Can we use a MOSFET/FET as amplifier at the first stage? [RF transceiver]

Discussion in 'General Electronics Chat' started by booboo, Nov 5, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    Howdy
    Why don't we use MOSFET or FET as amplifier at the first stage of the RF transceiver? in many cases if you be astute you will know that they use BJT to make them. Why? E.G.

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    And or

    [​IMG]


    For example can we use BSS138?
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    Theoretically it is possible to use a transistor BSS138, but it is difficult. There must be radically changed the electronic circuit.
     
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  3. booboo

    Thread Starter Member

    Apr 25, 2015
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    Thanks Bordodynov
    Would you give me an example?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    yes, you can use a mosfet or jfet as an rf amplifier, just have to match the impedance and bias.
     
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  5. booboo

    Thread Starter Member

    Apr 25, 2015
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    Then I can make an AM/FM receiver with a MOSFET(without BJT)?
     
  6. RichardO

    Well-Known Member

    May 4, 2013
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    The biggest challenge will be getting the correct MOS-FET. Common MOS-FET's have a very large amount of input capacitance making them impossible to use at radio frequencies, even AM.

    Another consideration will be how noisy the transistor is. I don't have a feel for this but I am sure others here are. :)
     
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  7. crutschow

    Expert

    Mar 14, 2008
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    MOSFETs also tend to have a higher noise level the BJTs and BJTs better match the low impedances of typical RF circuits. JFETs also generally have lower noise and can also be used but I don't think they have any particular advantage over a BJT (and BJTs are usually cheaper).
     
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  8. alfacliff

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    Dec 13, 2013
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    actually, most high end commercial and ham radios use a lot of jfets and mosfets. small signal mosfets dont have much input capacitance, see 40673 for instance, and work well into uhf as low noise amplifiers. not all mosfets are high current motor drivers. they can be used for all modes, am, fm, rtty, psk, ect. their high inputimpedance puts very low loads on input circuits too.
     
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  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Dual gate MOSFETs are pretty common in TV tuners and a fair number of Ham radio front ends, JFETs are not exactly rare in FM tuners - I've seen a number of projects for low SW transmitters with the common or garden IRF540 MOSFET as the transmitter PA stage.
     
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  10. tindel

    Active Member

    Sep 16, 2012
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    As pointed out before - BJT's are cheapest - which drives most designs. They also typically offer more gain than a JFET. Gain of 100 is usually obtainable.

    MOSFET's are pretty crappy for RF amplifiers because they have a input capacitance of a few 1000 pF.

    JFET's offer the best performance because they have a very small input capacitance so they don't load the antenna. But it's difficult to get much gain out of them. You're pretty skilled if you can get a gain of 20 or so.
     
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  11. alfacliff

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    Dec 13, 2013
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    look up the ratings on the 3n200, or 2sk99, typical rf low level amp mosfets, no where near 1000 pf input capacitance. you are thinking of large signal power mosfets, some of which are used for rf power amplifiers, but the ones designed for rf power amp use have less input capacitance than power mosfets designed for pwm or motor driver use.
     
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  12. ian field

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    Oct 27, 2012
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    Horses for courses - Power MOSFETs in TO220 or bigger have a few nF gate capacitance, RF DG MOSFETS like the BF991 etc have a few pF capacitance.
     
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  13. ian field

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    Oct 27, 2012
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    Some Ham transmitter projects I've seen used MOSFETs like the IRF540 as the PA stage, but the driver stage to drive its gate capacitance wasn't trivial - good enough for a QRP transmitter in its own right.

    Often a 2N3866 or bigger.
     
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  14. alfacliff

    Well-Known Member

    Dec 13, 2013
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    and thsat would not be the first start (rf preamp)? of a tranciever. my FT 817 uses a mosfet output from 1.8 to 450 mhz. I still dont see how they can get awsy with all that input capacitance in an OUTPUT stage over that large a frequency range, but it works.
     
  15. ScottWang

    Moderator

    Aug 23, 2012
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    I can find the 3N200, but I can't find the 2Sk99, do you have any datasheet of 2sk99?
     
  16. bertus

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    Apr 5, 2008
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  17. alfacliff

    Well-Known Member

    Dec 13, 2013
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  18. ScottWang

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  19. bertus

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    Apr 5, 2008
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    Hello,

    I can also not find the 2SK99.
    My databook jumps from 2SK97 to 2SK103.

    Bertus
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAICR: the S.E. Asian manufacturers, at least at first - had numerical ranges of part numbers unique to one company.

    Very popular parts like the 2SC945/2SA733 were multi sourced, but some numbers could only be found in one manufacturer's data book.

    Not at all like the European BC107 or the American 2N3053 examples that went beyond industry standard parts to almost become household names.
     
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