anything more? If you're using terms like "potential barrier" then what sort of children would you expect to be familiar with them?

Frankly, I think that just describing a common base amplifier, and how the signal becomes inverted, would be way too hard for most kids of secondary age (except a few enthusiasts who already have some experience of making up circuits). They may well be impressed with seeing an amplifier actually operate but, as for 'understanding', I doubt it.

What are the circumstances in which you are hoping to present these ideas?

I remember my Dad, in a one-to-one situation, explaining successfully (as I remember) how a thermionic triode works when I was about 11 yrs old. The triode is a much simpler device to describe and, being called a 'valve' it even sounded a bit like a tap.

 

[cabraham]

Agreed.



I disagree.

Yes, in general current does not require a voltage, and voltage does not require a current. For example, persistent currents in superconductors demonstrate currents at zero voltage. And the e-field surrounding an electron? That's existence proof of a voltage at zero current.

But here's a simpler case, offered in the goal of drilling down to the root of the disagreement.

Take a positive and a negative charged particle, and a distance separating them.

In Classical physics, in EM for beginners, we say that the force on one particle was caused by the e-field from the other. Newtonian "distant action" is an obsolete concept, instead we follow Maxwell and state that EM force upon a charge is caused by the local field experienced by that charge. It's not bi-directional, since the force upon that charge is not creating the pattern of local e-field it experiences.

With two opposite distant electric charges, the e-field causes the force, but the force doesn't cause the e-fields.
​

Yes, obviously there are more complicated situations where things aren't nearly as clear.

I hope we can agree on all of the above, since it's classical EM, straight out of beginner's textbooks.

Emphasis on Beginners. Photon exchange and gauge theory are inappropriate; we're not after ultimate truth, we're crafting a "beginner's explanation" intended for consumption by the general public and 11yr-old children familiar with electrostatic attraction, magnetic fields, etc. (And aimed at same audience reading this thread in the future.)

Ok so I have your agreement that voltage does not cause current, nor does current cause voltage? Agreed? Now you're stating that E field causes force, but force does not cause E field? Is that your point. First & foremost, nobody suggested causality here. I agree that force does not cause E, but I also hold that E does not cause F, merely that "E fields" are synonomous with "force fields".

Of course action at a distance is obsolete. Two charges exert mutual forces upon each other. That is bilateral. But if one charge is perturbed, the perturbed force felt by the other charge is not instantaneous, but delayed. So the field concept was developed. A charge is surrounded by a field with finite propagation velocity. The E field, or force field if you prefer, takes time to reach the charge it will act on. Ditto for the 2nd charge.

When the E field arrives, the 2nd charge incurs said force. Did E cause F? Well, that is pure semantics, but E is developed with the intent to describe charge interaction accounting for time delays. The force field takes a little time to reach the 2nd charge & the motion of 2nd charge is in accordance with this deleayed force field, or E field.

Also, the 2nd charge does act bilaterally w/ the 1st. E1 influences Q2 via force & E2 influences Q1 likewisde. My earlier point was that when an field imparts force & energy to a free charge, it is bilateral & mutual interaction. The force exerted on the free charge by the source field, integrated along the path of the displaced charge, equals the energy imparted to said free charge.

This energy is exactly equal to the decrease in the source E field energy. Also, the free charge E field alters the E field of the source. It is bilateral. Your whole crux is that E causes F, but not vice-versa, a rather trivial point. E is defined as F/q. Before E can be defined, we measured F as k*q1*q2/r^2. We then observed a time delay. We then discarded action at a distance & adopted field & finite propagation speed.

So when a battery powers a resistive heater. the charges constituting the current are moving because? If you say they move due to the battery's E field, well we have a problem. Let's use electron flow instead of positive convention. Electrons enter the battery +ve terminal & exit its -ve terminal. The electrons are moving outside the battery in accordance with the E field of the battery. They are flowing "downhill".

But inside the battery, electrons are flowing "uphill". They flow against the battery E field. THis uphill flow is needed to replenish the E field. Otherwise electrons entering the +ve terminal would cancel the +ve ions, & electrons leaving the negative terminal would reduce the -ve charge. As a result the E field decreases as current exists in the form of electron flow. In a very short time the current ceases. The E field is spent immediately.

But the chemical reaction inside the battery imparts energy to electrons & ions propelling them against the E field. This replenishes the E fields lost energy due to electron flow.

A similar case exists with ac generators. E fields are not what keeps charges moving, but rathert energy conversion. Have I explained this well?

Claude

 

[sophiecentaur]

That's fair enough. The field Potential as you go around the circuit is distributed amongst all the dissipating components and, at a small scale (piecewise for each section of the circuit), there must be field (volts per meter) which is keeping the charges flowing the right way from atom to atom. But there is no significant field 'along the wires' because there's no PD. All the field is where the energy is transferred. The field is not 'across the battery terminals', as people seem to imagine, at least whilst there is a complete circuit.

 

[Dmytry]

"children"??
If you really mean children then, apart from saying that a transistor acts a bit like a variable resistor with its value controlled by the third connection, what would be the point of anything more? If you're using terms like "potential barrier" then what sort of children would you expect to be familiar with them?

