Question about MOSFET acting as amplifier

Discussion in 'General Electronics Chat' started by tadm123, Nov 30, 2013.

  1. tadm123

    Thread Starter Member

    Nov 20, 2013
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    I'm reading that it acts like it in Saturation, but it's confusing to me since in Saturation there's no slope, rather the curve it's almost a straight line. While in Triode mode there's an upward slope that can serve to amplify the input signal that you put in the gate.

    Here are the characteristic curves.

    http://macao.communications.museum/images/exhibits/2_10_4_4_eng.png


    So my question is... In what region does the MOSFET operates as an amplifier? Thanks in advance.
     
    Last edited: Nov 30, 2013
  2. Jony130

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    Feb 17, 2009
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  3. LvW

    Active Member

    Jun 13, 2013
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    Correct - there is no (in reality: a slight) slope.
    However, please note that you are speaking about the ID=f(VDS) characteristics.
    Do you intend to use VDS as input signal to vary ID? No!
    The slope you are expecting to verify the amplifying characteristic can be seen in the ID=f(VGS) curve (input-output).
    In contrary, the slight slope of the output characteristic curves indicate the desired current output properties of the device (large dynamic resistance).
     
    Last edited: Nov 30, 2013
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  4. tadm123

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    Nov 20, 2013
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    I just noticed this, thanks for pointing it out it all makes sense now.

    Thanks a lot guys, I really appreciate it.

    Let's see if what I'm gonna say makes any sense now: Since we never intened to use VDS as input signal, because VGS is the control voltage, which is pretty obvious. We want the MOSFET in Saturation because that's the only mode where VDS value doesn't affect the amplification of the ID signal at all since it's almost exclusively dependant on VGS.
    Hence in Saturation, the more VGS we put in the more amplification we'll have. Am I correct?
     
    Last edited: Nov 30, 2013
  5. LvW

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    Jun 13, 2013
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    No - i depends on the MOS type you are using.
    The VGS dc value determines the operating point on the Id=f(VGS) curve.
    And it is the SLOPE of this curve (at this point) that determines the gain.
    For example, a depletion type PMOS transistor provides less gain (smaller slope) for rising positive VGS values (until the device is off for VGS,pinch-off).
     
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  6. tadm123

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    Nov 20, 2013
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    i see, so if we want to have a higher amplification we have to get an NMOS with a higher threshold votage then?
     
    Last edited: Dec 3, 2013
  7. crutschow

    Expert

    Mar 14, 2008
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    Not necessarily. You want a large transconductance (Gm) which is given in the data sheet.
     
  8. tadm123

    Thread Starter Member

    Nov 20, 2013
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    Thanks, looking around I just found the formula the overall gain, so yep it appears we want a large gm to have a large gain:

    Av= -Rin/Rin+Rs * gm (RD || RL || ro)
     
    Last edited: Dec 3, 2013
  9. LvW

    Active Member

    Jun 13, 2013
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    Normally, such a formula says nothing - if you don`t explain the meaning of the various symbols. OK - some are self-explaining (like RD perhaps).
    Nevertheless, do YOU know the meaning of the symbol ro ?
     
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