What does happen if the direction of the diode used for AM demodulation is reversed?

Discussion in 'Homework Help' started by SimpleKCLSimpleKVL, Mar 22, 2019.

  1. SimpleKCLSimpleKVL

    Thread Starter New Member

    Mar 3, 2019
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    Hello,

    In the lab, I built the basic radio receiver circuit that you can see in the attachment, and I used the germanium diode for the demodulation. Fortunately, it worked properly but I wonder that what happens if we change its direction ? Can we get sound from the headphone ?


    Thank you in advance.

    Note: After I built the circuit, I forgot testing its direction's effect so please don't ask me why didn't you try it. :D
     
  2. bertus

    Administrator

    Apr 5, 2008
    19,521
    3,977
    Hello,

    In the given picture, the negative tops of the signal are detected.
    The capacitor Cb will remove the DC part and the amplifier will work fine.
    When the diode is reversed, the positive tops of the signal are detected.
    The capacitor Cb will remove the DC part again and teh amplifier will also work fine.

    Bertus
     
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  3. Ylli

    Active Member

    Nov 13, 2015
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    Only caveat would be if there was some DC offset on the output of the RF amp. Otherwise, makes no difference.
     
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  4. SimpleKCLSimpleKVL

    Thread Starter New Member

    Mar 3, 2019
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    So if I understand correctly, you mean the cb capacitor blocks any dc off set (negative or positive) and we get the same signal at the output. Thanks for your rapid answers.
     
  5. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    What are you using for the "Amp" triangle block just before the diode?
     
  6. SimpleKCLSimpleKVL

    Thread Starter New Member

    Mar 3, 2019
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    There are cascaded op amps before the diode.
     
  7. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    What kind of op amps and what is the expected frequency of reception? (like 1MHz for example)
     
  8. SimpleKCLSimpleKVL

    Thread Starter New Member

    Mar 3, 2019
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    Their gbwp were 4 MHz.
     
  9. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Ok great, so what frequency are you trying to receive or maybe the whole AM band?
     
  10. SimpleKCLSimpleKVL

    Thread Starter New Member

    Mar 3, 2019
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    Tuner was designed to pass only 175 kHz signal.
     
  11. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Ok. Well then 4MHz op amps have to have reduced gain so they can pass that signal with enough amplitude to drive the next stage properly.
     
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  12. Ramussons

    Well-Known Member

    May 3, 2013
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    In this circuit, it makes no difference.
    But in commercial receivers, the DC derived after the diode (called detection) is used as an Automatic Gain Control (AGC) voltage on the preceding amps, normally at IF stage. In such circuits, reversing the diode will make a big difference.
     
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