Connect with us

Help with Varactor Diode

Discussion in 'General Electronics Discussion' started by abuhafss, Aug 11, 2014.

Scroll to continue with content
  1. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Here is a portion of a circuit in which I want to replace the variable capacitor with a varactor diode like MVAM108. I know a little basics about varactors. Please help how to incorporate MVAM108 in this circuit so that it can be tuned with control voltage.

    p.'s. http://www.tsf-radio.org/forum/im/133895mvam109.pdf Screenshot 2014-08-11 18.28.07.png
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Varactor diodes are explained here. See section "varactor circuits" for applications.

    Your circuit may not be suitable for the application of a varactor diode: A varactor diode's capacitance is controlled by the DC bias across the diode. Your circuit has an inductor in parallel to the diode. This inductor is a short-circuit for DC (apart from a typically low wire resistance of the coil). Any DC source that wants to apply bias to the diode will therefore be short-circuited by the inductor.
    You may try to put a capacitor in series with the inductor to de-couple it from the DC bias of the varactor.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Yeah, a simple option would be to break the connection from the bottom of the inductor to the 0V rail, put a capacitor (e.g. 0.1 µF) across the break, and apply the control voltage across that capacitor.
     
  4. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Is this your suggested configuration? Varactor 1.png
     
  5. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Is this your suggested connections? Varactor 2.png
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Not quite. Move the cathode of the varicap to the other end of the inductor, then it's right.
     
  7. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Hope this is OK. Varactor 3.png
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Now you have the resistor connected to the wrong place. Connect the resistor to the top of the 0.1 µF capacitor.
     
  9. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    You mean the junction of the inductor and the 0.1μF cap?
    In that case the varicap will be reverse biased thru the inductor?
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Yes, and yes.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Kris: For DC this should make no difference.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    With a 100k series resistor, it won't make much difference, no. I didn't actually suggest using a series resistor. My suggestion was to apply the control voltage straight to the capacitor. This avoids any effect on the Q of the tuned circuit. With a 100k resistor, that effect would only be tiny, but there would be a small delay waiting for the capacitor to charge. My suggestion was intended to avoid both problems.
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    O.K., I think I see
     
  14. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    And how the varicap would replace the capacitor in this circuit?
    And where the control voltage will be applied?
    Screenshot 2014-08-14 15.29.42.png
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-