colpitts oscillator problem

Discussion in 'General Electronics Chat' started by Amir Sarikhan, Jun 20, 2015.

  1. Amir Sarikhan

    Thread Starter Member

    May 29, 2015
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    0
    Hi all
    I have a problem with colpitts oscillator
    I have a circuit (I uploaded it).
    so I use 4700p caps and various amounts of inductor my transistor is 2SC3355 not on spice
    Problem is that when I remove inductor ,like second schematic I get oscillation about 200Khz .
    I calculated and I get that I must have 270uH inductance for this oscillation but where is that ?
    also when I replace inductor like 56u I get higher frequency but what is my total inductance ?
    Please help me I get confused .
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The feed back is determined by the ratio of C1 to C2. That is the most critical aspect of the Colpitts oscillator, with respect to starting, and stability.
     
  3. Amir Sarikhan

    Thread Starter Member

    May 29, 2015
    45
    0
    Hi
    Yes but why that oscillation I mentioned above occurs ?
     
  4. Amir Sarikhan

    Thread Starter Member

    May 29, 2015
    45
    0
    Hello
    I just want to say that I tested my circuit on pcb instead of bread board and I get the true results
    But I do not get the answer "Why on breadboard it oscillates on 200Khz ? "
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,786
    4,806
    Solderless breadboards have a lot of parasitic capacitance and tend to result in circuits with a lot of parasitic inductance, as well.
     
  6. Amir Sarikhan

    Thread Starter Member

    May 29, 2015
    45
    0
    Yes of course but 270uh is not a little large value for breadboard inductance ?
     
  7. Amir Sarikhan

    Thread Starter Member

    May 29, 2015
    45
    0
    Hi
    With values : c1=4700n c2=2200n and l=74u
    I get the result below
    Why the result is not sinusoid ?
     
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