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Why do crystal sets use diodes instead of transistors?

Discussion in 'General Electronics Discussion' started by dietermoreno, May 7, 2013.

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  1. dietermoreno


    Dec 30, 2012
    Why do crystal sets use diodes instead of transistors?

    This isn't the 1890s, so why use 1890s technology when we have invented the transistor?

    Transistors have been famously shown to have rectification properties in guitar amps at radio frequencies, and the regen radio uses a transistor instead of a diode.

    The transistor in the regen radio amplfies and rectifies at the same time, so why not use a transistor in a crystal set for better results?

    Does it have to do with the definition of crystal set?

    Is crystal set defined that the only semi conductor it uses is a diode?

    So if it used a transistor instead of a crystal, then it would be called ...a transistor radio instead of a crystal radio...?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Even in a normal commercial AM radio receiver full of transistors, a diode is still used as the detector.... why...
    cuz thats what the particular diode is designed for
    Transistors are NOT

  3. dietermoreno


    Dec 30, 2012
    Yes, I see that a commercial superhet consumer broadcast receiver has a diode.

    Okay, but then why does regen receiver use no diode?

    Last edited: May 8, 2013
  4. BobK


    Jan 5, 2010
    Because the regen receiver needs a transistor (or tube) to amplify. It just happens to do the demodulation as well, so there is no need for a diode.
  5. dietermoreno


    Dec 30, 2012
    Well but doesn't a tube or transistor not rectify RF very well, which is why the super het receiver uses a diode just like the crystal receiver to achieve better results since the super het receiver uses different means to increase RF gain compared to the regen receiver?

    Just a curiosity in me trying to understand the super het schematic that I posted: does it use a variable inductor for tuning on the AM loopstick antenna instead of using a variable capacitor for tuning, and the variable capacitor is instead used as a trimmer capacitor? I wonder if RimStar ever made a super het.

    Another super het question: Is that tube called a septode because it has 7 plates total? So the triode used by the regen receiver will not work for the tube used by the super het receiver?

    A regen receiver schematic question: Is that tube with 5 plates called a pentode? Will this schematic not work with a triode?

    Well here at this link I found some theory on using transistors to rectify in power supplies instead of diodes, but that sounds counter intuitive because I thought transistors only started to exhibit rectification at frequencies higher than audio if they are not designed to amplify RF.
    Last edited: May 9, 2013
  6. CocaCola


    Apr 7, 2012
    A bigger riddle, why does the US still hang a vast majority of it's vital power and communications lines on dead trees stuck in the ground?

    BTW, I see you are from Island Lake, small world as I went to school in Wauconda...
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The schematic indicates a variable capacitor in all cases.

    No, it's a heptode (or a pentagrid) because it has 1 plate, 1 cathode, and 5 grids = 7

    These are not as straightforward as you might think from the symbol. One of the grids is actually like another plate! See here.

    Not if the circuit required a pentode.

    cathode, grid, grid, grid plate = 5 therefore pentode.

    In neither case is the heater counted. Probably because the original heaters were cathodes and indirect heating is not considered an additional functional element

    Don't start making stuff up again. Call things by their proper names.
  8. dietermoreno


    Dec 30, 2012
    Okay. Thanks for the tutoring.

    From the link that you gave me, I found out that De Forest actually invented the triode to be used as an improvement over the diode for radio detection since the grid he intended it to control the flow of electrons to increase the sensitivity of the detector by negatively biasing the grid.

    I never knew that. I always thought that amplification was what was in mind.
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