Connect with us

difference between bipolar and mosfet

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

Scroll to continue with content
  1. The rest of what you say here, is even more amazing.
    No. Vbe is setting an *electric field* that attempts to accelerate
    charges from the emitter into the base region. This is truly 101
    physics.

    NO! NO! NO!. Absolutly not. You are wrong. It is not the flow of charge
    in the base that *causes* collecter current. This has already been
    explained in many posts.

    Charge flows because of:

    F=q(E+vxB)

    That is, excluding magnetic effects, it is *Electric Fields* that make
    chages move. Period. The flow of charge, excluding magnetic effects,
    cannot make other charge flow, other then by the change in electric
    field that such flow might cause.

    For the umpteenth time. Applying an *electric field* to the base emitter
    injects carriers from the emitter into the base region. Once the
    carriers are in the base region, they are attracted by the *electric
    field* of the collector and are swept up (collected) by the collector
    due to this *electric field*. Some of the emitter carriers just don't
    make it, and are picked up via the base terminal. This base current is
    an *effect* not a cause, and is incidental to the base emitter *electric
    field* injecting carriers.

    To repeat, it is not the *motion* of base charge that *causes* the
    *motion* of collector/emitter charge. It is the electric field at Vbe
    that causes both base current and collector/emitter current, as it is,
    now get this 101 physics, *ELECTRIC FIELDS THAT MAKE CHARGES MOVE*. End
    of story. Period.

    I have already given a link that diagrams this behaviour.

    http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page2.html
    No. This shows that base current is an effect, of an electric field
    cause.

    I am still amazed that after all these posts, you still don't understand
    the basic operation of the bipolar transistor.

    F=q(E+vxB)

    Its that simple.

    See above.
    And this is all wrong. I have already explained, a continuous resistance
    would mean that the base is connected to that resistance, hence, no
    transister action.

    Quite frankly, as noted above, you simply don't understand how a bipolar
    transistor works. Read and understand the above. Its the way it is. I
    can tell you that if you gave this description in an academic
    semiconductor physics class, you would get a zero grade.

    For some reason, you have picked up an erroneous understanding of
    transistor operation. You need to get to grips with the idea that you
    have been mistaken on this issue. This isn't debatable. Its in all the
    academic text books.

    Look, actually produce a detailed *physics* explanation as to *how* a
    flow of base charge can actually induce a flow of collector charge. This
    should tell you something.

    In all the semiconductor text books I have read, I have never seen this
    done, so why you can claim that this is the case, is pretty far fetched
    indeed.

    And this is wrong.

    Which isn't the case here. The error due to rbb' is usually small. It
    doesn't change the bipolar from its fundamental physics description as a
    voltage controlled device. Rbb' just makes the calculation of the
    voltage a little more involved.
    There is no conflict here.

    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.
     
  2. Miles Harris

    Miles Harris Guest

    Nope. You quoted this half-baked definition so it's quite proper for
    others to infer you're happy with it.

    [snip juvenile insults]
    That's rich, coming from someone who doesn't even understand
    transistor action. Your concept of it would have Shockley turning in
    his grave.
    Translation: "Holy shit! I've made the kind of dumbass error in that
    LED thread typical of a bonehead like me. Let's hope I can get away
    with this half-baked excuse before Miles spots it and I get exposed as
    an even *bigger* bonehead!"
    :p
     
  3. Miles Harris

    Miles Harris Guest

    It would, as you no doubt know, take me a great deal longer than one
    minute to write out everything I know about Beta. But were I so
    inclined, at least the OP would have an extensive, complete and
    accurate grasp of the subject, rather than the false sense of
    competence you attempted to instill in him by means of your own.
    HTH.
     
  4. Miles Harris

    Miles Harris Guest

    Nicely put.
    I doubt it'll be the end of the story, though. I suspect John Fields
    may be a troll. No rational person could conceivably be so obtuse.
     
