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Help with N-channel JFETs and MOSFETs ???

Discussion in 'Electronic Basics' started by [email protected], Jan 24, 2005.

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  1. Guest

    I have been looking at some books on JFETs and MOSFETs and have some
    basic questions.

    One of my problems is with the N channel JFET and MOSFET. I don't know
    how to get the negative voltage levels at the gate to get the JFET and
    MOSFET to turn off. I think that with a P channel JFET or MOSFET you
    would simply set up a voltage divider to get the positive voltage
    levels desired.

    I am having trouble finding an example of a simple circuit with DC
    power supply, light bulb, N channel JFET or MOSFET, and actual method
    of controlling the voltage levels at the gate to adjust the brightness
    of the bulb.

    My main goal is to begin to understand the JFET and MOSFET, not to
    build a light bulb dimmer. I know that there are more efficient ways
    of controlling the brightness such as pulse width modulation. I think
    that seeing a circuit like the one discribed above would help fill some
    voids in my understanding of basic electronics.
    Any help would be greatly appreciated. Thanks
     
  2. John Larkin

    John Larkin Guest



    +12v-------+---------------+
    | |
    | |
    | |
    | bulb
    | |
    | |
    / |
    / d
    pot /<-------------g n-mosfet
    / s
    / |
    | |
    | |
    | |
    gnd--------+---------------+



    Start with this. Experiment with how it behaves, measuring the
    voltages and currents in each situation, as you turn the pot. Compare
    the results to the specs and curves on the mosfet datasheet. I'd
    suggest a general-purpose TO-220 power mosfet for experimenting. The
    pot can be most anything, 1K to 1M, doesn't matter.

    N-Mosfets turn on with positive gate voltage, and are off with Vg = 0.
    Jfets are different.

    Try this, come back with results, and we can discuss it if you like.
    There are some fun variants to play with once you get this under
    control.


    John
     
  3. The main difference between jfets and mosfets (aside from the fact
    that a diode junction isolates the gate of a jfet and silicon dioxide
    isolates the gate of a mosfet) is that most mosfets are enhancement
    types and jfets are all depletion types. Enhancement means that the
    fet is normally (gate tied to source voltage) off, and the gate
    voltage has to be positive with respect to the source (for N-channel
    devices) to make the channel conductive. DEpletion mode devices are
    on when the gate voltage is at source voltage, and turn on less as the
    gate voltage gets more negative than the source (for N-channel
    devices).

    When working with a single supply, it can be difficult to use jfets in
    simple circuits as common source switches, because you have no voltage
    available that is more negative than the rail you tie the source to.
    They work well as amplifiers, though, if you add a resistor between
    the source and negative rail, so that the channel current drops some
    voltage across that resistor, raising the source voltage a bit
    positive. Then the gate can be biased to the negative rail with a
    high value resistor or other path (e.g. transformer winding connected
    to the negative rail).

    Just remember that the gate voltage must always be measured with
    respect to the source voltage, regardless of how the fet is connected
    to other things. There are smaller effects based on gate to drain
    voltage, but the dominant control effect is gate to source voltage.
     
  4. Jamie

    Jamie Guest

    you know, i was going to offer a good example, but your explanation hit
    rite on the money for the basic reader. i just wish others could reduce
    them self's at times to help the newbie's. i try very hard to keep my
    replies as simple as possible in this NG's, hence the name of the group :)
    i remember years ago (aprox 28 ago now :)) trying to get a simple
    description of a JFET & MoSFET biasing theories. what i got mostly was
    those trying to throw their college education at me for which a good
    sum of them really didn't understand, all they knew was, that was the
    way it was.! :)
    even the instructors in our electronics shop wasn't exactly 100%
    sure of them self's in that area. they admitted it.
    simply put, experience is a very good teacher.
     
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