Help understanding MOSFETs (or JFETs??)

Discussion in 'Homework Help' started by KansaiRobot, Jul 29, 2015.

  1. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    Hello and thanks always.

    I have used BJT transistors mainly. I dont understand JFETs or MOSFETS so I would appreciate someone explain this to me.
    In particular the following case:
    SLA7080.jpg
    You see the part that is surrounded by a red circle??

    1) Is that a MOSFET? or a JFET? or what... There are so many symbols for these kind of transistors (some with a circle, some without a circle) that I dont understand. Is that N-channel? or P-channel??

    2) I understand the purpose of the above circuit. What it wants to do is that when applying a "SLEEP" signal VDD is going to go to Ref (so as to activate the SLA7080x sleep mode but forget about that) and when no SLEEP signal all we are gonna have is a normal voltage divider... But isnt it inverted???
    I mean the current must go from 5V to Ref right? why the arrow on the other direction then???

    3) There is no suggestion at to what transistor to use here... Any examples of JFETs (or is it MOSFETs? ..confused) that I can take a look?

    Thanks a lot in advance
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    Have you ever try to search on Google?
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    1. It is a P-channel MOSFET.

    2. Cannot see the arrow clearly enough to know which way it is pointing, but the circuit only makes sense if it is a P-channel transistor, so the arrow points out. That is to indicate the polarity of the body diode that is between the drain and source.

    3. Use a P-channel MOSFET like the infineon BSS84P. Pay attention to leakage current if used at high temperatures.
     
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  4. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    318
    5
    Thank you for your very helpful answer. I appreciate it much.
    I will look for P-channel MOSFETs then but just to comment on #2: Yes the arrow is pointing out. So the upper part is the Source and the lower the Drain, right? That confuses me so much, since I have read that the Source is like the BJT Emitter and the Drain like the BJT collector. But in that case the current will go from the drain to the source , so from lower to upper... but but... we want the opposite no? to go from the upper 5V to the lower Ref.....

    Sorry for the confused question but that is how I am ... confused any help will be appreciated
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    KansaiRobot said: "So the upper part is the Source and the lower the Drain, right?"

    Correct.

    KansaiRobot said: "But in that case the current will go from the drain to the source , so from lower to upper... but but... we want the opposite no? to go from the upper 5V to the lower Ref....."

    It might be much easier to realize that when the gate is taken negative with respect to the source, current will flow between the drain and the source thus shunting R1 with the lower resistance of the MOSFET.

    The direction of the current is not important other than to note that for typical applications of P-channel MOSFETs the drain is negative with respect to the source.
     
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  6. Lool

    Member

    May 8, 2013
    117
    12
    I recommend "The Art of Electronics", for a nice summary of JFET, MOSFET, enhancement-mode and depletion-mode descriptions. This is one of the better summaries for having a working knowledge.

    One problem is many people are lazy about writing the correct symbols for the actual device specified. I know I'm guilty of that sin too. There are so many variants of symbols now, that it gets confusing, and putting a generic symbol with the part number is probably less ambiguous anyway. Alas, your schematic did not specify the part, but if you memorize the summary from the Art of Electronics, you will always be able to figure out the basic operation of the part shown.
     
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