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Bipolar vs FET: Def. and when to use one or the other?

Discussion in 'Electronic Components' started by default, Mar 10, 2007.

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  1. default

    default Guest

    It really depends on the application. The MOSFET is hard to beat for
    efficiency and power handling capacity in many cases. Without getting
    into actual circuit applications, which one is more suited is not
    clear - then you inevitably end up comparing two specific devices
    which can also impact your choice.

    The downside of the mosfet may be the current required to switch the
    gate capacitance rapidly for high frequency applications, or the way
    they fail with inductive transients that a bipolar will ignore - they
    require a little more protection in some cases.
  2. In school (if memory serves) I learned that a bipolar transistor is a
    current-controlled current "amplifier", and that a FET is a
    voltage-controlled current "amplifier". Replace "amplifier" with "switch" to
    complete the 2x2 matrix I learned about most uses for active devices.

    Is this -- on the basic level -- true?

    Without going into biasing configurations, when would I want to use a bipolar
    and when, for example, a MOSFET? I know FETs have high input impedance, so
    drive current is miniscule. Other reasons for using one or the other?

    Just trying to refresh my understanding of semi's.

  3. Eeyore

    Eeyore Guest

    Bipolars are cheaper than FETs !

  4. Lionel

    Lionel Guest

    <grin> Good answer!
  5. maxfoo

    maxfoo Guest

    Why do you want to know, is your tricorder broken again? I'm sure Scotty will
    fix it for you Bones...
  6. And you're making fun of his nym.....
  7. I am a Lostgallifreyan, but am I also a Doctor? Something to ponder if you
    have enough time...

    Re FET's, they are useful when you want a simple means to convert logic
    circuit voltage levels, and unlike BJT's (bipolar junction transistors),
    they can be used directly as voltage controlled resistors with extremely
    low minimum resistances, making them useful for shunt modulation of laser
    diodes, or as analog signal switches for AC or DC signals, or for power
    switching. They might cost more than BJT's but I think it's always worth
    considering them in new situations, on the offchance that they might do a
    job with a reduced part count. Sometimes they do, and so dramatically that
    the saving in parts, time and effort makes a FET extremely useful.
  8. chuck

    chuck Guest

    I don't know if this is a serious thread . . .

    On a more fundamental level, FET's will generally have higher (typically
    much higher) input resistance than BJT's, but in similar circuits, will
    generally provide lower voltage gains. BJT's are also vastly more robust
    in ESD space.

    If your interest is genuine, do a google search on something like "bjt
    vs fet" and see a lot more detail.

  9. Chris

    Chris Guest

    Hi, Doc. Here are a couple of differences between Bipolar Junction
    Transistors (BJTs) and MOSFETS when used as switches:

    * MOSFETS are voltage controlled whereas BJTs are current controlled.
    This can mean the difference when your output logic can't source any
    real current (such as with 4000-series logic gates -- they can
    typically only source 1/2mA at a Vdd of 5V). But even if you're
    driving the BJT with an HC output (4mA, 20mA, or 64mA depending),
    there's a limit on the current you can switch directly. It's best
    with saturated switching to supply 10 times the current the transistor
    current gain would suggest in order to achieve minimum Vce
    (saturation). So, let's say you've got a TIP power transistor with a
    DC current gain of 100 (typical). In order to switch 1A, the beta
    spec suggests you could supply 10mA of base current. But in order to
    saturate the transistor, you'd have to supply ten times that, or
    100mA. No logic gate can do that, so you'll have to add a second
    transistor to boost the base current. Needless complexity. Of
    course, you could use a darlington BJT transistor (with a DC current
    gain of 1000), but then Vce(sat) will be at least 1V, which might
    result in too much power disspation.

    * MOSFETs may require a higher gate voltage in order to turn on than
    your logic gate can supply. You should be careful with this -- even
    many "logic level" MOSFETs which allow use with a 5V gate drive may
    require something like 8V to 10V to achieve the miraculous Rds(on) in
    the data sheets.

    * Vds when the MOSFET is on is a simple product of Rds(on) times
    current. This can be a big advantage or a disadvantage, depending on
    the device and the current. Pd is the product of Vds and switching
    current. Ideally you want the switch power dissipation to be as small
    as possible.

    * MOSFETs tend to be more static sensitive than BJTs, and have a
    tendency to spit up and die unless you're a lot more careful with such
    things as circuit transients and momentary reverse voltages. In
    addition to being less expensive, BJTs have a reputation for being
    somewhat more rugged.

    I hope this has been of help with your homework.

  10. Jamie

    Jamie Guest

    Fets generally require higher voltages to bias on the gate.
    also FETS tend to act differently when it comes to current because
    a FET can actually act as a current regulator if you want it too.
    There are different types of FETs, JFET, MOSFET with enhanced mode
    and non enhanced mode (not common).
    JFets are on by default, you need -V (N-channel) bias voltage to turn
    them off.
    basically if you read the voltage specs. you might find a reference to
    pinch off voltage, this is the voltage that is required to turn it off.

    Mos in Enhanced mode works the other way around for gate control and is
    more favorable in many applications.

    Bipolar types are current control via the Base and Emitter current
    etc. the advantage there is it only takes an average of .6 volts between
    the base and emitter to get current flowing which governs the current
    flow from the Collector and emitter etc. There are other factors
    to consider but that is the basics. Fets require much higher voltages on
    the get but offer very high impedance with the cost of lots of
    capacitance verses the bipolar types.

    In R.F. circuits, the use of Fet's with their high gate capacitance
    plays a heavy roll in the resonating input circuit, that is why, when
    doing replacements you must get a fet that has gate capacitance close to
    what came out of there among other things.
  11. MOSFETs are easy to use in microcontroller based applications
    since they can be driven directly off the pins of many
    microcontrollers. The biggest single advantage that I have found for
    bipolars in my applications is their ability to block current in both
    directions. When using mosfets, the design gets more complicated when
    you have to protect against reverse battery connection.
  12. Conversely, FET's are better at withstanding ionising radiation. Weird.
  13. Forward based diode in series with power input? If the losses that causes
    are not wanted, wouldn't it be enough to put the diode across the supply
    terminals, reverse biased, after a fast fuse? If you used a Schottky diode,
    that could limit the FET's experience of reversal to a short surge of -0.4V
    or less. I don't know if any are vulnerable to that, but I suspect not.
  14. I hope this has been of help with your homework.

    Well, at 50-plus, I'm doing "homework" only assigned by myself.

    Thanks for your educational comments. (c:

  15. Rich Grise

    Rich Grise Guest

    His progenitors' names are Max Headroom and Foo Bar. ;-)
    (Or would that be "Bar Foo"?) ;-)

  16. Rich Grise

    Rich Grise Guest

    It's not that weird at all. A cosmic ray will introduce one excess current
    carrier. If that happens in a BJT's B-E junction, the resulting effect
    would be multiplied by beta (Hfe). In a(an?) FET, the current is
    controlled by expanding or contracting the electrical channel, and one
    current carrier more or less doesn't affect the controlled current all
    that much.

  17. Prajit

    Prajit Guest

    Compared to BJT....MOSFET has higher bandwidth.... higher thermal
    stability and also consumes less power....
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