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How is this Fet biased?

Discussion in 'General Electronics Chat' started by dr pepper, Aug 15, 2017.

  1. dr pepper

    dr pepper Well-Known Member

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    I have a depletion mode Fet Rf preamp circuit, the circuit appears to be unbiased.
    Here (half way down the page):
    http://www.vlf.it/fritz/pocketrx.html
    I get the 680k inline with the gate, its a Rc lowpass in conjunction with the gate capacitance.
    I dont get how this works bias wise, the gate needs to be for this Fet about 2 volts lower than the drain to be in the linear region, but the drain is grounded, so is the unused dual gate, and the input gate is too through a big resistor, to me this Fet is just going to pass its Idss and stay there, unless either its some kind of grounded gate amp with no bias or it relies on the Rf being high enough voltage tp overcome the gate threshold, which I doubt it will esp with Vlf signals which will be uV.
    Any ideas?

    Edit: Answered my own question, a jfet can only operate with a negative bias, and I assumed the Bf966 is a Jfet, its a dmos Fet and can work with very small amounts of positive bias.
     
    Last edited: Aug 15, 2017
  2. kubeek

    kubeek Well-Known Member

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    Well, it does have a mosfet symbol...
     
  3. Les Jones

    Les Jones Well-Known Member

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    I have never thought about biasing dual gate mosfets. When I used them (In the 1970s for RF amplifiers and mixers at around 145 Mhz.) I just built circuits from application notes. Seeing your post prompted me to Google "Biasing dual gate mosfets". I found this page which might help.

    Les.
     
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  4. dave

    Dave New Member

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  5. dr pepper

    dr pepper Well-Known Member

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    Good point kubeek.
    Yes I did find that unlike the jfet that can only go negative the dmos fet can go a couple of hundred mv positive and still be operating in its linear region, not a brilliant thing for a battery application as Id will be at its max.
    Jims circuits in the referenced page all have a bias resistor in the drain and even an Led!, and the unused gate is biased too, the vlf receivers preamp is a bit of a bodge.
    Interesting Jim also couldnt get a grounded fet preamp to work either.
    Another improvement I can make is to put a few pf mimmick cap inline with the Bfo, the o/p is 3 or 4 volts and blasts the rear end.
    Thanks guys.
     
    Last edited: Aug 16, 2017
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I found about 30 pictures where one of the gates is connected to a voltage source.
    upload_2017-8-16_6-37-45.png
    upload_2017-8-16_6-41-16.jpeg
     
  7. dr pepper

    dr pepper Well-Known Member

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    Varying the voltage on one of the gates acts as a variable gain or agc control, dual gate rf fets seem to be used for that a lot.
    I'm going to mod the design to work with a J310, I will need a drain resistor for that, maybe bypass it with a cap for good gain.
     
  8. dr pepper

    dr pepper Well-Known Member

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    Turns out the mos device has free electrons in its substrate, making the gate + pulls some of these into the channel making it conduct, the effect is more pronounced than the jfet and therfore gives very high gain in an amp, also with fewer components.
    I have got some Bf996's from Rs which seem to be the same but Smd's instead of To92's.
     
  9. schmitt trigger

    schmitt trigger Active Member

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    I think the key is the following sentence:

    "...the BF998 is a depletion-mode FET, similar to your common N-channel JFET."
    Italics are mine
     

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