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Error in textbook?

Discussion in 'Electronic Design' started by Paul Burridge, Aug 11, 2004.

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  1. Hi all,

    In his very skimpy explanation on basic DC biasing for FETs, Chris
    Bowick (in RF Circuit Design) gives the suggested bias network that
    I've posted to a.b.s.e under the same subject title as this message.
    I don't see how this arrangement can possibly work for any N-Jfet
    since for one thing at least, the gate is positive with respect to the
    source. I've tried to scan the page in and post that, but the
    scanner's messing about, so I've redrawn it as a spice schematic and
    posted that instead. If it turns out the arrangement is incorrect, as
    I suspect, I will endeavor to post his explanation for how he arrived
    at these resistor values.
    So: is he wrong or am I nuts?


    Note: for anyone using LTspice, the fet shown is not a working model;
    I'm simply posting this as a diagram for illustration.
  2. John Larkin

    John Larkin Guest

    Looks like the jfet will be saturated with the values shown, not good
    for RF work. Looks like he got the sign of Vgs backwards. The next
    example on the same page illustrates that Vg must be near zero, not
    +5. Really silly, putting these two circuits side-by-side.

  3. John Larkin

    John Larkin Guest

    Oh, the skin depth equation on p 10 is apparently wrong, too.

  4. Excellent, John. Many thanks. It's reassuring you've seen the actual
    book for yourself, but it would have been better if you'd have hung
    fire until Active8 or someone else impulsive jumped in and accused me
    of getting it all wrong. There are quite a few stoopid errors in this
    otherwise excellent book. This one was just a bit more obvious than
    the others. :)
    Thanks again for the prompt response.
  5. Joerg

    Joerg Guest

    Hi Paul,

    Seems like it was meant for a MOSFET or someone typeset a wrong resistor
    value and then all this got lost in the review process. Probably best to
    let the author know so he can correct in the next ed.

    I like this one! It reminds me of a mechanical engineering book I got from my late father in law, a book he received from someone even more senior. Under radio frequency waves it states that this is a wonderous and strange phenomenon that is yet to be explaineth. And here I am an RF guy...

    Regards, Joerg
  6. fellow

    fellow Guest

    Have a look on the inside flap to see if he's a reader in engineering at
    some University. If so, then they probably are errors.
  7. Fred Bloggs

    Fred Bloggs Guest

    Shhhh...don't tell the resident idiot, but it's going to be damn tough
    biasing that IDSS=5mA JFET to a quiescent ID=10ma....
  8. John Larkin

    John Larkin Guest

    Good point. Bowick seems to be applying the jfet gate-voltage equation
    backwards to enhance it! The other example on page 120 is even

    Just shows you that an RF expert can't always handle DC.

  9. Active8

    Active8 Guest

    Hey POS troll. How would you see it if I'm kill filed?
  10. Active8

    Active8 Guest

    Well, when you rearrange the Id eq to get Vgs, you get

    [ ]
    | |
    | ( ) |
    | | I | |
    | | D | |
    V = -Vp| sqrt| ------- | - 1 |
    GS | | I | |
    | | DSS | |
    | ( ) |
    | |
    [ ]

    You can swap the terms inside the brackets by moving the negative
    sign of Vp inside, which gives you Bowick's version. Either way, you
    get the wrong answer unless you recall that

    sqrt(4) = +/- 2

    So you have to apply some reasoning.

    Thus (3 dots in a triangle) Burridge = idiot^100.

  11. Active8

    Active8 Guest

    LOL. That was an astute observation, not that I'm surprised. Either
    I didn't read that part of the book ( I have a NOV '82 Siliconix
    data book that sufficed) or I blew it off.
    I *do* remember reading *that*. 0 + 2.48 = 0 for sufficiently small
    values of 2.48, yup. I better compare all his refs to my own
    collection of app notes in der future and check the math.
    It's his math, actually. See my other post and while you're at it,
    reply to my reply to the idiot so he can see it. A blank post will
    suffice >-)
  12. Active8

    Active8 Guest

    Strike that. I'd didn't work. Gots me wondering WTF now.
  13. Active8

    Active8 Guest

    On Thu, 12 Aug 2004 19:12:04 -0400, Active8 wrote:

    Otay. You write the node eq for Vgs, expand Shockley's eq., equate
    the two, rearrange, solve the quadratic and pick the correct root.

    Sorry I thrashed around on that.
  14. John Larkin

    John Larkin Guest

    Now *that's* funny!

    Too bad Paul won't see it!

  15. Active8

    Active8 Guest

    Based on some recent posts, I suspect that duplicitous white trash
    POS is reading my posts despite his blasting JT for "not sticking to
    his [ctrl-k] guns" just to see what I'm saying behind his back. It's
    not really backstabbing since it's out in the open and I'd say it to
    his face before I rearrange it like so much algebra.

    I think he needs a good old fashioned hillbilly ass-whoopin' what
    with the way he's flaming a few of us and making hillbilly slurs.

    Apologies again for the multiple replies to self while working this
    out. That Siliconix book used design curves and iterative stuff.

    I'd be impressed if SFB Burridge (rhymes with porridge - like the
    space between his audio sensors) could solve the bias net (or any
    net) on his own.
  16. Ken Smith

    Ken Smith Guest

    Theres no problem getting 10mA to flow in a FET with Idss of 5mA. Just
    apply a positive bias to the gate. I've had as much as 2 or 3 A flow
    through a JFET this way.
  17. How many mS did the Fet last? I suppose you could always stand there
    with a can of arctic spray directed on it, but I doubt the customer
    would be impressed. ;-)
  18. Fred Bloggs

    Fred Bloggs Guest

    Actually there have been systems produced that did run quite hot and
    were arranged with a liquid nitrogen drip onto the electronics to keep
    things cool.
  19. Mike Andrews

    Mike Andrews Guest

    One model of Seymour Cray's computers ran with the logic immersed in
    a bath of chilled Fluorinert or some such, with a fairly hefty pump
    to keep the coolant recirculating through the chiller.

    Paul Raj Khangure mumbled:
    From frustration, if nothing else.
    DaZZa, in a.t-s.r
  20. John Larkin

    John Larkin Guest

    Jfets make pretty good pA-leakage diodes, although the series
    resistance tends to be higher than for a "real" diode. Never tried one
    as a power rectifier.

    Jfets don't enhance much, but gaasfets - especially phemts - enhance a
    lot, typically 1.6 * Idss just before the gate begins conducting in
    earnest. That can be useful.

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