How Transistor works - verifying

[cabraham]

OK, thanks! And yes, comparing narrow base to extremely wide base is an excellent teaching strategy of which I previously hadn't heard.

Regarding canon, earlier you stated that the Ebers-Moll model is a CC model, and their original paper employed current-controlled current sources. Correct?

Are you certain that you're not misinterpreting something? N.b. that your assertion regarding Ebers-Moll being a CC model goes entirely contrary to numerous undergrad Uni books which give the following CV (large signal transconductance) equation as the central feature of Ebers-Moll model:

Ic=Is*(exp(Vbe/Vt)-1) (for large hfe of course, alpha ~=1)
​

Is this not canon? If the above CV equation isn't "Ebers-Moll model," then you've discovered a vast flaw in an enormous number of textbooks.

Send me a message, & I'll email you the Ebers-Moll 1954 paper. It shows the bjt collector current being controlled by emitter current, i.e. a CCCS. Alpha is the parameter which determines the Ic value from Ie. They show a schematic w/ a CCCS controlling Ic.

The exp( ) relates the Vbe to Ic, but alpha is all important. The exp( ) covers the relation between Vbe & Ie. To get Ic we need alpha. The diode equation for the b-e jcn is Ie = Ies*exp((Vbe/Vt) - 1)> But Ic = alpha*Ie. So Ic = alpha*Ies*exp( ). The collector current is controlled by alpha*Ie. But Ie has a direct relation w/ Vbe as well as Ib.

All 3 eqns are relevant. A bjt offers both current gain as well as voltage gain. To compute current gain we use eqn 1) for the common emitter & emitter follower topologies. For the common base current gain, we use eqn 3). To compute voltage gain we use eqn 2). Again, all 3 eqns come into play when thoroughly analyzing bjt behavior.

We cannot make more than one quantity the controlling quantity. We generally control Ie, & Ic = alpha*Ie. Sometimes we control Ib, w/ Ic = beta*Ib. Usually this is not good due to beta dependency. We want networks that rely on well defined parameters like alpha & resistor values. Resistors have tight tolerances.

Send me a message, & I'll email you the E_M paper, the horses' mouth on the E-M eqns. BR.

Claude

 

[wbeaty]

I'm trying to understand the details of Ebers-Moll.

Based on your semiconductor expertise, would you say that the
Ebers-Moll model depicts the BJT as a current-controlled device?

Yes, that is what figure 5 on page 1765 depicts.

I'll take a look.

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.) See 2008 Microwave Journal, 23 years: Acceptance of Maxwell Theory http://bit.ly/qRQNCH

 

[Studiot]

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.)

Kirchoff's original statements are also quite different from what is often now presented under his name.
Maxwell's translation of K is quite interesting to read since if Professor Lewin followed the original he would not be able to present his famous 'conflict between K and Faraday lecture'.

This type of situation is actually not uncommon in the physical sciences.

 

[cabraham]

I'll take a look.

Beware, this could be a case similar to Maxwell's Equations. Go to Maxwell's original work and you won't find his four equations. Maxwell never actually wrote those four, and probably wasn't aware they existed. Maxwell attacked the problem in an obscure way, employing magnetic vector potential, quaternions, and writing twenty equations. Later scientists came in and revised everything, producing the four equations known today. If you rely on Maxwell as the "horse's mouth," then you'd be in big trouble. (Officially the four equations are today called the Hertz/Heaviside equations. But Maxwell discovered the original mathematical form which describes EM fields.) See 2008 Microwave Journal, 23 years: Acceptance of Maxwell Theory http://bit.ly/qRQNCH

So Bill, is this what you're saying? Your site claims that the CC model of bjt operation is wrong, & you are right. Then you use Ebers-Moll as the source for your claim that the bjt be classified as VC, not VC, because Ebers & Moll seem to suggest that. I just showed that the E-M paper published in 1954, depicts the bjt as CC, not VC. Now you're suggesting that Drs. E & M should not be viewed as the horses' mouth.

