colpitts oscillator lc circuit

Discussion in 'General Electronics Chat' started by Reyo, Feb 27, 2013.

  1. Reyo

    Thread Starter New Member

    Feb 13, 2013
    Hey experts,

    im trying to construct a single transistor colpitts oscillator circuit using LC circuit as shown in the figure, to detect the change in frequency when varying the value of inductor

    According to f=1/2*pi*(L*Ct)^1/2
    Where Ct= C1*C2/C1+C2

    so C1=C2=47ηF
    and L1=47μH

    Hence oscillating frequency should be 150 Khz

    However, the circuit didnt oscillate when connecting it in bread board

    i have tried many schematic circuits on the internet but it seems that the feedback circuit is not connected to the transistor (( amplifier circuit )) for some reason

    may any of the great engineers help me connecting this circuit
  2. Wendy


    Mar 24, 2008
    The Completed Projects Forum is for Completed Projects only. It is meant to allow members to show plans for projects they built so other members can duplicate them if desired. New threads are also automatically moderated per Moderator review for this reason. Your thread does not belong in this forum, and was moved here.
  3. bertus


    Apr 5, 2008

    I found your thread in "the completed projects" forum, where it will stay invisible.
    As the title says it is the showcase for members who have a completed project.

    I moved it over here and made it visible to the others.

    On the circuit, I do not see how the circuit is powered.

    On this page of the EDUCYPEDIA you will find some links with schematics on the colpitts oscillator:


    PS I noticed that some of the links do not work.
    You can have a look in the internet archive if the pages are stored:
    You can copy the not working URL into the search-bar.
  4. #12


    Nov 30, 2010
    The plastic plug-in boards have a lot of capacitance between their terminals. That might be your problem.
    Also, there is no power supply represented in the drawing.
  5. Brownout

    Well-Known Member

    Jan 10, 2012
    Nor are there any bias resitors. If you're going to post a schematic, please post the circuit you built. We can't help if we don't know what it is.
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Lots of parasitic inductance as well.

    Is the OP trying to use this circuit to find the value of an inductor?

    Please clarify all of the above please.
  7. Reyo

    Thread Starter New Member

    Feb 13, 2013
    thank you guys for answering that quick, well my problem is the connection of the circuit !!

    all of the circuit i constructed have a 5 v power supply from the collector and i tried both diagrammes shown below

    #12 and brownout
    as you can see in graph the values of c and l are different , however the bias resistor values are 10k for both and the load resistor is 50 , i didnt know how to get values for the dc blockers capacitors (C3 & C5 in 1st graph)
    just to clarify im using BC107 transistor with the rl tank circuit to detect the change in oscillating frequency when varying the inductor value
  8. Reyo

    Thread Starter New Member

    Feb 13, 2013
    bertus: sorry for choosing the wrong section ,, the website seems great and usefull, however my most wanted page there is not working even when using the internet archive web
    but thanks anyway
  9. Reyo

    Thread Starter New Member

    Feb 13, 2013
    its my uni's final year project and i tried my best to make the oscillator work but i failed ,, please guys i would appreciate it if someone try to find the problem here