problem testing 2N5486

Discussion in 'The Projects Forum' started by flp2mm, Feb 20, 2015.

  1. flp2mm

    Thread Starter New Member

    Feb 18, 2015
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    I'm trying to build a very simple preamp using the 2N5486 transistor. Following this web page
    https://sites.google.com/site/hpftechllc/home/technical-diy-stuff/jfet-study
    instructions for testing the transistors. He says with 9 volts on the drain, gate grounded and a 3.3k resistor between the source and ground you should get about 2.7 volts between the source and the gate. I bought 10 of these and the range I am getting is 0.4 to 1.0 volts with most around 0.7 .
    He uses the surface mount. Mine has the pins. I checked the pin out several times to make sure I was hooking it up right.
    Why would I be getting such low values?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. DickCappels

    Moderator

    Aug 21, 2008
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    According to the Fairchild datasheet, 0.7V across 3.3k is about right.
     
  4. flp2mm

    Thread Starter New Member

    Feb 18, 2015
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    I haven't given up. I've been exploring the net trying to find explanations of the JFET operation that I could understand. I found several which allowed me to then return to the links bertus suggested and I think I've learned a thing or two. I have pages of notes and just a slight headache. Then I experimented with the preamp circuit trying various Rs and taking measurements. The supply voltage is 9v. This circuit has no Rd. Rg is 10M. I used resistors for Rs from 10 ohms to 10k ohms. The source-gate voltage went from .013 volts with the 10 ohm resistor to 0.69 volts with the 10k resistor. Current at the drain went from 1.29 mA with Rs at 10 ohms to 0.07 v with Rs at 10k. The preamp worked with every Rs value. In no place did I encounter a source to gate voltage of 2 volts. Then by mistake I took out the 10M Rgate and then I got the voltages he was talking about. 3.3 k giving me 4.1 v and 1.5k giving 1.9v
    Boy or boy have I learned a lot. Thanks bertus for poking me with the teachers ruler.
     
  5. flp2mm

    Thread Starter New Member

    Feb 18, 2015
    11
    0

    Well not all is well. I just discovered the preamp does not work unless I remove the Rg of 10M ohms. R1 on his circuit diagram

    https://sites.google.com/site/hpftechllc/home/technical-diy-stuff/quick-and-dirty-jfet-buffer-rev-2
     
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