BJT Vs MOSFET V vs I controlled(Simple Trick companies don't want you to know... - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: October 28, 2017, 12:42:21 pm
content: So this is the video I was looking for. Makes sense to me:BJT is current controlled because that's how the depletion layers overcome the resistance to currant. A MOSFET is voltage controlled because it just needs potential to overcome the current barrier.I'm sure a debate will still ensue but I'm curious now that I understand this as to why people would claim other wise.Here is the one simple trick that companies don't want you to know about:NPN stands for Not Pointing In*Source is like a tap that *Emits watera *Drain *Collects waterThe *Gate to the *Base opens and closes

author: A Hellene
author_rank: Frequent Contributor
timestamp: October 28, 2017, 01:55:58 pm
content: Quote from: Beamin on October 28, 2017, 12:42:21 pmSo the BJT is.Else, why would we need a B-E in-parallel resistor in order to turn the transistor off as soon as possible? Because, by zeroing the B-E junction current the transistor WILL NOT turn off until we drain any residual voltage charges from the B-E junction (parasitic) capacitance; which means that the BJT is voltage driven device also --a transconductance device, in particular, with a parasitic P-N junction (for NPN polarities) in parallel to the input of the device that draws parasitic current when being driven into conduction.The unapproximated Ebers–Moll model equations are used to describe the three currents in any operating region of a transistor; those equations are based on the transport model for a bipolar junction transistor. If you look at them they are all exponential relationships of Vbe/Vt. The current through a diode is exponentially related to the voltage relative to Vt (V/Vt). This is the way the junctions work. Transistor currents work the same way. If you look at the above models you see that the currents are a result, not a cause. I'm not saying that the currents are not part of the process. I'm saying that the currents are a result of Vbe/Vt, and that's what causes things to happen. Obviously Ic = aplha*Ie, but look at what causes Ie. It's exponentially related to Vbe/Vt and the reverse saturation current.Yet, the starting equation for BJT design is the Shockley Diode equation which led to the Ebers-Moll model; this is only an approximation to reality. A much better approximation is the Gummel-Poon model; look it up.Perhaps it is not so obvious that it is base-emitter voltage that produces an electrical field across the junction between the base area and emitter area. The junction between base and emitter is very thin, so a small voltage can produce a very large electrical field. It is this electrical field that cause carriers to migrate from the heavily doped emitter region into the base region. Without the field, some do wander into the base region by thermal motion, where they neutralize oppositely charged carriers, creating a depletion region whose thickness is then diminished by a forward biasing electric field. What happens as a result of this migration of carriers from the emitter is complicated, but the end result is collector current that now flows through the reverse-biased base-collector junction. The fact that there is a base current is almost incidental. It is the base-emitter voltage causing an electrical field to be produced across the base-emitter junction that is responsible for the collector current.The BJT is best thought of as a voltage-driven device. In the common-emitter configuration for example, the Vbe causes the B-E junction to be forward biased. This generates a diode-type current of electrons into the base. The "efficiency" of the BJT is determined by how much of those electrons are swept across the base into the reverse-biased C-B junction and are collected in the collector region (hence the names "emitter" and "collector"). The electrons lost in the base come out the base terminal. The measure of the efficiency is the ratio of collector current to base current (ie the beta). This is why the base must be very thin in order to have a BJT - just putting 2 diodes back to back does not create a BJT.The analogy for gain can be considered by thinking of a waterfall that is controlled by an open or closed valve. If the valve is open, all the water flows. If the valve is closed no water flows. So a small amount of energy (that needed to open and close the valve) controls a large amount of energy (the water flow). I assume the analogy is clear - the water flow is the collector/emitter current and the valve is the base-emitter voltage.As to the arrow directions on the BJT symbols, it refers to the flow of "positive" current --which is opposed to the flow of electrons. Back in the old days, before people realized that current consisted of the flow of negatively-charged electrons, they chose the flow directions assuming the flow consisted of positively-charged things. We still use that flow assumption for current. So, even though electrons flow from the emitter to the collector via the base, the current is assumed to flow in the other direction. Once that's understood, it's no big deal.As for holes (the absence of electrons), it is a useful concept. If you picture a sea of electrons and one hole and by applying a voltage, the electrons move to fill that hole, the hole thereby moves from one end to the other. When you understand the original assumption of the positively-charged particles flowing, you can start to understand (not completely) the usefulness of the hole concept. It is just a conceptual way of looking at things - if you don't like it you can replace the word "hole" with "the electrons flow the other way, effectively moving the absence of the electron in the other direction". It is more complex than that, since it really refers to the outermost electrons on the silicon atom.-George

