FET as VVR

Discussion in 'General Electronics Chat' started by SSij, Nov 2, 2014.

  1. SSij

    Thread Starter New Member

    Oct 27, 2014
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    How does FET work as Voltage Variable Resistor?
    I would also like to know about working of Automatic Gain Control(AGC).
    Is there any other application of FET which uses its ohmic characteristic region?

    Thank you.
     
  2. SSij

    Thread Starter New Member

    Oct 27, 2014
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    I am also confused with the conventional current flow direction in FET . Please help me
     
  3. blah2222

    Well-Known Member

    May 3, 2010
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    A quick Google search led me to this article that does a great job answering your questions: http://www.vishay.com/docs/70598/70598.pdf

    The article is specific to JFETs, as they are commonly used as voltage controlled resistors, and that is what I assumed you were talking about.

    JFETs are symmetrical devices with conventional current flowing from drain to source (electrons flow in the opposite direction).
     
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  4. crutschow

    Expert

    Mar 14, 2008
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    That's true for N-channel JFETs.
    The current would flow in the opposite direction for P-channel JFETs of course.
     
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  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    It doesn't; it acts like a FET.

    At any given Gate to Source voltage, the Drain to Source current plotted vs Drain to Source voltage is anything but linear, which it would have to be to act like a resistor.
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    There is an old application note by National Semiconductors on JFETs. Amongst other uses it shows how to make them acting as variable resistors.
     
  7. MikeML

    AAC Fanatic!

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    The V/I curve of a JFET is not like that of an NFET or PFET.
     
  8. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Hola Mike. Is the above a reply to my post?

    Just in case, I was wrong. It was not National Semiconductors; it was Siliconix, year 1997.

    Here it is.
     
  9. ramancini8

    Member

    Jul 18, 2012
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    There is a circuit configuration that use a JFET as a variable resistor in an AGC application.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I dont dispute that a JFET can be used to emulate a voltage-controlled resistance, albeit only for small signals. I am saying the Drain to Source current/voltage characteristic of an NFET or PFET is different, and that makes those unsuitable for this purpose.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    The JFET is just a bar of silicon with a P/N junction alloyed onto it - if you increase the reverse bias on that junction, the electric field spreads the depletion region further across the bar of silicon, eventually you reach a condition known as "pinch off" - which is just what it says, the electric field/depletion region has pinched off the conducting channel.
     
  12. blah2222

    Well-Known Member

    May 3, 2010
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    Good catch, thanks! It's so rare that I use a PFET, that I got ahead of myself.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    Lately it has become fashionable to use MOSFETs the wrong way round to serve as polarity protection diode.

    If you put the MOSFET in the supply lead such that the body diode is conducting, it serves to prevent reverse supply - but like any diode, it has a Vf drop.

    However if you bias the gate with the correct polarity to the channel, the channel conducts with lower volt drop than the diode Vf.
     
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