Frankly, I think that just describing a common base amplifier, and how the signal becomes inverted, would be way too hard for most kids of secondary age (except a few enthusiasts who already have some experience of making up circuits). They may well be impressed with seeing an amplifier actually operate but, as for 'understanding', I doubt it.

What are the circumstances in which you are hoping to present these ideas?

I remember my Dad, in a one-to-one situation, explaining successfully (as I remember) how a thermionic triode works when I was about 11 yrs old. The triode is a much simpler device to describe and, being called a 'valve' it even sounded a bit like a tap.

I recall reading how BJT works in an old book when i was about that age. It was explained that it would normally not pass any current, as two diodes would not, but as the base is so thin, injection of other type of charge carriers into the base by making one of diodes conduct current makes the second diode leak.
It previously explained how diode works with charges being pushed away from midline, and had a drawing of transistor with charges.
Not a very accurate explanation I know, but it made sense to me back then, and is not grossly invalid.

 

[sophiecentaur]

Ummm.
You may feel that such an explanation worked for you but my opinion is that there are many ideas in electronics which are more appropriate and timely for a student to get straight than leaping in with an incomplete model of a very complex device. Let's face it, if they aren't familiar with all the basics then what use is it to have an arm waving model of an object that they may, in fact, never see in their lives as a discrete circuit element?

 

[Dmytry]

Well, I seen it as a real circuit element.

Also, that model is not so bad. It is a common explanation that minority carriers in the base region are responsible for conduction. Of course it does breed misconception with two diodes for some people but I don't recall ever thinking that I can connect two diodes and get this because I knew exactly how 2 diodes work (having learned about rectifier) and the book stressed the bit about carrier injection into thin base. edit: Really, the only difference it had vs the stuff you can hear taught to university students is that this old book didn't define a bunch of terms like "minority carriers" but instead used descriptions of what those are in the text. Which was probably because it was in russian and its easier to be descriptive in this case.

 

[sophiecentaur]

Yes it is a real circuit element and the model only 'creaks' a bit but there aren't many three legged devices seen these days (except for regulators, perhaps). Then again, there are other technologies than the junction transistor.

But in any case, I asked what was meant by "children". Whilst there may be one or two 'nerdy' (no offence) lads in school, teaching a class full of kids of (any) school age about semiconductors in circuits would be way off syllabus and probably a big yawn for them. I say that as a one-time boy-home-constructor. But that was in the 60s. In the last 20 years of teaching, I only came across one student who showed that sort of interest within School and that was only because someone had given him an ancient home constructor kit. He only wanted to do electronics at the 'system level', in any case.

Things have changed recently, at least, in my experience.

 

[Dmytry]

Well the point of education is twofold: teach everyone to read and write and do very basic math, and get started those who will become professionals in complicated fields (who will be few). It would seem to me that west often neglects the second goal nowadays or sacrifices it for the first. Furthermore certain cultural attitudes seem to make it so that only social outcasts become sufficiently invested in technical topics nowadays in the developed countries.
Which is of course helpful for me because otherwise I'd have real trouble competing with the people raised in the western countries who had far more opportunities than I did, and would perhaps have to underbid them instead of being able to set my own price.
I don't really know what you would need to change about the attitudes though. It seems to you self evident that it is important to entertain the majority of students if only minority can benefit from a boring course. It doesn't seem self evident to me. Sure, all the topics beyond basic literacy and numeracy are going to have incredibly small yield, but that small yield is all the highly qualified non-foreign professionals that your country will get.

 

[sophiecentaur]

You are right, in many ways, about the purposes of education. I would say that education is also supposed to 'cultivate' where possible; this is at least as important as teaching basic skills and preparation for a job.
You say "It seems to you self evident that it is important to entertain the majority of students if only minority can benefit from a boring course." but that is not actually true. In the UK we have a National Curriculum which has to be delivered and teachers are answerable if students, their parents, Schools and inspectors are not satisfied with their subsequent exam performance. Of course, many students under-perform because the courses are hopelessly mis-matched to ability and culture for many students. But one has to cover one's arse by making sure that as much of the course content is delivered. If I had been allowed to take classes and deliver material and experiences to them that I deemed 'appropriately interesting' for them, then they would have been a lot less bored and more receptive. That would have been at the expense of following the dreaded Scheme of Work. Teachers who do that will often find themselves being disciplined and told that it is just not their choice.

Our Great Comprehensive Education System is aimed to provide every child with an equal opportunity to do their best in life. That's Motherhood and Apple Pie; one can't argue with it. EXCEPT that it is assumed (and frequently stated) that anyone can do anything if they just get the right education. The National Curriculum is a total One Size Fits All, despite the politicians claiming that it is precisely Not that. Everyone is judged against the standards of everyone else and, thus, half of them will fail to achieve Median Performance (by definition).
Whatever we claim to do in our Schools, LIFE is not comprehensive. Once you leave School or College, you may think that you deserve all the goodies in life but, without indulgent parents, that sort of thing comes to an end. The same thing applies to disabled kids. They may acquire funding for education and care whilst they are minors but many of them find themselves at the bottom of pile for the rest of their lives.
I shall stop my left wing ranting and pirating this thread now! Sorry chaps.

 

[uminair]

here is a video presentation that describes the principle of work of transistor.
http://www.youtube.com/watch?v=fodfowmdfvy&feature=related

nice