  5. John Fields

    John Fields Guest

    On Wed, 12 Jan 2005 08:06:17 GMT, "Kevin Aylward"
    ---
    Well, Kevin, it's only analogy...
    ---

    ---
    Yes, it is. I don't think anyone has said, yet, that it _isn't_ the
    electric field which is making the electrons move, i.e. "pumping
    charge", so It seems strange to me that you would think it has.
    ---
    ---
    I said that the flow of charge between the emitter and base causes
    charge to flow between the emitter and collector, and you say that's
    wrong because the flow of charge between the emitter and base causes
    charge to flow between the emitter and collector [because the flow of
    charge between the emitter and base causes the electric field between
    the emitter and collector to be changed to the point where charge can
    flow between the emitter and collector.] Now where was I wrong,
    exactly?
    ---
    ---
    It doesn't matter how many times you say it, if it's not right it's
    not right. ;)

    If it weren't for the base current there would be no collector
    current, so it _is_ a cause, regardless of what's causing _it_ to
    happen.
    ---
    ---
    I don't disagree with that.
    ---
    ---
    Again, I don't disagree with that.
    ---
    ---
    And I'm more than equally amazed that you don't seem to be able deal
    with analogy.
    ---

    ....
    ---
    I don't know why you keep claiming that I said that the thing is a
    slab of of either P or N type material, when what I said was that it
    starts to _look_ like that between the collector and emitter when base
    current starts flowing. Much like the region between the cathode and
    the plate of a vacuum tube starts to look like a resistor when the
    tube is conducting. Is there a resistor between the plate and the
    cathode? No. Does it _look_ like there is? Yes.
    ---
    ---
    Perhaps, but since this is sci.electronics.basic, a rigorous treatment
    of the matter is seldom apropos. In this instance, a simple reply to
    the OP (which, BTW, you said you had no problem with) and some
    analogies which you either misunderstood or decided to take issue with
    for your own reasons seems to have led us up to where we now sit.

    I, for one, no longer have the desire to continue with this
    "discussion", so I'll excuse myself and bid you good day.

    Thanks,

    John
     
  6. John Fields

    John Fields Guest

    ---
    I _am_ happy with it, regardless of whether you choose to take issue
    with it or not. After all, your objections to it are largely
    unfounded in that from the context of the definition everything being
    described was quite clear.
    ---
     
  7. John Fields

    John Fields Guest

     
  8. John Fields

    John Fields Guest

     
  9. Indeed you did.
    No I didn't. I specifically denied that. I said the *ELECTRIC FIELD*
    between the base and emitter causes charge to flow from emitter to the
    base region.

    *THE FLOW OF CHARGE BETWEEN EMITTER AND BASE DOES NOT CAUSE THE FLOW OF
    CHARGE BETWEEN EMITTER AND COLLECTOR*

    I can't say it any plainer.

    All of the above. I repeat what I said here.

    "Applying an *electric field* to the base emitter
    injects carriers from the emitter into the base region. Once the
    carriers are in the base region, they are attracted by the *electric
    field* of the collector and are swept up (collected) by the collector
    due to this *electric field*. Some of the emitter carriers just don't
    make it, and are picked up via the base terminal. This base current is
    an *effect* not a cause, and is incidental to the base emitter *electric
    field* injecting carriers."

    *Nowhere* does this claim that the flow of charge causes another flow of
    charge.

    Not at all. A correlation does not have to be causal. This is basic 101
    statistics.

    For example, suppose those that have cats have less stress, should we
    recommend that people get cats? Or is it that less stressed people
    simply have a side effect of liking cats.
    No. This is why the tobacco industry had a legal claim against smoking
    causing cancer. It might be that those more likely to get cancer also
    had a side effect of just wanting to smoke.

    One needs to show that a correlation is *actually* causal. In this case,
    it is Vbe that causes both collector current and base current. Base
    current does not cause the collector current, but it is correlated.


    Your argument above says that you do disagree.

    Your argument above says that you do disagree.
    Because this analogy must account for the fact that there is a direct
    connection from the base to the emitter.
    Its not like the tube. The tube has no connection from grid to cathode
    or anode.
    Fair enough.

    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.
     
  10. Bob Myers

    Bob Myers Guest

    Sigh. If I might interject a couple of thoughts, just to hopefully
    redirect this little p***ing contest into something more productive.