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 02:13:27 pm
content: If you want the wider truth, it's that the device can be represented as a general 2-port.What is a 2-port?It is a block with two ports, and some relationship between them.What's a port?A port is a pair of terminals which has voltage and current local to it. That is, there is no common mode interaction between a port and any arbitrary voltage or current elsewhere. It's a floating two-pin component (a resistor or capacitor, or any general 2-terminal circuit, is a one-port). Or, if you like: a port is an ideal 1:1 transformer to the rest of the circuit.The currents must always be balanced: current into the positive terminal, and flows out the negative terminal.A two-port, with the ports shorted together, is just that, an ideal transformer: no capacitance between windings, no magnetizing inductance, and whatever current flows in through one port, flows out the other.You have six choices for how to model a linear two-port at a given frequency. These are the possible permutations of voltages and currents at the two ports. Every combination has been accounted for, and named.For example, if you want to know the port voltages as a function of their currents, you use the z (impedance) matrix. \$\vec{v} = \mathbf{Z} \vec{i}\$, where each vector is an ordered array of the ports' voltages and currents. Or if you want port 2 in terms of port 1, you use the transmission matrix (ABCD parameters). Or if you want a mix of currents and voltages, you can use h parameters (which BJTs have traditionally been written in terms of), or y parameters (usually FETs), or g, or uh, whatever the last one is, I forget.As long as the system is not ill-conditioned, simple linear algebra formulas convert between these various methods. They are all equivalent.Of course, a transistor has three terminals, so it happens that the two ports do share a connection: the negative terminals are connected. This saves a pin, and doesn't change anything about the ports themselves (because a port doesn't care where you connect it). If you like, you can consider an optotransistor as a transistor with the ports still isolated (though the isolation defeats many of the useful transistor-y properties, so it's not nearly fair enough of a comparison).Tim

author: A Hellene
author_rank: Frequent Contributor
timestamp: October 28, 2017, 02:47:50 pm
content: Very nicely put, Tim!Yet, (advanced) modeling comprehension is heavily based on the very basic principles internalisation of the operation of any device/component/concept/idea in question; which, when absent (or even incomplete) can only lead to greater lengths of confusion...Dear Beamin, you can begin your BJC/MOSFET acquaintance fromthis thread; go through the whole discussion thread and we will be here to patiently answer your questionings.-George

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 28, 2017, 04:55:31 pm
content: This is at least the second time Beamin has brought up the same question.For some reason some people passionately believe one point of view or the other.Usually the Shockley equation is brought up. Probably not helpful. Where did that exponential term come from?This may clear it all up:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 05:31:22 pm
content: Gosh, I wonder how much math/physics is hidden in P&tB... I should watch it some time (it's been forever).Indeed, the exponential tail of thermal energies is key to any three terminal amplifying device. Here's a plot showing 5 decades of exponential current flow in the 6AL5 vacuum rectifier:It flattens off at high current as classical field behavior takes over (Child–Langmuir law), much as the semiconductor FET flattens off at high currents, going from exponential (subthreshold region) to quadratic. (The response with respect to grid voltage is very similar; the grid-cathode system looks like a virtual cathode of variable current flow, so the same law follows.)Tim

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: October 28, 2017, 08:19:52 pm
content: A BJT begins to conduct at a relatively predictable Vbe of around 0.6 to 0.7v. The base current increases exponentially with applied voltage. This is because you are effectively placing a voltage across a forward biased diode.Meanwhile, the collector current is reasonably proportional to the base current over most of the operating range.Current gain can vary considerably between samples of the same type. Surprisingly, the gain when voltage-driven seems to vary less between samples. I think this is because higher gain samples also tend to have higher b-e diode impedance, so the consequence is a smaller base current but the same resultant collector current.Though if you try to drive a BJT with a voltage signal, you will end up getting tied in knots with mathematics trying to work out what collector current will result.You will also end up with a relatively high level of harmonic distortion. Many early BJT circuits were designed like that because the designers didn't know any better, and just copied valve amplifier design principles, substituting positive bias for reverse bias of course.If you drive a BJT with a basecurrent, the result is a nearly linear amplification. The trick to doing so is in the overall circuit design; typically the use of a negative feedback loop to 'bootstrap' the emitter so that it follows the base voltage. This is why single transistor amplifiers have dreadful performance; adding a second BJT transforms the linearity and makes the gain easily calculable. (Cray supercomputer not needed, thanks)MOSFETs on the other hand, are purely voltage driven. They start to conduct at a threshold voltage typically between 2v and 5v, which varies not only between types, but also quite widely between samples of the same type.Meanwhile, the drain current is nowhere near proportional to the gate voltage. In some types the gain is actually greater at high drain currents than at low. Thus, their audio performance tends to be poorer than the current-driven BJT in terms of harmonic distortion, but better than the (dreadful) single BJT voltage driven stage.The gate current being mainly due to capacitance, MOSFETS have much smaller drive requirements at DC or low frequencies. At high frequencies, not so much difference.Both types suffer from 'secondary breakdown' considerations (Basically that current handling is more limited at higher Vce or Vds than simple power dissipation would suggest) but MOSFETS arefarmore restricted in this respect. This is the gotcha that leads to a lot of homebrew MOSFET power amps blowing.

