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JFET Common Source DC Amplifier Temperature Compensation

Discussion in 'Electronic Design' started by D from BC, Nov 19, 2007.

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  1. D from BC

    D from BC Guest

    My app uses a JFET..

    What are some ways to temperature compensate a common source JFET DC

    +------------Vout (biased for 50% Vdd)
    signal--->[ N JFET

    'DC' amplifier.

    1) The signal does contain a stable bias voltage.
    2) Temp range: 20C to 50C
    3) Vout thermal error of +/-10mVdc.
    4) BW: DC to 2Mhz

    I'm guessing compensation by:
    1) PTC or NTC resistor
    2) Use another JFET and feed in anti drift.
    3) Find a special jfet (IC??) << not an op amp!

    Possible ??

    D from BC
  2. Jim Thompson

    Jim Thompson Guest

    What does "temperature compensate" mean? Bias point, gain, both?

    ...Jim Thompson
  3. D from BC

    D from BC Guest

    Let's say the gate voltage is set for Vout = 2.5V...
    Vout can vary with temperature..Especially with no JFET source

    For a DC amplifier like this, doesn't DC bias thermal drift at the
    output also look like DC gain thermal drift?

    IIRC the power supply drifting with temperature can cause bias drift

    D from BC
  4. Joerg

    Joerg Guest

    That would be my avenue. There are duals such as this one, pretty low noise:

    Buying those might be a challenge though.
  5. Jim Thompson

    Jim Thompson Guest

    Except for the "sweet spot" bias condition, gain (small signal) and
    output Q-point probably vary independently.

    Instead of your vague "specification" what is it you are trying to

    ...Jim Thompson
  6. Tim Williams

    Tim Williams Guest

    Isn't the slick solution (given in AoE) to put another one in the source,
    so source current (and thus Vgs) is always equal between the two?

    Hmm...but that's for a source follower. Constant source current doesn't do
    you much good. Well, it would work for setting DC bias nonetheless, with
    the source bypassed to ground. Doesn't do anything for transconductance.

  7. D from BC

    D from BC Guest

    Ahhh... I think you've caught on that I dumb down problems to the
    point of making it the wrong problem :)

    Gee...I dunno if I can devague it more...
    I'll try...
    I need a fast DC amplifier with an input range of 0 to
    -200mV(negative) with a response up to 2Mhz.
    The output of the amplifier has to reach 2.5V out for -200mV in.
    There is no negative supply present.

    I thought a JFET would be best..but spoiled by thermal drift.....
    My alternatives were:
    Use resistive network + op amp
    Create a negative supply + op amp
    Put one of those 'beyond the rails' op amps to the test.
    Negative rail IC + op amp

    D from BC
  8. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    Are you sure that they are independent?
    For a given jfet model gm is strongly linked to the drain current (and
    essentially free from IDSS).
  9. Jim Thompson

    Jim Thompson Guest

    If -200mV Input => +2.5V Output, what does 0V Input => ?? Output

    ...Jim Thompson
  10. Jim Thompson

    Jim Thompson Guest

    They're not "independent", but I don't think incremental gain and bias
    Q-point will exactly track.

    ...Jim Thompson
  11. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    Ah, a bit of french meaning slept in there.

    Now you said they'd probably vary independently, which they don't (for
    ex. ).

    But gain and bias point sure can't exactly track. For this to be so,
    you'd have to have gm proportional to Id, which you can't ignore to be
    the hallmark of exponential :)
  12. Joerg

    Joerg Guest

    Many regular opamps can work down to about 300mV below the negative rail
    but you'd have to find one that's fast enough and low enough in offset.

    If you have a spare inverter somewhere you could make in inverting switcher.
  13. D from BC

    D from BC Guest

    It's allowed to clip.. :)

    After some thought ..and to answer your question earlier..
    I'd say I'm looking for bias point thermal compensation.
    Not small signal gain thermal compensation.

    I believe it'll be ok for the thermal compensation to move the Q point
    in order to maintain Vout at the bias level of 2.5V.
    The Vout bias level is not to drift with temperature.
    The new Q point due to thermal compensation and the resulting new
    small signal Av.... is probably ok.

    For my app..
    The Avdc thermal stability (at only one Q point) is more important
    than the small signal Avac thermal stability..

    D from BC
  14. John Larkin

    John Larkin Guest

    The r-r opamps do work. Sounds like a lot less hassle than trying to
    work around a discrete jfet.

  15. Jim Thompson

    Jim Thompson Guest

    Is this part of your "alternating noisy edges" problem?

    Seems you're requesting assistance at too small a scale... a larger
    over-view might produce a more elegant solution when we can see loads
    and continuing functions ;-)

    ...Jim Thompson
  16. D from BC

    D from BC Guest

    Bummer... :( No app note at the bottom.. :)

    I get suspicious when parts are hard to get...It usually means I'm
    doing something the wrong way..or old way.. :)
    On occasion, it can be it being a very new way..but I don't think so
    in this case..

    D from BC
  17. Joerg

    Joerg Guest

    Looks like he doesn't even need output RR. Isn't there a LM324 on
    steroids somewhere?
  18. D from BC

    D from BC Guest

    Good guess! :)

    I know what your saying..
    It's like Dr. Frankenstein putting bits and pieces of people together
    to make Frankenstein..
    IIRC Frankenstein wasn't an elegant solution.. :)

    I'm just chicken to post my whole project. Also, there's the pride
    factor too..
    And...I'm trying to have a mostly original project..

    So... it might wander around like a zombie and scare people...I made
    it and it works...well kinda.. :)

    D from BC
  19. Winfield

    Winfield Guest

    Well, as I understand it, and based on my measurements as well,
    gm does mostly track Id, more or less independent of Idss,
    especially at currents well below Idss. But I don't get the Q
    discussion y'all are having here. To my mind the Id operating
    point is best set independently from the particular miserable
    JFET's Vgs vs. Id, etc. I mean, part-to-part, sheesh! Bummer!

    But anyway, whatever, I suggest that D just go ahead and use a
    JFET opamp. I mean, I'm a "big" fan of JFETs and use them as I
    can, for when they're best, but what an unholy pain they are!
  20. Phil Hobbs

    Phil Hobbs Guest

    How about an ordinary single-supply op amp wired as an inverter? Ground
    the noninverting input, and bob's your uncle.


    Phil Hobbs
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