    Could everyone please note the sign on the door, here? This is
    sci.electronics.BASICS. While a number of us here are, in fact,
    Professional Electrical Engineers of Long and Revered Standing
    (at least, we have a diploma and someone actually pays us to do this
    stuff), the questions here are more than likely going to come from
    people who do NOT have such a background, are never going to
    get into these phenomena down to the quantum-mechanics level,
    and whose questions will be more than adequately satisfied by the
    simple, "classical" explanations.

    Are those explanations, in many cases, "wrong" in the sense that
    they give what is to some degree a false or misleading understanding
    of the fundamental physics underlying the operation of these
    devices? Of course they are. But so what? If you're trying to
    answer a question posed by someone who does not now and likely
    never will care about his or her ability to derive the ideal diode
    equation from basic principles, these simple explanations are very
    likely still going to be the right choice. I strongly suspect that just
    about all of us who currently have some sort of "Engineer" title on
    our business cards started with just the same level of understanding, and
    it didn't stop any of us from getting where we are today. As long
    as it is made clear that the explanation being given IS a simplified
    look at things, and that later on - IF they choose to go further -
    they will learn more about what is really going on, I for one do not
    see the harm in starting out at this level, as opposed to effectively
    dumping a graduate-level solid-state physics texts in their laps and
    saying "there you go." That sort of approach, IMHO, does NOT
    serve the purpose of this newsgroup.

    All too often, we seem to have requests for such basic information
    posted by someone who is quite clearly an amateur/hobbyist, followed
    by a response by someone apparently interested primarily in
    demonstrating their own command of the intricacies of the topic
    in question, rather than actually saying something that the original
    poster would find helpful. I would like to respectfully suggest that
    such an approach is, to say the very least, idiotic.

    We now return you to your irregularly-scheduled chest-beating...

    Bob M.
     
  11. Hurray Bob! :)) I started to write something along these lines, but
    couldn't make it sound as nice as you just have. I think it's important
    to also note that everyone's model is wrong in the strictest sense of
    definition of correctness. They are all just models, and like all
    models, they serve to be a "reasonable" facsimile of the real and
    absolute truth; whatever that may be. They are not perfect or they
    wouldn't be called models. I'm reasonably sure that there are people
    that could come in and make a mockery of every model that has been
    presented here by demonstrating their shortcomings and failures. I
    suspect some of those people are actually present here reading this, but
    not wishing to add fuel to the fire.
     
  12. Kevin Aylward wrote:
    <snip arguments for and against>

    The fact that current through a bipolar transistor and a diode are
    described by basically the same equation points to the fact that a
    bipolar transistor is really just a diode, in which many or most of the
    electrons that would normally escape through the base are diverted to
    the collector by the geometry and chemistry of the device and voltage on
    the collector. Since a diode is clearly a voltage controlled device, it
    is clear that the voltage across the PN junction of the base to emitter
    that causes electron flow.

    However, saying that current through the base has nothing to do with
    this is just wrong. Because of the fact that a diode is involved, the
    only way to keep the voltage of the base up is to pull electrons out of
    it through the base lead. The ratio of electrons out the base lead to
    electrons out the collector is 'fairly' stable, enough to be printed in
    datasheets as the famous hfe or beta parameter.

    Because of this, the base current is yet another feature of the bipolar
    transistor that can be used to roughly predict the collector current,
    and thus to design circuits with. It is also easier to use, in my
    opinion, owing to the fact that the relationship between base and
    collector current is generally linear, within certain ranges.

    On the other hand, in my experience, the voltage to current equation is
    far more less use, since Is is not generally published in datasheets,
    and is severely temperature dependent to boot (just like beta).
    Predicting the current through the collector of a a 2N3904, given a base
    voltage, is practically impossible. Predicting the current through a
    2N3904 using beta is simple, if somewhat imprecise. By guessing a beta
    of 100, one can easily see that 10uA through the base will give about
    1mA through the collector.

    Both beta and EM have their place in one's toolkit. Why toss out tools
    that can be useful?

    --
    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.
     
  13. John Fields

    John Fields Guest

    ---
    Speaking of obtuseness, you seem to have (after all the ass-kissing)
    missed the part that was missing. That is, the role that Vce plays in
    generating the collector current.

