connecting a JFET

Discussion in 'The Projects Forum' started by gerases, Dec 7, 2012.

  1. gerases

    Thread Starter Member

    Oct 29, 2012
    177
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    Hi,

    A basic question about an NPN JFET. The theory says that when the gate voltage is 0V, the source-drain current is at its maximum. And when more negative voltage is applied to the gate, the source-drain current decreases.

    If I have a regular power supply, say 10V, how do I make the gate more "negative" than 0V?
     
  2. mrmike612

    New Member

    Dec 1, 2012
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    0
    positive to ground
     
  3. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    sorry, don't understand. can you clarify?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,101
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    A simple way to get a negative voltage for testing is just to use a AA battery, battery + to power supply ground, battery - to the device, to drop an additional 1.5V at the device.

    I'm not commenting on your experiment.
     
  5. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    I get it. Interesting solution. What if I didn't want to involve a battery?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    You have an N-channel Jfet. It is not an NPN.

    To make it biased as an amplifier, simply connect a resistor in series from the source pin to 0V. The current in the Jfet creates a positive voltage on the source pin. Then when the gate pin is made 0V with a resistor to ground the gate voltage is more negative than the source.
     
  7. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Point taken.

    Can you give me an example with some numbers for me to see what more negative really means?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Maybe this schematic explains it.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    This is true of a depletion-mode JFET.

    An enhancement-mode JFET needs a positive gate-source voltage in order to create a conduction region.
     
  10. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Thank you for putting the numbers in. Remember though I'm a newbie and when I see something like that it seems very complicated to me. Can you explain to me how the base becomes more negative than 0 in this case?
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    With the gate at 0v, a depletion-mode FET already conducts current. Current flows through the source resistor. The source of the FET is at a positive voltage and hence is at a higher positive potential than the gate.

    The gate is not at negative potential. It is negative with respect to the source.
     
  12. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Thank you, I'm beginning to understand. So, if there are no resistors on the DS path, the JFET is fully ON. When you put a resistor on the source, the gate becomes more negative in relation to it. Beautiful.

    So, as soon as the drop across that resistor is 1.5V the DS current will stop in this case, right?

    Also, in the book I'm reading it says that JFETs are preferred to transistors in applications where high input impedance is needed and, it says, the transistor has low input impedance. What the heck does that mean?
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    A bipolar junction transistor (BJT) is a current device. The resistances across the junctions are relatively low and hence the currents are relatively high.

    A field effect transistor (FET) is a voltage device. The resistances across the junctions are relatively high and the currents are low. It responds more to voltages than to currents.

    We sometimes use impedance and resistance interchangeably.

    To be more precise, resistance should be used when we are referring to DC conditions whereas impedance should be used under AC conditions.
     
  14. gerases

    Thread Starter Member

    Oct 29, 2012
    177
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    I see. But what would be their use -- very simply?
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    What use are you referring to?
     
  16. gerases

    Thread Starter Member

    Oct 29, 2012
    177
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    I mean what would be their application so that it would require specifically a JFET and not a BJT?
     
  17. MrChips

    Moderator

    Oct 2, 2009
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    Do you mean where would you use a low impedance device versus a high impedance device?

    If you were using a high impedance microphone, you would want to use a FET amplifier, for example.
     
  18. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Ok, thank you all for your help!
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    A Jfet does not have a base, instead it has a gate.
    The current in the Jfet is reduced when the gate is more negative than the source.
    If the gate is connected to 0V through a resistor and the source has a positive voltage due to current in the source resistor then the gate is more negative than the source and the current in the Jfet is reduced.
    SEE?
     
  20. #12

    Expert

    Nov 30, 2010
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    Another way to put it is that depletion mode jfets are born "on". They always have what I call "idle current" when the gate and source are the same voltage. The datasheets call it "Idss" which means, "amps through the drain terminal, source shorted to gate". You basically can not bias a jfet to a higher idle current than it was born with.

    The gate of the jfet is actually a reverse biased diode junction. It, being off, has very high resistance. In the case of that J201 in the drawings, it is guaranteed to have not more than 100 picoamps gate current with -40 volts gate to source. That calculates to resemble 400,000 megohms, so don't bother thinking you're going to move much current in or out through the gate terminal.

    The way you use them as an amplifier is to design the circuit so the gate is more negative than the source, find that the idle current is less than it was at zero volts gate-to-source, and wiggle around in that region.

    If you find my style informative, there is a "Thanks" button in the lower right corner of this box. If you realize that I am stretching the limits of imaginative writing, don't throw rocks. They will damage your monitor.
     
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