Mosfet gives linear results???

Discussion in 'General Electronics Chat' started by rougie, Dec 20, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    2
    Hello,

    A simple thought!!!!

    I am using the 2N7000 mosfet.

    I am varying the input voltage from 2V to 4V which allows 0 to aproximately 35ma through the mosfet's drain to source.

    Can it be that the variations of the input voltage vs output current relation is linear instead of exponential!!!

    Can this happen due to the fact that I am using a very small part of the exponential curve in that to me it seems linear?

    thanks for all feedback!
    r
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Why do you think it's linear? You only have two data points!
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    well, i am varying from 2-4V but within this range
    I have logged down 8 coordinates for voltage/current. The graph seems to be
    linear. Well okay I actually logged the measurements of voltage
    to distance perceived by my sensor... (on a v/inches graph).
    And this does not seem exponential at all!
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    You shold log the output current vs input voltage. There is no distance. You should not find an exponential function. The device should obey the square law for transconductance. Make your measurements from cutoff through the entire active region.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    4,802
    You need to provide the circuit you are using. The behavior will be determined by the intersection of the MOSFET's loadline and the loadline of the rest of the circuit.
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    okay... then it's not a parabola... it's more of a square
    so ignoring the load .... so to speak, for ex:

    so 1v over the threshold = 1^2 ma = 1ma
    so 2v over the threshold = 2^2 ma = 4ma
    so 3v over the threshold = 3^2 ma = 9ma

    and so on... is this correct?
    thanks
     
    Last edited: Dec 21, 2012
  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Someting like that. The output current is proportional to (vgs - vt)^2. It's equal to that times a constant factor in the active mode. There should be lots of sites that gives the equation.
     
  8. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    yes I have seen that equation on various sites.

    thanks for replying brownout.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    When you gave your data you referred to the voltage as being the "input voltage". Is this the "gate-source" voltage on the transistor, or just the gate voltage relative to ground? It makes a huge difference, which is why it would be helpful for you to provide a sketch of your circuit.
     
  10. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    2
    the gate voltage relative to ground?
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    Voltage is always a measurement between two points. When you say that the gate voltage is 4V, what is that the voltage between? The gate and what other point in the circuit?
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You need to post a schematic of how you are measuring the voltage AND current. Show the transistor, the power supply (with its voltage), the ammeter, and the voltmeter, and any other components in the circuit (resistors, etc.).
    The make and model of your ammeter is also important.
     
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