    Kevin states that: "It is the electric field at Vbe that causes both
    base current and collector/emitter current"...

    While it's true that Vbe causes Ib to flow and is, of course, a
    contributor to the flow of Ic, the implication is that all of the
    collector current flowing through the base region is due solely to
    Vbe, and that with a given Vbe and no Vce, the emitter current will be
    what it would have been had the collector been connected to a source
    odf voltage. In other words, the collector current which would
    normally have been making its way to the collector because of the
    attraction of Vce drawing it away from the base will now make its way
    to the base because of the lack of Vce. Such is not the case.


    Consider this case with Ic flowing:

    183mA--> 2N4401
    [HP6285A]-----[FLUKE 8060A]----------C E-----GND
    B
    1.8mA--> |
    [HP6216A]-----[WAVETEK 27XT]-----------+




    and this, with no Ic flowing:

    2N4401
    C E-----GND
    B
    15.1mA--> |
    [HP6216A]-----[WAVETEK 27XT]-----------+


    Both power supplies were set up as voltage sources with no current
    limiting and adjusted to give the readings obtained in the upper
    example. The collector supply was then disconnected from the
    collector and the reading shown in the lower example was obtained.
    About a tenfold increase in base current because the collector current
    was no longer causing a voltage drop across the base-emitter diode,
    but certainly not the hundred-fold increase one would expect if Vbe
    were the sole contributor to the cause of Ic.



    BTW, since I find you mildly annoying I decided to take a look at your
    posting history to see what you're about, and I found that other than
    about the 20 posts archived at Google (where you also seem to have an
    attitude) you seem only to have posted here, so welcome to the swamp.

    While looking, I found this rather interesting post on this NG:

    <QUOTE>
    Um, yeah, but they drop voltage _according_ to current! If they're
    dropping 0,7V., they're not passing much current! Diodes are a crap
    way to drop voltage unless the load is light and predictable!!!

    miles
    <END QUOTE>

    which seems to belie your claim that you're an electrical engineer
    with 35 years of experience.

    Continuing on, we find, from Danny T:

    <QUOTE>
    http://www.kpsec.freeuk.com/components/diode.htm

    says

    "There is a small voltage across a conducting diode, it is called the
    forward voltage drop and is about 0.7V for all normal diodes which are
    made from silicon. The forward voltage drop of a diode is almost
    constant whatever the current passing through the diode so they have a
    very steep characteristic (current-voltage graph)."

    --
    Danny
    <END QUOTE>


    To which you replied:

    The forward voltage drop is entirely dependent on temperature (the
    junction temp. of the p/n junction; which is in turn dependent upon
    the current passed.) Higher currents equals higher temp. equals lower
    voltage drop. It's a well known effect which can eventually destroy
    the diode altogether. The physics of diodes is actually more complex
    than a lot of texts would have you believe.

    ---
    Indeed, but if you think the forward voltage drop is _entirely_
    dependent on temperature, you seem to have missed reading some of the
    more fundamental ones.

    For example, while it's certainly true that the voltage across the
    junction can be described by:


    kT / If \
    Vf = ---- ln ( 1 + ---- )
    q \ Ir /

    and that when T is equal to zero at 0°K, Vf will be 0, you've
    neglected to mention that current passing through the bulk resistance
    of the diode, at any temperature, will cause a drop across the
    junction which is dependent on the resistance and the charge flowing
    through the diode.

    More importantly, perhaps, you pooh-pooh'd Danny T's idea to use a
    diode as a bad one merely because of your opinion, which was
    erroneous. Diodes are _often_ used as voltage dropping elements in
    the real world because of the small change in Vf caused by If.
    Moreover, your example of the negative TC of a diode destroying it
    would more closely describe a diode with a voltage source connected
    across it allowing the diode to get into thermal runaway. Such a
    condition would not happen with the load limiting the current through
    the diode and the diode sized to carry the required current under the
    required environmental conditions. Furthermore, depending on the
    diode, above a certain current the tempco becomes positive, something
    else you "neglected" to mention.