author: A Hellene
author_rank: Frequent Contributor
timestamp: October 28, 2017, 08:54:04 pm
content: Quote from: rfeecs on October 28, 2017, 04:55:31 pmI am sorry I have missed that, since it seems that I am not such a frequent member to have come across that one; could you please point me at that thread in question?QuoteBy substituting the term 'passionately' with the more accurate term 'factually,' I will plead guilty (as I have already demonstrated at my previous message)!QuoteThe Shockley equation has been brought up in my previous message because no semiconductor forward biased P-N junction can escape or overrule the Shockley Diode Equation; not even the heretic Tunnel Diode...So, how could that Shockley Diode (universal) equation be irrelevant to the lowly Bipolar Junction Transistor's Base-Emitter junction?QuoteAh! I can see the invocation of my currently used avatar heroes,Pinky and the Brain!Well, I think that Tim has already sufficiently covered this aspect of the argument, just right in his message above!Really, could this argument have any grounds if I changed my avatar picture to my scary and ugly mug, below?-George

author: A Hellene
author_rank: Frequent Contributor
timestamp: October 28, 2017, 10:21:09 pm
content: Quote from: T3sl4co1l on October 28, 2017, 05:31:22 pmI am under the impression that MOSFETs arefour-terminal devicesby design, no matter if the Bulk (should be or) is already shunted to the Source terminal for convenient purposes...Quote from: IanMacdonald on October 28, 2017, 08:19:52 pmIan, I think that the BJT small-signal and large-signal behaviour calculations can be largely different, just because of the inconsistentcurrent gainavailable models: in some cases it is useful to consider the BJT as a voltage-driven device while in some other cases as a current-driven one.My objection is straightforward: A model IS NOT the absolute behavioral rule of a device; it is just a practical simplification, which might be inadequate to meet ALL of the practical requirements.Now, the fault of the existing Academic misconceptions should not be counted as unbendable rules but as the measure of the current learning system incompetency: The resourceful Academic outcomes will eventually find their way to success, while the incompetent ones that will blindly parrot anything they have been told will be used by the establishment to perpetuate any of the myopic Academia malpractices (since the Academia CAN NEVER be wrong about anything...). Just see the perpetuation of the 'conventional' current flowing direction doctrine instead of a graceful Academic back-stepping and the amendment of their own mistakes about that; or about every other mistake of theirs that they still perpetuate as an unbendable truth, no matter if it has already been factually debunked...-George

author: Beamin
author_rank: Super Contributor
timestamp: October 29, 2017, 01:55:11 am
content: Quote from: rfeecs on October 28, 2017, 04:55:31 pmI kind of regard eevblog dave as the authority on this board and the subject and he states what I said which is what he said. Not rehashing shit I just find that he said what it is and why it is. I really hope he responds to this thread and we can all agree ad put the debate to bed.So do some of you think Dave is wrong? He seems to be right about all of his teaching videos. Also I forgot that this came up in another thread.

author: Cerebus
author_rank: Super Contributor
timestamp: October 29, 2017, 03:04:34 am
content: The problem is that some people insist on transistors being either voltage driven or current driven. They are neither, they are charge driven. There fixed that for you.No, but seriously. Try this. Explain to yourself how an electric current moves through a wire. If you did it without mentioning both charge and electric fields to yourself then you've got a problem - you don't understand how and why basic electricity works. I suspect that in excess of 95% of people who want to debate whether a transistor is voltage or current controlled can't adequately explain how and why a simple current moves through a wire. If you can't, don't panic, I've known a lot of people quite competent at electronics who couldn't answer that question, but you will have just realized that there is a significant hole in your knowledge that needs filling.Don't get me wrong, if you want a long lecture on semiconductor physics then I'm not the man to come to. I barely understand how a transistor works but I know enough to know that a proper understanding of basic electricity in terms of charges and fields is necessary to understand them. You are going to need a proper grounding in the very basics of electricity (not electronics, electricity) before you're really equipped to realise that 'charge controlled' is the only tenable basic answer to the question posed.As far as modelling transistors for everyday design is concerned, my experience is that your life is made much easier if you treat BJTs as basically voltage controlled and deal with the relationship between currents after dealing with the voltage controlled part. Anybody who wants to insist that 'current controlled' is the only proper way to approach the problem is invited to design a log or anti-log amplifier using a 'current controlled' paradigm and to show their workings.

