-- Well, Kevin seems to subscribe to the policy that one should learn to run, then walk. That is, first throw Ebers-Moll at a newbie then, later, beta. If you agree with that philosophy, then I'll have to disagree with you both.
Well, its a bit more subtle:-)
Depletion and enhancement both work exactly the same way, which is what you actually said, but not so obviously. Increasing the gate voltage will increase the current in both type of devices. The essential difference is that at OV an enhancement device is off, where as a depletion device needs a negative voltage to get it off. That is, its
*only* Vto that is different.Kevin Aylward snipped-for-privacy@anasoft.co.uk
Really? What formula do you use to predict Id? The one I use for the active region is
Id = k * (Vgs - Vgs(th))^2
where
k = Ids(on) / (Vgs(on) - Vgs(th))^2
Since k and Vgs(th) are constant for a given FET, Id is quadratic in Vgs, right?
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.
-- Beta doesn't exist? That's news to me!
-- I brought up gate capacitance to illustrate that there's no ohmic contact between the gate and the channel, not to change the direction of the thread for the pupose of showing off.
-- Then, in fact, you were remiss in not further including _all_ FETs in your "exposition" and expounding on them at length. After all, why leave any tern ustoned?
In view of the fact that you brought up gate capacitance, it was entirely apposite, IMV.
The voltage gain of a FET is lousy compared to a BJT at the best of times. If you're only going to paritally charge/discharge Cg., then you'll only exacerbate that failing and be lucky to get any useful gain at all.
The subject line refers to MOSFETs, certainly. However, in the body of his original post, the OP refers to FETs., which could just be innocently (but wrongly) intended as an abbreviation for MOSFET, or it could actually mean JFET. We don't know if the OP knows the difference between JFETs and MOSFETs, so I think it's entirely appropriate to point out they are different devices with different characteristics. Had this not been pointed out, the OP might well assume that everything that he has read here WRT to MOSFETs applies equally well to JFETs - and of course it doesn't!
Your approach would lead the OP into a false sense of competence over his grasp of the subject. At least now he has a much better idea of the scope of this surprisingly complicated question and if he trawls through all the replies and studies them intently, he will be a better man for it. Plus he will have no unpleasant surprises further down the line in his studies.
More like hyperbolic, actually.
Then presumably you're the type of person who tells his kids Father Christmas exists. All very nice and well-intentioned, but not fair on the child when he finds out the real truth and starts to question everything he's ever been told. I'd sooner be straight with people right from the start.
I dont see johns orginal post here
I most certainly dont.
Not at all. One need only state that the collector current is a direct function of base emitter voltage, and that when this voltage is applied, there is some base current, which is typically much less than the collector current.
This correct description is no more complicated that giving the *wrong* base current controlled one.
I don't agree with a philosophy of giving false technical information, if the correct information is just as easy to give.
Yep. It is very rare that I think white lies are the way to go. In this particular case, kids should informed from the outset that ideas such as santa claus, god, elves, etc are simply made up fantasies.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
Your approch gave an inital *wrong* grasp of the subject, i.e. no grasp at all.
No. There is no reason for not giving a *correct* description if that correct description is simple.
To wit:
The bipolar transistor is a voltage controlled device,. Its collector current is a direct function of its base emitter voltage. Incidentally to this process, the base terminal requires current in order for the transistor to work correctly.
If the reader can not understand such a simple idea, then there would be no point in giving any description at all.
If you think that you have helped by reinforcing erroneous notions on how the bipolar transistor operates, I suggest you have another think.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
No. They are a transconductance device because applying a voltage across the base emitter junction injects carriers from the emitter to the base
*region*. This charge essentially *all* flows out of the collecter, not the base terminal.This is not an accurate description of the bipolar transistor. This description is more relevant to operation of the mosfet. The npn junction simply does not act like a slap of N type. If it did, base current would be huge.
Indeed it is.
This is not too much detail at all. Its can't get any simpler. vbe controls the collector/emitter current. End of story.
No. No. No. It most certainly doesn't.
Referring to the bipolar as "a current controlled device" causes never ending confusion that is a bloody nightmare to correct. This is a case in point. You yourself are trying to put forward the idea that that idea has merit. It doesn't.
Since this is the actual truth to the matter, this is what should be said. Lying doesn't help one iota.