    What's most disturbing, however, is that with Danny T admittedly being
    a newbie and asking for information, you deliberately sidestepped the
    issue when he presented you with the [valid] information he found
    which supported Andrew Holmes' suggestion to use a diode in order to
    keep from having to admit that you were wrong in stating that: "Diodes
    are a crap way to drop voltage unless the load is light and
    predictable!!!

    For shame, sir! :-(
     
  14. John Fields

    John Fields Guest

     
  15. Not in this context it isnt.
    No its not stable. Hfe varies all over the place. In contrast, the gm
    equation of a transistor is the same for all transistors.

    Any decent design has to be done such that the huge variations in hfe
    are overcome.
    No it most certainly cant be used to predict the collector current. For
    switching circuits it is used to calculate the *minimum* base current
    required, with the collector current being set by the collector loop not
    the base loop.

    For ac circuits, hfe is designed out.

    Ho hum...you don't do much transistor design do you?

    This is a non starter.
    Oh dear...in your experience...well, yet again, at the risk of sounding
    arrogant, my experience is rather more extensive then yours in the
    design of complex transistor level circuits. I have been professionally
    designing very large transistor count i.c. and board level circuits for
    well, some time now.

    The gm equation is absolutely indispensable. It forms the heart of
    serious transistor level design. For example, see
    http://www.anasoft.co.uk/EE/bipolardesign1/bipolardesign1.html

    e.g.

    re = 1/40Ic

    Av = Rc/re

    max gain Av=Va/Vt

    or the design of current mirrors, multipliers, bandgap voltage
    references. The list is truly endless.
    Sure, if you just apply a raw voltage for DC conditions, but one doesn't
    usually do this. For ac conditions, ic= vb.40.Ibias
    Oh dear, this approach is useless for anything but switching circuits,
    and as noted above, it doesn't determine the collector current. This is
    shown here
    http://www.anasoft.co.uk/EE/bipolardesign3/bipolardesign3.html
    No one is claiming that beta has no use, one is pointing out that the
    bipolar transistor is a voltage controlled device.

    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.
     
  16. Miles Harris

    Miles Harris Guest

    Oh, your assignment question? Listen, Junior, that's for your teacher
    to explain to you. I owe you no such favors. Stick with your studies,
    though; you may make it one day - if *you* work through your own
    homework instead of trying to trick others into doing it for you.
     
  17. Miles Harris

    Miles Harris Guest

    "Of course they're wrong, but so what?"????
    The mind boggles.
    I do appreciate the dilemma, though. On the one hand we'd like to
    convey a complete understanding of the subject to the questioner. On
    the other hand, we suspect that if we did so, they'd find it all too
    much, be turned off and simply find another hobby to pursue.

    Perhaps the answer is to provide the simpler explanation, based on the
    questioner's level of knowledge, but spell out the caveat that there
    is more to the topic than has been explained in the follow-up. IOW,
    tell the questioner that the answer provided is sufficient for their
    current purposes, but they may need to take more on board as they
    advance in their studies.
     
  18. Miles Harris

    Miles Harris Guest

    Translation: "My ass can't take no more whuppin' so I'm outa here!"
     
  19. Miles Harris

    Miles Harris Guest

    On Wed, 12 Jan 2005 15:20:16 -0600, John Fields

    [snip pointless, uncalled-for lecture on base current but maybe some
    newbie can make use of it]
    I've been posting to Usenet for many years, but normally use
    X-no-archive or my nickname or whatever. Some of the views I put
    forward on political matters are sadly not regarded as acceptable
    these days, so when needs must....
    Really? So what is it you're taking issue with? Taken as a
    generalization for most silicon diodes I can't see real problem with
    it.
    It can hardly be described as "disturbing" FFS.
    Excuse me for not having the time or inclination to wade through your
    entire post and answer every individual point; but if you're trying to
    suggest that I'm as guilty as you in giving 'easy' and expedient
    explanations to newbies that don't reflect the full picture, then I
    guess I'd have to plead guilty. None of us are perfect.
    If you don't like it, sue me.
    :p
     
  20. Miles Harris

    Miles Harris Guest

    You've pre-empted my considered view on the matter (see my posting of
    earlier today further up the thread).
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-