Need help trying to match some J107 JFETs by Idss

Discussion in 'General Electronics Chat' started by foxfire3, Nov 21, 2014.

  1. foxfire3

    Thread Starter Member

    Jan 29, 2013
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    I've been trying to match some J107s by Idss using the attached circuit, but I get varying and somewhat inconsistent results.

    The datasheet states a Idss of 100mA at 15V Vds using a "pulse test" I assume to help control thermal changes.

    Can someone suggest any changes to this circuit or another circuit that may work?

    Thanks...
    Measuring_N_JFETs2.png
     
  2. foxfire3

    Thread Starter Member

    Jan 29, 2013
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    Thirty nine views and no one here knows how to match higher Idss JFETs??
    Interesting...
     
  3. ScottWang

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    Aug 23, 2012
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    Using a 1MΩ or upper corss between G and Ground, and in series with an 0.1 uf to input signal, and remove the Rs, changing Rd and try again.
     
  4. Jony130

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    Feb 17, 2009
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    Hi, to measure JFET Idss you can use this simple circuit

    2.PNG

    You use the first circuit if you want to measure Vgs(off). And you use the second circuit if you want to measure Idss. In practice all you need to measure Vgs(off) or Idss is to switch your multimeter between voltmeter/ammeter.

    And tell me why you want to measure Idss in the first place?
     
    Last edited: Nov 23, 2014
  5. Papabravo

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    Feb 24, 2006
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    I was going to say that matching Idss is something design engineers seldom do because the things we design are supposed to work over a wide range of Idss values.
     
  6. foxfire3

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    Jan 29, 2013
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    Matching devices results in much less DC offset in audio circuits that are DC-coupled. That's why it's important to me.
     
  7. foxfire3

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    Jan 29, 2013
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    With these simple circuits, the device heats up quickly and the readings start to drop like a rock.
     
  8. ronv

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  9. Papabravo

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    Feb 24, 2006
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    It's a poor long term solution. Even if they start out matched they seldom stay that way because the surrounding components age differently and it all falls apart. I'll bet you're one of the golden ears too.
     
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  10. foxfire3

    Thread Starter Member

    Jan 29, 2013
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    If you use quality resistors(Dale/Vishay) around the FETs in a cascode configuration, offset remains stable for a long time.

    What exactly is a "golden ears"?
     
  11. GopherT

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    Nov 23, 2012
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    Golden ears - aural aurum. Sounds good together. However, a more appropriate but less poetic name would be oxygen-free copper ears.
     
  12. foxfire3

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    Jan 29, 2013
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    If the term "golden ears" is implying sarcasm in some way, I can do without it!

    I'm not familiar with this forum, but if we want to start stereotyping people and playing name- calling games, then I'm certainly up to the challenge.
     
  13. ronv

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    I suppose in a way they are making fun of someone who would match components in favor of ac coupling. But hey, they haven't seen the circuit, maybe it makes perfect sense.
     
  14. Papabravo

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    "Golden Ears" is a term for audiophiles who make extraordinary claims about their ability to hear things that few other people are able to duplicate. They go to extraordinary lengths to match components and pay big bucks for things of allegedly superior quality. Trouble is most people can't distinguish the things they can. It is either a blessing or a curse depending on your point of view. It was not meant to be sarcastic, just a characterization of someone willing to go to extraordinary lengths for elusive gains.

    My real point is -- why not pursue a design that does not require matching components?
    Is it your contention that no such thing exists?
     
  15. foxfire3

    Thread Starter Member

    Jan 29, 2013
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    There are many circuits requiring matched devices on the input stage...ever heard of Nelson Pass and studied any of his relatively simple designs?

    Anyone can stick a coupling capacitor(s) in a circuit to block the DC.
     
  16. foxfire3

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    Jan 29, 2013
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    Point taken...I'm not one of those people who believes they can hear differences in wires or cables and other snake oil theories.
     
  17. Papabravo

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    I'm not familiar with Neson Pass. The wiki on him leads me to believe that he is one of the "Golden Ears". Nelson is entitled to his opinions, just the same as you and me. Ever heard of Wadia, or Audio Research?
     
  18. Jony130

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    Simply use DC offset circuit or add a pot in your amplifier to bring DC-offset to 0V.
     
  19. ronv

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    They make some real nice op amps.

    The 5532 is not in the forefront for DC accuracy, though it's not actually that bad. The offset voltage spec is 0.5 mV typical, 4 mV max, compared with 3 mV typical, 6 mV max for the popular TL072. I have actually used 5532s to replace TL072s when offset voltage was a problem, but the increased bias current was acceptable.
    With horrible inevitability, the very popularity and excellent technical performance of the 5532 has led to it being criticized by subjectivists who have contrived to convince themselves that they can tell op-amps apart by listening to music played through them. This always makes me laugh, because there is probably no music on the planet that has not passed through a hundred or more 5532s on its way to the consumer.

    "The best cap is no cap" is claimed by some. I would much prefer to ensure that no DC flowed where it is unwelcome by using a cap than allow a fully DC coupled system to try to destroy speakers given the chance. Perform all the blind tests you can with capacitors used in real circuits. Having done this, if you still think there is a difference (and can demonstrate it to others in a double-blind test), then you will probably be the first to do so.
     
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  20. Papabravo

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    I personally prefer the Class D listening experience.
     
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