do any linear mosfets exist!

Discussion in 'General Electronics Chat' started by rougie, Dec 15, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    hello,

    I currently using the pn27000 ( I bielive the
    part number is) bu the problem the
    vin vs I out is exponential.

    I was wondering if any mosfets exist
    that are linear ???

    thanks for all feedback
     
  2. crutschow

    Expert

    Mar 14, 2008
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    All MOSFETS have a exponential relationship between Vin vs Iout but you can obtain a linear relationship by adding an op amp in a constant-current configuration such as this.
     
    #12 likes this.
  3. #12

    Expert

    Nov 30, 2010
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    That is a really good example of how to use google to get exactly what you want.
     
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    hi crutschow,

    I haven't had yet a chance to dive into op amps, but Q1
    seems to be what would work for me. The exponential relation
    complicates things. But if I may ask you a question
    how does the op amp attenuate the voltage's input's
    amplitude so that it makes the MOSFET linear ....???

    Also, can I use an Lm324 as opposed to an OPA333?

    I should really put some time in testing op amps !!!

    anyways thanks so much for your help.
     
    Last edited: Dec 15, 2012
  5. #12

    Expert

    Nov 30, 2010
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    The basic concept of an opamp is that it does whatever is necessary to make both of its inputs equal. The opamp is responding to the voltage from the potentiometer and providing whatever is required to get its other input to the same voltage. In this case, the opamp is changing the voltage on the gate of the mosfet just enough to get it to pass enough current to get the right voltage across the load resistor.
     
  6. nepdeep

    Member

    Sep 14, 2011
    139
    0
    Now if I may ask you a question, where do you want to use mosfet in linear region....can you share the schematic please??
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The op amp doesn't really make the MOSFET linear, but it does make the output versus input linear as stated by #12. It uses negative feedback to make the non-linearity very small (basically divided by the open loop gain of the op amp).

    Yes, an LM324 should work for this purpose.
     
  8. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    After all the schematics i have posted and worked out
    with this forum during the months of August, Sept and
    Nov... I must admit I am a little exhausted.... You guys
    have been a tremendous help and very patient with me
    as I am not a very good hardware guy.

    I know if I post the schematic, there will always be something
    wrong with what I am doing. Right now though everything
    works as I need it. So I would rather go on with what I have
    in terms of hardware for now and continue my software
    part of the project.

    Rest assure though, I try every weekend to get an hour
    or two in on some electronics theory.... and will get back
    with schematics regarding my issues. For now not much
    time in my hands for this though... and therefore I try my best...

    thanks for replying...!
    r
     
    Last edited: Dec 16, 2012
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Very well explained #12. This gives me at least an idea
    of what's happening. During christmas holidays I will
    *try* to put some time in basic op amp theory...

    thanks again!
     
  10. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    okay.... thanks so much for your circuit... really appreciated

    r
     
  11. #12

    Expert

    Nov 30, 2010
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    Just a note...the first mistake noobies make with an opamp is in choosing the polarity of the output, and you choose that at the input. The opamp doesn't have the sense to know if it should hunt at a higher voltage output or a lower voltage output to get its inputs to equal each other. You are solely responsible for designing the feedback circuit and choosing whether the "+" input or the "-" input should be attached to the incoming signal. If you get it wrong, the output of the opamp will be "stuck" to one of the voltages you gave it from the power supply. Now, if you see this condition, you will know where to look.
     
  12. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    thanks for the warning #12

    r
     
  13. MKCheruvu

    Member

    Nov 20, 2012
    30
    6
    A MOSFET when used as switch will be operated in cutoff(OFF) - Vgs<Vt
    and Ohmic(ON)- Vgs>> Vt conditions . and when operated as Amplifier is operated in in current Saturation region - Ids = K{(Vgs-Vt) Square}/2 ie the Ids follows Square relation but in reality it gets raised to a value between 1 & 2.

    As an Amplifier Ids = Gm. Vgs, ie a small change in Vgs(input) will be magnified linearly by Gm times (to realize Ids of the MOS FET Current.)
    So a MOSFET when used as Amplifier(Ids=K Vgs) is used as Linear Element (with certain amount of built in nonlinearity due to second order effects)between input(vgs) and output (Ids)
     
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