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JFET on resistance?

Discussion in 'Electronic Basics' started by Eeyore, Apr 11, 2009.

  1. Eeyore

    Eeyore Guest

    Measure one ?

    Graham
     
  2. Eeyore

    Eeyore Guest

    Can't you get samples ? I've rarely had a problem for a genuine enquiry. The
    only company that gave me trouble actually was IR who said 10k devices p.a.
    wasn't enough.

    Graham
     
  3. Eeyore

    Eeyore Guest

    On resistance is measured at Vgs = 0. Which gives @ 5V Id = ~ 0.07mA ( Fig 3 )

    Roughly 70 k ohms.

    What do want it for ? I can get you 8 ohms if you want.

    Graham
     
  4. Jamie

    Jamie Guest

    Well maybe you're looking at a generic JFET which is on and you must
    turn it off. If you can't find what your looking for, look for the pinch
    off voltage.

    Unless we're talking about enhanced mode here?

    http://webpages.charter.net/jamie_5"
     
  5. tempus fugit

    tempus fugit Guest

    Hi;

    Is there a way to figure out the R(on) of a JFET when this parameter isn't
    specified in the data sheet?

    Thanks
     
  6. tempus fugit

    tempus fugit Guest

    I mean before I place the order.

    Thanks
     
  7. tempus fugit

    tempus fugit Guest

    Thanks John. I was thinking there must be some way using Idss, but I wasnt
    sure which V to use. So for example from this data sheet:

    http://www.fairchildsemi.com/ds/FJ/FJN598J.pdf

    it would be 1.5V/350uA = roughly 4.3 megohms.

    I must be doing something wrong. Can you set me straight?

    Thanks
     
  8. tempus fugit

    tempus fugit Guest

    Wow thats awful..

    I'm looking for a JFET to switch a guitar signal in and out of effects
    pedals. I'll need 8 in total, but the signal will only ever be passing
    through 4 JFETS at any 1 time. I am going to bias the drain and source at
    1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
    using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
    will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
    completely off to isolate the effects from the signal chain when I want them
    bypassed. I realize the Ron doesn't need to be extremely low (although the
    lower the better, I suppose) but I also need the lower Vgs off. I'm trying
    to find a nice balance between those parameters.

    Thanks
     
  9. tempus fugit

    tempus fugit Guest


    I'm trying to get the switching to be click free, and I've tried a MAX4622
    CMOS switch, with no success. I tried using an MPF 102 and the switching was
    much quieter (only a tiny thump in one direction), and I've read that it's
    easier to ramp up the control voltage on a JFET than it is on CMOS.
    Which has led me to try ordering up a bunch of suitable JFETs to see if I
    can get them to behave.

    Thanks
     
  10. Eeyore

    Eeyore Guest

    5V is really too low a pinch-off voltage when you consider some guitar pickups
    can deliver volts of output.

    Also, there tends to be a relationship between a higher pinch-off voltage and a
    lower on-resistance.

    Typical jfets used for switching audio are the J09 and J175 families.

    Graham
     
  11. Eeyore

    Eeyore Guest

    Actually I did seem I dropped a decimal place somehow. I get ~ 6k now. With Idss
    as low as 100uA it's always going to be a high resistance device. Note the
    application. Capacitor mics etc. It's designed for low current consumption.

    Graham
     
  12. Eeyore

    Eeyore Guest

    Possibly, at insane prices.

    Turn-off time is quite long.

    Graham
     
  13. Eeyore

    Eeyore Guest

    You'll almost certainly need a seperate supply to drive the things off or the
    audio may simply break through by modulating the gate-source or gate-drain
    voltage. Beware, pinch-off voltage can vary widely too.

    Graham
     
  14. Eeyore

    Eeyore Guest

    The shunt / series configuration is very good but if you happen to have a higher
    voltage handy for gate drive, series only will normally give you adequate isolation.

    Graham
     
  15. Rich Grise

    Rich Grise Guest

    If it's not fully spec'd, don't buy it.

    Good Luck!
    Rich
     
  16. tempus fugit

    tempus fugit Guest

    Actually I breadboarded my design using an MPF 102, since it was all I had
    lying around. Unfortunately, it has a Vgs off of 8V, so it didnt provide
    great isolation when it was off. You're suggesting a MOSFET also? I've got
    some BS170s lying around so I'll do some work with them.

    Thanks
     
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