But they were your source for your thesis. My CC model is affirmed by every semicon OEM I know of, i.e. Tex Instr, Natl Semi, Fairchild, Intl Rectif, etc. Ebers-Moll is pretty accurate, but Gummell-Poon is an improvement. The G-P model includes an additional factor to account for Early influence. The Early voltage is denoted as "Va". So Gummell-Poon is as follows:

Ic = alpha*Ies*(exp((Vbe) -1)))*(1 + (Vce/Va)).

At large values of Vce, the collector current for a given Ib, or Vbe as well, is larger than that obtained at low values of Vce. I'm sure you are well aware of the Early effect, no need to elaborate. That is the main difference between E-M & G-P.

Again, let me re-iterate that we seem to agree that the current control bjt model is a good external model when internal physics need not be considered. But when the internal charge profile & device geometry is relevant, the EE canon uses charge control as the correct bjt model. You insisted on voltage control, relying on E-M as the source, which does not support such a claim.

So when we look inside the device, & we need a better model than current control, what do we use? I say charge control, as does every semicon OEM, & uni. I say "QC", & that is my final answer. Any questions? Anyone?

Claude

 

[wbeaty]

So Bill, is this what you're saying? Your site claims that the CC model of bjt operation is wrong, & you are right.

No, not exactly.

I still haven't stated my position. Such enormous messages, and all without knowing what my arguments are. I'm very confused.

I haven't clearly stated my reasoning yet, so do you want to hear it? That's my second question.

I could be wrong, but it looks to me like this is the problem here:

http://www.nizkor.org/features/fallacies/straw-man.html
Description of Straw Man
The Straw Man fallacy is committed when a person simply ignores a person's actual position and substitutes a distorted, exaggerated or misrepresented version of that position. This sort of "reasoning" has the following pattern: ...

​

I've been remaining silent on this issue because I'm waiting for you to notice that something important is missing. Also I'm waiting for you to stop repeatedly putting words in my mouth while constructing extensive counterarguments, over and over.

Looking back on this thread, it obviously hasn't worked.

So, what would work? You got me... I have no idea.

Suggestions from everyone would be welcome.

 

[cabraham]

Bill, please state exactly what you differ with in the EE canon. Your site makes some pointed criticisms regarding the way bjt operation is presented in engr colleges & semicon OEM app notes.

Where is the error in the common view of bjt operation? Where did I (Claude) err in presenting my understanding of bjt basics. Let's not be cryptic. Anything that I said that is not clear, or appears mistaken, or requires elaboration, is no problem at all AFAIC.

Again, I won't presume to know what your position is. So I'm asking you to state very plainly in detail the specific issues you have, if any, with OEM bjt models. We can take it from there. Best regards.

Claude

 

[wbeaty]

Bill, please state exactly what you differ with in the EE canon. Your site makes some pointed criticisms regarding the way bjt operation is presented in engr colleges & semicon OEM app notes.

I can't answer that without first giving background. My article is aimed at the general public, meaning ~13yr old kids. My goal was to explain simply the inner workings of bjts, and do it without relying on a single equation. Kids have no use for advanced models which cover high frequency operation, high power, etc.

Look, I couldn't explain resistors to the public if I was forced to include the complete high-freq LC model; all we really need is Ohm's law (although even Ohm's law is quite a bit too advanced.) Isn't that obvious? And I certainly wouldn't explain diodes to them using a charge control high-freq model. And Gummel-Poon CC model of the bjt is completely inappropriate for my audience.

That's why I keep asking, did you notice who my audience was? Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, it's just ridiculous.

Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job. (If you're looking for a tool which always works in every situation, well, good luck with that.)

So, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to the math-phobic general public?
​

Above is the whole point of my article.

 

[cabraham]

I can't answer that without first giving background. My article is aimed at the general public, meaning ~13yr old kids. My goal was to explain simply the inner workings of bjts, and do it without relying on a single equation. Kids have no use for advanced models which cover high frequency operation, high power, etc.

