What is a dual gate MOSFET?

Discussion in 'General Electronics Chat' started by ubercool, Dec 24, 2012.

  1. ubercool

    ubercool

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    What is a dual gate MOSFET? Why do we need two gates in a MOSFET?
     
    ubercool, Dec 24, 2012
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  2. ubercool

    davenn Moderator

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    A dual gate MOSFET is a FET with 2 gates

    In the work that I do .... RF field, dual gate MOSFETs are often used as a mixer device in a receiver or transmitter circuit.

    for example ... in a receiver, the incoming signal goes into one gate and the local oscillator goes into the other gate and mix within the FET to give an IF output signal which would normally be the received freq minus the LO frequency.....

    OK here's a sample circuit with the receiver frontend amplifier and mixer transistors

    [​IMG]

    The 3SK73 -- arrowed -- is the dual gate MOSFET

    cheers
    Dave
     

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    davenn, Dec 24, 2012
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  3. ubercool

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you're old enough to remember valves (tubes) then a dual gate mosfet is a bit like a tube with multiple control grids.

    If you're not old enough to remember them, then I haven't helped at all.
     
    (*steve*), Dec 24, 2012
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  4. ubercool

    gorgon

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    I suppose that is true if you are too old to remember them too :D
     
    gorgon, Dec 24, 2012
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  5. ubercool

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    We have some older members, but none that old :)
     
    (*steve*), Dec 25, 2012
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  6. ubercool

    ubercool

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    Nah I came even after the transistors..somewhere around microcircuit time :) but anyways thanks for the help..I was actually wondering I understand how a single gate mosfet works but what happens internally in a single gate mosfet which makes it behave like a mixer and if this technology is old then what do we use nowadays for mixing RF and oscillator signals?
     
    ubercool, Dec 26, 2012
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  7. ubercool

    davenn Moderator

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    The use of dual gate MOSFETS for mixing has been around for ~ 30 years
    its still used because its very effective. The big advatage over a passive mixer like say a SBL-1X, is that it has gain where the passive mixers dont, rather they have substantial losses and need additional RF amplifier transistors/other devices to boost the signal

    Dave
     
    davenn, Dec 26, 2012
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  8. ubercool

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Once apon a time, dual gate mosfets were the rule rather than the exception. I have an early mosfet circuit book and it uses dual gate mosfets for everything, often either grounding or applying a fixed bias to the gate that is not otherwise used.

    These days if you want a dual gate mosfet you need to go searching a bit (but they're still available)

    Another option for mixing is a long tailed pair, but bandwidth is harder to achieve.
     
    (*steve*), Dec 26, 2012
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  9. ubercool

    davenn Moderator

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    yeah I fondly remember the days of the metal cased 40822 and 40673
    they were in just about every receiver commercially built. Probably still have a few in the parts drawer ;)

    D
     
    davenn, Dec 26, 2012
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