Why is the current in this Colpitt oscillator distorted ?

Discussion in 'General Electronics Chat' started by silv3r.m00n, Sep 14, 2019.

  1. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
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    I am fiddling with a colpitt oscillator in falstad simulator.
    Here is the simulation link: https://bit.ly/2kgtRID

    The current in the inductor of the tank circuit is nice sine wave, but the current in the capacitor (right side) is distorted in the negative half.
    Here is a screenshot,
    [​IMG]

    Why is this distortion happening ?
     
  2. SamR

    Well-Known Member

    Mar 19, 2019
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    What is the value of the resistor? 1u?
     
  3. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    its a fake resistor to act like some natural resistance.
    the distortion persists, even on removing it.
     
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  4. Danko

    Active Member

    Nov 22, 2017
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    Try something like this:
    upload_2019-9-14_16-58-25.png
     
    Last edited: Sep 14, 2019
  5. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    thanks. that looks like a good circuit. let me try it.

    but i am also curious to know the cause (and maths) behind the current distortion in my circuit.
     
  6. danadak

    Distinguished Member

    Mar 10, 2018
    3,504
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    Is it Vbe of transistor in reverse bias, limited by its breakdown,
    that is causing this ? Seems like step size is ~ brkdwn of a NPN
    emitter base junction.

    Not sure.

    Regards, Dana.
     
  7. Danko

    Active Member

    Nov 22, 2017
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    Distortions of tank capacitor current are from too big and hard positive feedback.
    In my circuit feedback current is lowered and smoothed by resistor R4.
     
  8. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    The distortions go away if i add a 3K resistor between amplifier and tank.
    Falstad: https://bit.ly/2kh973v

    [​IMG]
     
    Last edited: Sep 14, 2019
    SamR likes this.
  9. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
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    During negative current cycle, the phase shift between voltage and current seems to be higher than the phase
    shift in the positive part.

    it seems like the current flows quicker (sudden loss, due to amplification) than it should, around the negative peak, which causes a sudden break path to 0.

    May be this phenomenon, is reduced by the 3K resistor, as far i can guess.
     
  10. SLK001

    Senior Member

    Nov 29, 2011
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    The distortion is happening in ALL the sine waves that are shown. It is greater at the output, because of AMPLIFICATION!
     
  11. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    so why is the distortion only in the negative half, why is it not symmetric ?
     
  12. SLK001

    Senior Member

    Nov 29, 2011
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    That probably has something to do with the biasing of the transistor - it looks like it momentarily cuts off.
     
  13. Danko

    Active Member

    Nov 22, 2017
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    @silv3r.m00n:
    Your circuit 1kHz, enhanced.
    Added R1 and R6.
    C tank = C4 and C5 in series = 1uF
    upload_2019-9-14_23-17-9.png
    It is how your circuit works now in web simulator:
    circuit-20190915-0402.png
     
    Last edited: Sep 14, 2019
  14. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    The distortion happens during the negative current cycle in the tank capacitor
    During that time, the transistor is in high conduction (low output).

    So transistor cutoff may not be playing any role in the distortions.
     
  15. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
    3
    Thanks, the circuit is indeed much better now. Falstad link: https://bit.ly/2kOFWF9

    What does the 200 ohm resistor in series with the bypass capacitor do ?
    My first guess is that it reduces AC gain by increase the emitter resistance.
    But couldn't that be done by increasing the capacitive impedance ?

    I tried removing the 200 ohm capacitor and increase impedance by altering the value of capacitor,
    but that brings back the distortions.

    Whats the difference between increasing impedance by adding a resistor VS by increasing capacitive impedance alone ?
     
    Last edited: Sep 23, 2019
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  16. Danko

    Active Member

    Nov 22, 2017
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    Resistor 200Ω reduces AC gain up to 10 and linearizes transfer function of amplifier.
    It is not good idea to remove resistor 200Ω and reduce emitter capacitor 10uF to 0.77uF (200Ω impedance),
    because circuit simple will not oscillate at all (due phase shift at 1 kHz).
    Diagrams below show how shapes of collector current and collector voltage are affected by resistor 200Ω.
    upload_2019-9-17_4-35-48.png
     
    Last edited: Sep 16, 2019
  17. silv3r.m00n

    Thread Starter Active Member

    Apr 15, 2010
    48
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    guess i was thinking along similar lines. would it be correct to say -

    1. adding a resistor reduces ac gain without inducing phase shift, which keeps closed loop phase shift to minimum.
    2. if no resistor is added, and capacitor reactance is reduced alone, then a significant phase shift is induced, inhibiting oscillations.
     
  18. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    With oscillators there are also concerns about fast starting and frequency stability. Of course increasing the feedback to the point of running the active device into it's non-linear areas is always going to produce distortion. That is inescapable. An optimum design includes negative feedback that tends to keep the oscillation in the linear area of the active device, presuming that the intention is to produce a low distortion sine wave. Oscillators intended to produce strong harmonics are a different case.
     
  19. Danko

    Active Member

    Nov 22, 2017
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    Yes, it is correct.
    I slightly modified your circuit.
    No capacitors, except for tank caps.
    No phase shift.
    Quite clear, 11Vp-p sine:
    upload_2019-9-18_0-0-57.png

    EDIT:
    @MisterBill2
    upload_2019-9-18_2-49-13.png
     
    Last edited: Sep 17, 2019
  20. MisterBill2

    Distinguished Member

    Jan 23, 2018
    3,709
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    Based on the harmonic content shown in post 19, just because I don't see the distortion does not mean it is not there. BUT certainly there is no obvious tendency to break into a higher order oscillation either.So the circuit is an improvement. Probably changing R1 to 100K, R 2 to 27K, and R4 to 1K will reduce the harmonics quite a bit.
     
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