Look, I couldn't explain resistors to the public if I was forced to include the complete high-freq LC model; all we really need is Ohm's law (although even Ohm's law is quite a bit too advanced.) Isn't that obvious? And I certainly wouldn't explain diodes to them using a charge control high-freq model. And Gummel-Poon CC model of the bjt is completely inappropriate for my audience.

That's why I keep asking, did you notice who my audience was? Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, it's just ridiculous.

Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job. (If you're looking for a tool which always works in every situation, well, good luck with that.)So, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to the math-phobic general public?
​

Above is the whole point of my article.

I agree with that except for the part I highlighted in bold. So we agree that the discussion is limited to external mAcroscopic modeling, not internal physics. But then you state that CC "utterly fails", to which I say "say what!" The CC model is very accurate as long as we're dealing w/ speeds well below ft, & not saturating the device.

Of course there is no tool which works in every situation. I stated that repeatedly. Even QC (charge control) does not always provide a precise result. CC has its limitations, but since we're discussing external models, no internal physics, how is it that CC untterly fails?

Examine the following 2 current eqns:

1) Ic = beta*Ib.
3) Ic = alpha*Ie.

Again if "ft" is the device transition freq, & beta is the current gain, then fb (f_beta) = ft/beta. This "fb" value is where the beta value is 0.707 times the low frequency beta value. At freqencies below this value, eqns 1) & 3) are valid. You can rely on them provided the device is not used as a saturated switch.

The following eqn relates Vbe to Ic:

2) Ic = alpha*Ies*exp((Vbe/Vt) - 1).

Again, this is a perfectly valid relation at speeds below fb, & with the restriction that the bjt not enter saturation. These are the same restrictions for eqns 1) & 3).

For higher speeds and/or use of the device as a saturated switch, we must use QC. Stored minority carriers, distribution in space, reverse recovery charge, reverse recovery time etc. come into play. None of these useful parameters appear in eqns 1), 2), & 3).

Eqns 1), 2), & 3), are simply what I call the "terminal eqns". They are not a failure, just limited in scope. They are conditionally valid.

My final point, forgive me for repeating myself. Neither I nor the OEMs ever claimed that the CC model was valid for internal physics. We have presented the CC model as a superficial external estimate. It works under the conditions given above. No need to belabor the limitations of CC, as it is universally acknowledged.

For high speed operation, saturation, and internal physics analysis, the CC model is too oversimplified and cannot provide much help. But your whole point is to avoid complex math, & theory. On one hand you claim that CC fails, then you state that you wish to avoid heavy math & physics.

The eqn 2), Ic in terms of Vbe, is just as limited as are eqns 1) & 3). When the CC model is shown to be inadequate, you say we should use the VC model, I & the OEMs say QC model. That is where we differ. It's not about the limitations of CC model. We agree that CC is limited.

Our difference is in which model is more precise for the conditions where CC fails. Your view tha VC takes over has no support from any solid state physics theory. Only QC can handle the speed & saturation conditions. Neither Ic eqn, using Ib, Vbe, or Ie, can handle these conditions.

I'm at a loss to make it any clearer. Is my point clear?

Claude

 

[wbeaty]

I agree with that except for the part I highlighted in bold.

We've utterly failed to communicate.

I said this:

Yes Gummel-Poon obviously is required for VHF design, for accurate spice simulations, etc. But for explaining transistors to people with zero math skills, [ Gummel-Poon (CC) ] is just ridiculous. Is this clear? CC is wrong. It utterly fails. It's a complete mismatch for the task at hand. It's the wrong tool for the job.

I never said this:

CC is wrong. It utterly fails.

Please don't take my sentences out of context. That's close to being straw-man.

OK, one more time.