author: IanB
author_rank: Super Contributor
timestamp: October 29, 2017, 03:32:27 am
content: Quote from: Beamin on October 29, 2017, 01:55:11 amDave is neither right nor wrong. He is simply giving a perspective and a viewpoint. This viewpoint is "right" in as far as it is applicable, and it is "wrong" where it is not applicable.Let me ask you a question: Are Newton's laws of motion and gravity right or wrong?

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 29, 2017, 05:31:13 am
content: Quote from: Beamin on October 29, 2017, 01:55:11 amHere's the thread, from three weeks ago:https://www.eevblog.com/forum/beginners/differance-in-bipolor-mosfet-and-jfets/msg1321635/#msg1321635You've seen many examples in these threads of people who think the answer is current control, people who think voltage control and people who think both. So who's right and who's wrong?I have to say that working as an electrical engineer for more than thirty years and actually designing transistors, I don't recall this question ever coming up until I saw it on this forum. But then my memory is probably as bad as yours. And questions like this are what the internet is for?The fact is, if you are designing circuits, all you need to know are the mathematical models that relate voltage, current and charge. The Ebers–Moll model is one example. Those are mathematical equations. They work both directions. For a given voltage you get a current, for a given current you get a voltage. So the question of voltage controlled vs current controlled is nonsense.If you want to look at the physics, go ahead. Here's a place to start:https://ecee.colorado.edu/~bart/book/book/contents.htmYou will see these are also based on mathematical models that finally result in the same equations as before. So current controlled vs voltage controlled is still nonsense.

author: Beamin
author_rank: Super Contributor
timestamp: October 29, 2017, 10:56:54 am
content: Quote from: IanB on October 29, 2017, 03:32:27 amwrong. He models them in flat space and not curved space time with no consideration that time and motion are relevant to some observer. IE He doesn't take into account that mass distorts the space around it causing spinning bodies to create ripples due to their spin which accounts for energy his laws don't take into account.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2017, 11:25:24 am
content: Quote from: Beamin on October 29, 2017, 10:56:54 amMissing the point: the answer is contextual.Newtonian gravity is an approximation (v << c, or c --> infty), and is suitable anywhere that approximation is acceptable. Astronomers got by just fine for centuries on that; but when it came time to make more precise predictions, and to explain anomalous observations like the precession of Mercury, or the beginnings of particle physics, it was necessary to invoke (and invent) Relativity.Tim

author: A Hellene
author_rank: Frequent Contributor
timestamp: October 29, 2017, 02:41:00 pm
content: Quote from: Beamin on October 29, 2017, 10:56:54 amWell, leaving aside for the momenttheCantorian gangperennially failed 'Distorted Spacetime'as well as theEuclidean Geometrythat dominates our universe (even NASA admitted to it by vastly usingLagrangian mechanicsfor space travelling), I believe that this could be an enlightening starting point regarding your original question:How do Transistors REALLY work?&Part 2, fromWilliam Beaty's fine site that might also help you get a better grasp ofwhat electricity isas well as lots ofcommon misconceptions.-George

author: Beamin
author_rank: Super Contributor
timestamp: October 30, 2017, 08:25:49 pm
content: Quote from: A Hellene on October 29, 2017, 02:41:00 pm"Electricity is a mysterious substance flowing through the ether the like of which mankind will never understand"-That is from a grade school science text book from about 120 years ago. I still think that 99% of people really have no idea what electricity is. Even more disturbing is the veritasium video where he asks people does the sun orbit the earth and more then half the people thought that was true. How the hell can you not know that? I had a quite smart 9th grade English teacher that said "is it the sun around the earth or the other way around? I always get that mixed up"I think when most people look up its just a blue ceiling with a bright spot or a dark ceiling with little dots on it and nothing more. I am always in awe at how little and small I can see when I look up at night knowing even light years are a tiny % of what's out there.I also think people have no idea they are looking into a time machine when they see stars.