Help understanding project :(

Discussion in 'The Projects Forum' started by Engin_Nerd, Apr 7, 2011.

  1. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    0
    I've attached my schematic and basically, I'm trying to simulate a micro signal going into the Gate of a mosfet (Vg = 5vdc or 0vdc). R_load and C_load is the load impedance where i measure Vout. I need to determine what values I need for R1, R3, C2, and if I need any other components to be able to ensure that when the circuit is switching on, the Rise Time for Vout to transition from .5v to 3.7V will be 40us ...and that when the circuit is switching off, the Fall time from when Vout transitions from 3.7V to .5V will be 20us.

    I'm not sure how to choose my values or how I will know if I need more resistors/caps.

    Also, can anyone suggest a good Mosfet for this application.

    Thanks in advance.

    mos.jpg
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    You might notice on the data sheet that the threshold Vgs is 2 - 4 volts. With 5 volts applied, it will have some conduction, but less than full conduction. You need to be able to increase Vgs to at least 10 volts.

    When the FET switches on, the voltage between R2 & 3 will get pulled towards ground.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Their are logic MOSFETs, but the big problem is all MOSFETs handle the in between parts of on/off poorly.
     
  4. russpatterson

    Member

    Feb 1, 2010
    351
    16
    I've been using the IRL3714ZBPF. It's not the cheapest MOSFET but it's logic level, low RdsOn and handles a lot of current. Check it out.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I suspect the OP is trying to use a MOSFET as a linear amplifier. This can be done, but there are better components out there for linear jobs (including BJTs).

    MOSFETs are generally used only in the on or off modes, as their conductances while on are superior to most everything else. This means they don't generally get very hot when they are carrying a lot of current.
     
  6. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    0
    Thanks for the feedback. Is the 2N7002 a good choice as well for logic level applications? I'm going to look up the IRL3714Z BPF

    Also, how would I simulate or calculate the input impedance (impedance going into R_load and C_load) at 0Hz.

    I think the 0Hz is throwing me off because this is a DC circuit with the FET being switched by a logic level 5v.
     
    Last edited: Apr 24, 2011
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Impedances are determined by frequency. At DC, a capacitor is pretty close to an open circuit. An ohmmeter will give the DC resistance.
     
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