Do you understand who is the intended audience of my article? My article is for children and the general public. My transistor article is for children. As I said in my previous message, Gummel-Poon and CC is wrong for children. When explaining the inner workings of transistors to children, CC fails. Using it is just stupid. It's completely the wrong tool for children.. It's like using a screwdriver to hammer nails.

If I tried to use CC and some equations to explain the inner workings of the BJT, and my audience is children, then I'd be a failure.

So we agree that the discussion is limited to external mAcroscopic modeling, not internal physics.

No we do not. The goal of my article is to explain the inner workings of transistors to children.

The goal of my article is to explain the inner workings of transistors to children.

Perhaps I didn't explain this before?

:)

CC has its limitations, but since we're discussing external models, no internal physics

We are not discussing external models.

We are explaining the internal physics of transistors ...to children.
Again, how would we answer the following question?

What's a good way to explain the inner workings of the BJT to children?​

This question reveals the entire point of my transistor article.

 

[cabraham]

Bill, you ask "What's a good way to explain the inner workings of the BJT to children?"

My answer is, of course, the 2 diode back to back model. After diodes are explained to children, then the 2 diodes can be explained. A thick base region behaves like 2 back to back diodes & nothing more. As the base region is made thinner, transistor action is then observed.

The CC model, namely Ic = alpha*Ie, works very well. For 2 back to back diodes, the Ic value should only be a small value, that associated w/ the c-b jcn reverse leakage current. Namely Ic = -Ics*exp((Vbc/Vt) - 1). After all, the c-b jcn is reverse biased, so little current can flow.

The emitter current Ie is given by Ie = Ies*exp((Vbe/Vt) - 1), of course. So we have

Ic = -Ics*exp((Vbc/Vt) - 1).

Ie = Ies*exp((Vbe/Vt) - 1).

For 2 diodes, that is the truth, whole truth, & nothing but the truth.

But when the base is so thin that the 2 junctions are in extreme proximity, another term is added to the eqns.

Ic = alpha*Ies*exp((Vbe/Vt) - 1) - Ics*exp((Vbc/Vt) - 1).

The added term is in bold font. Personally, this explains bjt action w/o quantum mechanics. The "alpha term" accounts for the emitted carriers from the emitter, transporting right through the base before most can recombine, then continue onward into the collector. The better the transfer, the closer alpha is to unity.

Honestly Bill, if the target is children, & QM is off limits, is there a better explanation than the 2 back-back diodes w/ ultra-thin base width? If there is, please enlighten us. BR.

Claude

 

[wbeaty]

Bill, you ask "What's a good way to explain the inner workings of the BJT to children?"

My answer is, of course, the 2 diode back to back model.

A bit too sophisticated for this audience.

the Ic value should only be a small value, that associated w/ the c-b jcn reverse leakage current. Namely Ic = -Ics*exp((Vbc/Vt) - 1).

Way overboard, we need total, total simplicity. E.g. we'd assume this Ic ~=0.

After all, the c-b jcn is reverse biased, so little current can flow. The emitter current Ie is given by Ie = Ies*exp((Vbe/Vt) - 1), of course

No equations allowed. Again: the audience is children. Even Ohm's law is too much. To construct equation-free explanations, one first must learn to think in equation-free terms: pure concepts, and visual/verbal/intuitive language.

 

[wbeaty]

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? :)

 

[Studiot]

So, how can we explain BJTs to children?

I said before and I'll say again I like the picture of man operating a tap.

Children can easily understand how this can compass either polarity or FETs etc.

Otherwise you have all sorts of words they only half understand such as voltage, charge, current carrier etc.

(Are physicists really any better? )

 

[cabraham]

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? :)

I believe the problem lies in #3. How does the potential barrier "set the rate of charges crossing the b-e junction"? I'm afraid that we are right back at the endless chicken-egg riddle. Does voltage "set the current" or does "current set the voltage"? Until we grapple with that vexing riddle, we will argue ad infinitum, which I am not going to do with anybody.