They are different, in part, in that the bipolar requires base current, but that this base current is simply a nuisance.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
I would question your idea about GOD, but are you really trying to me that Santa Claus is a myth ? don't tell it so! who the hell has been eating those cookies and drinking that milk then! :)
--- Not no. From:
"Transconductance is an expression of the performance of a bipolar transistor or field-effect transistor (FET). In general, the larger the transconductance figure for a device, the greater the gain (amplification) it is capable of delivering, when all other factors are held constant.
Formally, for a bipolar device, transconductance is defined as the ratio of the change in collector current to the change in base voltage over a defined, arbitrarily small interval on the collector-current-versus-base-voltage curve. For an FET, transconductance is the ratio of the change in drain current to the change in gate voltage over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
The symbol for transconductance is gm. The unit is the siemens, the same unit that is used for direct-current (DC) conductance.
If dI represents a change in collector or drain current caused by a small change in base or gate voltage dE, then the transconductance is approximately:
gm = dI / dE
As the size of the interval approaches zero -- that is, the change in base or gate voltage becomes smaller and smaller -- the value of dI / dE approaches the slope of a line tangent to the curve at a specific point. The slope of this line represents the theoretical transconductance of a bipolar transistor for a given base voltage and collector current, or the theoretical transconductance of an FET for a given gate voltage and drain current."
---
--- yes, were it not for the current limiting resistance external to the base the base current could become huge. After all, the base-emitter diode is just that, a forward biased diode operating on the far side of the VI knee. The intent, in both devices, is the same. That is to cause a non-conductive region in a semiconductor to become conductive. In a MOSFET it's accomplished by treating the channel like the plate of a capacitor and making it _seem_ like it's composed of the same material as the drain and the source by influencing the charge distribution in it using the gate metalization as the other plate of the capacitor, while in a BJT it's accomplished by forcing dynamic charge into the base ["base region" if you like ;)] and using that charge flow to make it seem like the base region material is becoming more and more like the emitter and collector material as the base current increases.
---
--- Hardly. Here this newbie asks "What makes a BJT different from a FET?" and you reply "If you put a voltage across the base and emitter terminals of a BJT current will flow between the collector and emitter, while if you put a voltage across the gate and source terminals of a FET current will flow between the drain and the source." So, while your description may be true, its utter simplicity leads the newb to think they're the same same thing with differently named terminals.
Here is my original exchange with Skeleton Man:
--- Essentially, yes. But, the voltage applied to the base must force charge through the base-emitter junction before collector current can flow.
---
--- Yes, but it still requires current to charge the gate capacitance. However, once that capacitor is charged up, current can flow through the drain-to-source channel with no further current required into the gate.
Do you have a problem with that?
---
--- Yes. you're right. That was poorly stated. See my original reply, above, to the OP for clarification.
---
--- The problem which arises here, I think, is that the change in base voltage required to affect a change in collector current is so tiny that it becomes easier to consider what happens on the other side of the change in base voltage. That is, the collector-to emitter current change due to the base-to-emitter current change.
---
--- I agree. See my original reply, above, to the newb.
---
--- Hardly a _nuisance_; it won't work without it.
Kind of like that we are different from corpses, in part, because we are required to breathe, but that that breathing is simply a nuisance.
-- John Fields
-- Precisely! ;^)
-- And in others is eminently useful. In the real world, for instance, what's important when driving, say, a relay is the beta available and forcing that beta to a value which will never change regardless of tghe environment into which the circuitry is placed.
-- What I told Kevin was that I had misstated something, not that I was prepared to argue that the beta model was the be-all and end-all. If you'd bother to go back and read my original post to the OP, instead of playing your stupid posturing game, you might find that your entire argument is baseless and, basically, noise.
It can't by any stretch of the imagination be described as "showing off" since this is all pretty fundamental stuff - and fundamentals are terribly important. Would you happily build a house on a flawed foundation?
Nitpicking isn't going to help the OP.
If you actually measure, in the real world, a real FET's Vgs against Id at several points up to Idss., you'll find the curve you get is closer to hyperbolic than to quadratic. Your method may well be arithmetically correct, but like so many things involving maths in electronics, it's simply an approximation with an inevitable degree of inaccuracy.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.