PJFET circuit symbol

Discussion in 'General Electronics Chat' started by CD-RW, May 13, 2011.

  1. CD-RW

    Thread Starter Member

    Feb 26, 2011
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    Hi. I have a circuit that uses a P Channel JFET - a 2N3993 in a TO72 case.

    Just a few questions really.

    I have added a screenshot of the circuit diagram symbol, but I'm not sure which is the Source lead, and which is the Drain. One of the channel wires is crossed with a bar. Does this mark the S or D lead please? I have done a Google image search for this but cannot find a definate answer.

    Also is it correct that for a P-JFET the S and G are connected to positive, and the D to negative?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What program uses that symbol?
    I know it's nice to get the symbol placed properly, but almost all JFETs are symmetrical, so source and drain are interchangeable. The circuit will work the same either way.
    For PJFET amplifiers and digital switches, the source connects to the more positive voltage, and the channel is controlled by the gate to source voltage. JFETs can also be used as analog switches, in which case the source-to-drain voltage can change polarity, but is generally on the order of millivolts.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Wow, where did you find THAT part? I can only find distributer listings from obsolete houses, and the only data sheet assumes a "standard" wiring for a TO-72.

    A p channel J FET is full on for Vgs=0, and conducts less with a negative Vgs until the threshold voltage is reached and the device is off.

    One curious property of JFETs is the drain and source are typically interchangeable. The last job I used them on we use them in bare chip form, and the pads were named D/S and S/D, and of course G.

    I have no idea what the slash on your part symbol means.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The gate cutoff voltage is positive relative to the source.
     
  5. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    @RonH
    Thanks for that reply :)

    http://qucs.sourceforge.net/components.html - about the 58th symbol down. I've asked about this on the mailing list, but it is not that active.

    I'd rather not take any chances with connecting them round the wrong way. As you say most FETs are interchangeable with the S & D leads.

    @ErnieM
    Thanks for your reply.

    It was difficult to locate the 2N3993 , however can find the part here: http://www.utsource.net/2n3993.html

    IIRC The 2N3993 was invented and first made by InterFET:
    http://www.interfet.com/Home/

    Motorola also made 2N3993 PJFET's as well, in Korea.

    And the datasheets are here:
    InterFET: http://search.datasheetcatalog.net/key/2N3993
    Motorola (and others): http://www.datasheetarchive.com/2N3993-datasheet.html


    This is the circuit I'm building, and I want to use the 2N3993 in place of the J176, as that will allow me to power the circuit with 12vdc, as I have that power supply already.

    http://www.electro-tech-online.com/...7405941-agc-preamp-distortion-agc-circuit.png
     
    Last edited: May 13, 2011
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
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  7. CD-RW

    Thread Starter Member

    Feb 26, 2011
    33
    1
    Looking at the FET symbols there, is it safe to assume that in either N or P JFET's, the lead with the line crossing it is the Source lead?
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yeah my first guess would be the source has the slash. Good thing all you can expect is simulated smoke.

    Most are, I seem to remember some that are not. If the data sheet says "Simon Sez" then I do it just that way too.
     
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