The power/signal source driving the amp stage is what sets the current. Then when carriers cross the junction recombination takes place for a small minority of carriers. Ionization occurs, & the local E field increases slightly due to the slightly increased charge distribution at the barrier zone. Vbe increases slightly.

This results in a slightly greater Vbe drop at the increased current level. The important point is that the rate of charges crossing the junction is determined by the power incident on the signal source driving the whole network. Using my singing Susan example w/ a microphone works well. Sue imparts acoustic energy to the mic diaphragm. Mechanical/acoustic power is converted to electrical power. The voltage drops due to cable resistance, the rbb' base region resistance in the bjt, the Vbe drop, & re, all subtract from the mic generated voltage.

The Vbe drop does not solely determine Ie. Rather, Ie is the net voltage after drops, divided by the total loop impedance. Vbe does not control Ic, but they are intrinsically & indirectly related. Here's how the events take place.

1) Sue sings into the mic.
2) Current & voltage are generated at the mic diaphragm. The ratio of V to I is the mic cable characteristic impedance, Zo.
3) The I & V move along the cable & encounter the bjt amp stage. Along the way cable resistance results in collisions & a charge distribution forming a potential barrier. Signal gets attenuated.
4) At the amp stage input carriers incur base side collisions due to rbb', & emitter side collisions due to re. Attenuation occurs.
5) Then the carriers cross the b-e junction. Most nake it to the collector but a few recombine in the base & ionize local atoms.
6) This changes ther barrier charge density & potential. The current is again attenuated.
7) If Sue cranks up her volume, the additional charges outputted by the mic add to the b-e barrier increasing Vbe.

Bottom line, Sue is the prime mover, she makes everything happen. Her volume determines all currents & voltages. She can sing loud, soft, or in between. Nothing happens until Sue makes it happen.

Rbb', re, cable R, cable C, b-e diffusion capacitance, b-c Miller cap, & barrier potential play a role. But none of them exclusively determine the current crossing the junction. Sue determines that current mostly, but there are drops due to rbb, re, & Vbe that diminish Sue's output. If Sue's mic outputs Vmic, each quantity cable R, rbb, re, & Vbe drop a portion of Vmic, leaving maybe 0.78*Vmic, or 0.63*Vmic, whatever.

If all drops were zero. Sue dictates the current. But nonideal parameters mentioned above rob a portion of Sue's signal. Thus, Vbe, rbb, re, & others do indeed play a partial role in "setting the current", but all are minor roles.

Sue is tha main entity that "sets" & determines the current crossing the junction. Is my explanation clear. BR.

Claude

 

[wbeaty]

Once again again again: THIS IS FOR CHILDREN. Complexity is verboten, and complex-ifying a simple situation is not any sign of competence. "Complexifers" are extremely valuable as graduate textbook authors, as RF chip designers, and for writing the best SPICE models. But for talking to children, "simplifiers" are who we need to hire.

We simplify everything by removing every smaller effect we can think of: remove the AC-source (microphone) and inject a Base current, unless you prefer a DC voltage source. Assume that rbb and re is zero, remove high-power phenomena, use the magical zero-resistance cable found in all intro courses, treat the DC case alone while ignoring dynamic AC issues such as Miller and other parasitic capacitance/inductance. We want a clear view of the most important phenomenon, so we wipe away all smaller details from our window. We don't even mention these details. (If something forces us to do so, we can add some details back in afterwards.)

 

[wbeaty]

I believe the problem lies in #3. How does the potential barrier "set the rate of charges crossing the b-e junction"?

It's a diode.

:)

Is there really a huge controversy about how PN junctions actually work? Is there some thread on physicsforums with a long battle over explaining the diode?

And pay close attention to your question above: how does a potential barrier set the rate of charges crossing [a] junction? If textbooks are fairly unanimous regarding explanation of PN junction potential barrier and resulting currents, then they've answered your question.

 

[cabraham]

It's a diode.

