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difference between bipolar and mosfet

Discussion in 'Electronic Basics' started by Skeleton Man, Jan 8, 2005.

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  1. John Fields

    John Fields Guest

    ---
    Have you paid any attention at all to the subject line? It reads:

    "difference between bipolar and mosfet", not "How many different
    flavors of FETs are there?"

    As far as confusion goes, I'm sure that your refusal to KISS have done
    little to alleviate the OP's.
     
  2. John Fields

    John Fields Guest

     
  3. Miles Harris

    Miles Harris Guest

    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.
     
  4. Miles Harris

    Miles Harris Guest

    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.
     
  5. Miles Harris

    Miles Harris Guest

    More like hyperbolic, actually.
     
  6. Miles Harris

    Miles Harris Guest

    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.
     
  7. 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.
     
  8. John Fields

    John Fields Guest

    ---
    Beta doesn't exist? That's news to me!
    ---
    ---
    I rather doubt whether you have the "real truth" on tap to hand out,
    and all children should be taught to question everything they've been
    told no matter how much you may cherish the truth you think you're
    being so benevolent in bestowing on them.
    ---
     
  9. John Fields

    John Fields Guest

     
  10. John Fields

    John Fields Guest

    ---
    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?
    ---
    ---
    My _carefully measured_ approach would allow the OP an initial grasp
    of the subject matter which could later be broadened, if he chose to,
    without the undue confusion forced on him by your rather heavy-handed
    insistence that he eat more than he asked for.
    ---
    ---
    The point is that there really was no reason for him to have to trawl
    through anything until you started pumping out your bilge. Plus, if
    you think that you've shielded him from any unpleasant surprises
    farther down the road because of your "contribution", I suggest you
    have another think.
     
  11. 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

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  12. 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

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  13. 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.

    http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page2.html
    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

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  14. Miles Harris

    Miles Harris Guest

    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.
     
  15. Miles Harris

    Miles Harris Guest

    On Sun, 09 Jan 2005 17:05:58 -0800, Robert Monsen

    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.
     
  16. Miles Harris

    Miles Harris Guest

    The beta model isn't even suitable for under-tens to study. Sooner or
    later the OP will discover it falls apart in certain circumstances.
    Then some kind soul will introduce him to Ebers-Moll and the
    transconductance model and he will damn your hide for spinning him
    such snake-oil early in his studies.
     
  17. John Fields

    John Fields Guest

    ---
    Not no. From:

    http://searchsmallbizit.techtarget.com/sDefinition/0,,sid44_gci214200,00.html

    "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:

    <QUOTE>
    ---
    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.
    <END QUOTE>


    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.
     
  18. John Fields

    John Fields Guest

     
  19. John Fields

    John Fields Guest

    ---
    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.
    ---
    ---
    You obviously have a reading comprehension problem if you _assume
    that's what I did. Here is the exchange to which you seem to have
    taken exception:


    <QUOTE>
    ---
    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.
    <END QUOTE>


    Can you point to where I advocated the beta model for him to study?
     
  20. Jamie

    Jamie Guest

    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! :)
     
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