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Probably a stupid question...

Discussion in 'Electronic Design' started by Dave, Aug 31, 2007.

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  1. Dave

    Dave Guest

    But what determines the gain, or amplification factor, of an N-channel JFET?
    Hate to expose my ignorance, but... Is it the biasing? Or what? What I
    have doesn't specify such on the packaging, and I am wondering.


  2. Bob Myers

    Bob Myers Guest

    Transistors, etc., themselves don't have a "gain" or
    "amplification factor," unless you're talking about
    some very specific parameters related to the inner
    workings of the component (such as "beta" for a bipolar
    transistor, which is sort of "gain" figure that relates the
    collector current to the base current). But those
    parameters are not directly related to the "gain" figure
    that will be established for the complete circuit which
    uses that device; THAT comes in from other factors,
    such as the particular amplifier configuration involved,
    the values of particular components used within that
    circuit (which will, among other things, set the biasing,
    so yes, that is involved) the impedance of the source
    and load, etc., etc..

    Bob M.
  3. Dave

    Dave Guest

    Hmmm. Okay. Thank you very much for the reply and the information. It's
    been so long since I dabbled in such I am beginning to wonder what I *do*
    remember. I have a copy of Electronic Principles and am trying to work my
    way through the transistors sections, but it is slow going. Thanks again,
    and thanks for taking the question seriously. :)

  4. Jim Thompson

    Jim Thompson Guest

    My take... Bob M. = amateur BS artist ;-)

    ...Jim Thompson
  5. Don Bowey

    Don Bowey Guest

    Aren't you into the grape a bit early today. ;-)
  6. Jim Thompson

    Jim Thompson Guest

    Nope. Tell me, Is there anything intelligible in that, "Transistors,
    etc..." ??

    ...Jim Thompson
  7. Fred Bloggs

    Fred Bloggs Guest

    Your packaging should specify things like BVdss, gm, Idss, and Vp at a
    minimum. The "gain" of the standalone device is understood to mean gm,
    the transconductance, and this is the small signal ratio of
    drain-to-source current per unit of gate-source voltage. The JFET is
    ideally a voltage controlled current source: the current is
    drain-source, the control voltage is gate-source, and gm is the control
    factor. Very simple.
  8. Dave

    Dave Guest

    COOL! Thank you, very much. I was ready to have to analyze the entire
    circuit of a project I built based on a commercial schematic that doesn't
    seem to amplify small-signal RF very much. Your words are most encouraging.
    Much appreciated.

  9. Dave

    Dave Guest

    Forgot to add "and I may still have to do that..."

    Bottom line: I now have a starting point, from which to proceed forward.

    Again, Thanks.
  10. Joel Kolstad

    Joel Kolstad Guest

    How much is "not very much?" And at what frequency?

    Unlike say, the op-amps that Jim designs that give you something absurd like
    90dB open-loop gain, many RF amplifiers only provide some 10-20dB (which is
    ~3x-10x voltage) gain.
  11. Dave

    Dave Guest

    "Not very much" means a barely perceptable difference in signal quality when
    used with a standard portable shortwave radio, if any perceptablel
    difference at all. At any freq in the shortwave spectrum, but mainly from
    approximately 6 MHz to 12 MHz. The commercial product I was copying is
    supposed to help pull in weak signals, but I have heard it doesn't do this
    very impressively, so suspected the schematic I was working with was just
    not designed very well. Makes use of two N-channel JFETs and a UHF high
    speed switch, but left a lot to be desired on my part. I added a
    Q-multiplier taken from Joe Carr's Practical Antenna Handbook, which
    improved things, but I'm still not satisfied as I want it to work with the
    on-board whip as well as it does with the 110' longwire antenna I currently
    use it with. I am guessing I need another 20 or 30 db out of it, at least.
    I am still fairly new to RF however, so it goes slowly.


  12. Jamie

    Jamie Guest

    Hows it going Jim? that's not like you :)
  13. Jamie

    Jamie Guest

    Well, I'll tried to shorten it and in simple terms..
    JFETS have to be biased to shut them down, in other
    words, they normally are in their state of flow/low-resistance with
    the gate pulled to common for example.
    With JFETS, you must lower the gate voltage below
    the source voltage. This figure is in the specs of the
    transistor data sheet of where the pinch off point is.
    Since the FETS are voltage biasing devices and not
    current biasing devices like bipolar, this is where
    signal source types dictate's as to how the circuit
    should be designed.
    In the case of FETS, those with lower voltage specs
    on the Gate-source (Vgs) that places it in the pinch
    off state is generally those that will give you a higher
    gain when designing around them.
    So basically, a small (Vgs) for pinch off should give a
    higher gain.
    Now since fets have very high impedance on the gate, this
    device is very good in cases where you need to reference
    signals that generate a higher voltage but hardly not enough
    current to drive a bipolar type input circuit with out effecting
    the performance of the reference device.
    There are other benefits to using FETS over bipolar like switches
    and so on, but won't get into that.

    an Enhanced mode fets work the other way.
  14. John Larkin

    John Larkin Guest

    The datasheet should specify a typical transconductance at some drain
    current, and probably has a graph of same. The common-source voltage
    gain is nearly equal to the transconductance (gm, in Siemens)
    multiplied by the load resistance (in ohms) in the drain circuit.

    The operating current is a function of the gate-source voltage,
    another spec or curve. Actually, gm is the derivative (slope) of the
    Id versus Vg curve. In general, the higher the drain current, the
    higher the transconductance.

    A typical small-signal jfet might operate with the gate a half volt or
    so negative relative to the source, and might have a gm of, say, 0.003
    Siemens at 4 mA or so drain curent. If it dumps into a 2K drain
    resistor, the voltage gain would only be about 6.

    Jfets have low gains and, usually, terrible part-to-part
    repeatability, with Idss sometimes specified over a 5:1 or even 10:1
    range, which makes design tricky.

    "Amplification factor" for a tube is transconductance times plate
    resistance, which is the voltage gain you'd get with a
    constant-current (very high impedance) plate load. The same idea
    exists for a jfet, namely gm times the slope of the drain curve. I'm
    guessing numbers like 25 or so for a typical jfet.

    So, in general, jfets suck. They are handy in some niche applications,
    like low-noise, high-impedance amplifiers.

  15. Eeyore

    Eeyore Guest

    You need to know the transconductance of the device. Also, the voltage gain will
    vary according to the circuit in which it is operated.

    A full treatment of the subject can be found in text books. There is no simple
    one-line answer. You'll need to study to understand.

  16. Eeyore

    Eeyore Guest

    Bwahahahaha ! Your 'answer' was anything but short and didn't even answer his

    He didn't ask about 'shutting them down' did he ? It's called 'pinch off' by
    electronics engineers btw.

    More stupid waffle from Jamie the clown as ever.

    I notice you couldn't even provide a simple answer like gm.Rl

  17. D from BC

    D from BC Guest

    The gain depends on which spice model you are using. :)

    D from BC
  18. Dave

    Dave Guest

    Thank you, all who provided information on the necessary details of what I
    was ultimately asking about. More complex than I realized, but now I have
    some idea as to what I am asking. And thank you specifically, Jamie, for
    getting to the heart of the matter in a cogent and intelligable form.

    I have some studying to do, but I at least know better what I am looking

    Again, many thanks.

  19. Eeyore

    Eeyore Guest

    It's also a lot more complex at Radio Frequencies. Device capacitances come
    heavily into play.

  20. Bob Myers

    Bob Myers Guest

    No, Jim, actually I'm a pro at it....;-)

    Bob M.
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