:)

Is there really a huge controversy about how PN junctions actually work? Is there some thread on physicsforums with a long battle over explaining the diode?

And pay close attention to your question above: how does a potential barrier set the rate of charges crossing [a] junction? If textbooks are fairly unanimous regarding explanation of PN junction potential barrier and resulting currents, then they've answered your question.

Please elaborate. I don't want to be accused of building straw men, but is this what you're saying?

1a) Id = Is*exp((Vd/Vt) - 1), for a diode. Are you claiming what most contrarians claim that Vd "sets" Id? If so, I need to remind you that the same equation can be expressed as :

1b) Vd = Vt*ln((Id/Is) + 1).

Vd does not set Id. The 2 quantities are inclusive. A change in Id takes place ahead of the change in Vd. Vd does not determine Id. If you doubt me, I'd suggest setting up an experiment w/ a fast scope & observe which quantity changes first. A pulse generator plus a resistor is to be placed in series w/ the diode. A current probe & voltage probe are used to measure Id & Vd.

On his web site, contrarian Kevin Aylward bases his whole position on the "fact" that diode current is "controlled" by diode voltage. He refers to Shockley's diode equation, per 1a) above. I emailed him telling him that eqn 1b) is just as valid as 1a). He didn't respond.

Vd does not "set Id". Nor vice-versa. The power/signal source energizing the network along w/ device parameters are what "set the current".

Claude

 

[wbeaty]

Please elaborate. I don't want to be accused of building straw men, but is this what you're saying?

1a) Id = Is*exp((Vd/Vt) - 1), for a diode. Are you claiming what most contrarians claim that Vd "sets" Id?

Ah, so there *is* a controversy about diode explanations!

OK, then before anything else we need to get clear on some very basic physics. Not voltage sweeping charges out of silicon, that's controversial stuff apparently. Not resistor operation (too controversial?) Not the nature of conductors. Not charges accelerated by fields. All the way back...

Cut to the chase ...I'm "claiming" the same thing any intro physics text "claims" ...that e-fields cause charges to experience a force.

This simple basic fact is built into all of classical fields concepts: a gravity field applies a force to a point mass, a b-field applies a force to a magnet pole, an e-field applies a force to a point charge.

Do you object? Are these "contrarian?"

Conduction, resistors, diodes, transistors, all that stuff comes later. First let's get the above out of the way. Do you accept that, if we apply a uniform e-field to a region of space containing an electrically charged object, the object experiences a force?

 

[cabraham]

Of course F = q*E. But the E field is provided by an external source. The charges along the depletion layer do NOT "sweep" anything. Examine the polarity & it is clear that the local E field due to accumulated charges in the depletion region oppose charges from crossing the junction. When you say that E fields exert forces on charges, please be more specific. Which E field in which location due to which charges acting on which other charges.

You take a well known concept, such as F = q*E, then spin & interpret said concept in a contrarian manner. If your theory was valid, it would be what is taught everywhere. Sure E fields exert forces on charge. But remember that when an E field imparts force & energy to a charge carrier, it loses the same amount of energy & is replenished w/ displacement current from the exernal source maintaining said E field.

The depletion zone has a local E field which opposes the flow of charges across the junction. It is the external source of E field which makes conduction happen. You sound as if the local E field associated w/ Vbe is what exerts force on the charges. Is this your theory? The E field associated w/ Vbe opposes charge motion across the junction.

PLease explain how charges move through the silicon. Start w/ the external source, say a microphone. Then cover the charge motion through the mic cable, emitter & base regions, then motion across the junction. Thanks in advance.

Claude

 

[wbeaty]

OK, then before anything else we need to get clear on some very basic physics... I'm "claiming" the same thing any intro physics text "claims" ...that e-fields cause charges to experience a force... a gravity field applies a force to a point mass, a b-field applies a force to a magnet pole, an e-field applies a force to a point charge.

Do you object?

Of course F = q*E...

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.

 

[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.