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N channel Fet v npn

Discussion in 'Electronics Homework Help' started by Rajinder, Feb 27, 2018.

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  1. Rajinder


    Jan 30, 2016
    Hi all
    I was after some help regarding n channel fets v bipolar npn.
    I want to conduct a simple experiment so I can understand the differences when using these transistors as switches.
    I want to switch an led powered from 9V using both transistors.
    So basically 2 different circuits.
    One with an fet and the other with a npn.
    I want some help with the mathematics.
    I know the npn will need a base resistor and collector resistor. I want to use 3v to switch the transistors on.
    I think for the fet I need a gate voltage greater then the threshold voltage and I might need a logic level fet.
    Can anyone help?
  2. Kabelsalat


    Jul 5, 2011
  3. kellys_eye


    Jun 25, 2010
    Switching 'on' resistance in MOSFETs can be considerably lower than any bjt device with a resultant low power dissipation in the device (less requirement for a heatsink).

    Switching speeds may be an issue as gate capacitance needs to be considered.

    These days there are MOSFETs for practically every switching application and bjt's are increasingly relegated.
  4. duke37


    Jan 9, 2011
    You will need a resistor in series with the led to set the current. Take the voltage supply, subtract the led voltage and the voltage across the transistor and calculate the resistor to get the current required.

    A power fet will have a very low resistance and hence voltage drop if turned on fully.
    A bjt will have a voltage drop of up to about 1V depending on how much base current is supplied.
  5. Alec_t


    Jul 7, 2015
    If you have only 3V to control the FET gate then the FET must be a logic level type with a Vgs(thr) well below 3V to ensure the FET turns on fully.
  6. BobK


    Jan 5, 2010
    Did you mean MOSFET, as everyone seems to assume or did you mean FET. They are two different things.

    FET usually refers to a junction FET or JFET. A typical N-channel JFET requires a negative voltage to turn it off.

  7. Rajinder


    Jan 30, 2016
    Yes i meant a n channel FET.
    I just want to know if i have a logic Mosfet n channel controlled from a PIC I/O line.
    What do i need to consider for the gate drive? How do i control ID drain current to light the LED?
    I was thinking off puttibg a resistor to the gate pulled to 0V. So it doesnt turn on when firing the PIC up.
    But how do i control different amounts of drain current.
    Thanks in advance
  8. duke37


    Jan 9, 2011
    "But how do i control different amounts of drain current."

    I gave this information in #4.
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The answer (for all of these devices) is "place the correct voltage on the control terminal."

    However, that voltage will vary between devices (even if the same type), change with temperature, and be vastly different between devices of different types.

    Typically, you either use the device as a switch or employ feedback to ensure the exactly correct voltage is applied.

    The simplest way is to bias the device so it can allow a much larger current than is required and use some external device to limit the current (it may be as simple as a resistor).

    If you are using a jfet, be aware that these are depletion devices and tying the gate to the source will allow the current defined as Idgss to pass through the device. This is typically the maximum current for the device.
  10. Rajinder


    Jan 30, 2016
    If i drive it greater than vgs does that mean more drain current?
  11. Alec_t


    Jul 7, 2015
    If Vgs is only at the threshold, Vgs(thr), then only a few microamps of drain current flow (check the datasheet for the actual value). Higher Vgs = more current.
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