Colpitts Oscillator

Discussion in 'The Projects Forum' started by Bill B, Dec 23, 2015.

  1. Bill B

    Thread Starter Active Member

    Nov 29, 2009
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    Hello all,

    I'm working on a 150 kHz Colpitts oscillator. I have an oscillation at the frequency I desire but the negative portion of my sine wave is clipped and distorted. Getting a perfect sine wave isn't really necessary for this application as I intend to use it as a noise source, but, my perfectionist side won't let me get away with saying the heck with it. I've attached a schematic of the circuit but the tank circuit values aren't accurate with what I have on the perf board (L = 6.8 uH, C = 330 nF each). I don't need more than 100 mV on the output. I tried adding some resistance to the emitter which gave me a better signal but significantly reduced the amplitude. I also attached a screenshot of the waveform. Any thoughts on how I clean up the waveform?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Bill B

    Thread Starter Active Member

    Nov 29, 2009
    61
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    This is a design I found on the internet. I have found that I get a better signal on the emitter than the collector. Granted it is at a much smaller amplitude, but the smaller amplitude is what I need for my particular application.
     
  4. bertus

    Administrator

    Apr 5, 2008
    17,159
    3,015
    Hello,

    Have a look at the following circuit:

    colpitts_oscillator.png

    It has a fully decoupled collector.

    Bertus
     
  5. Bill B

    Thread Starter Active Member

    Nov 29, 2009
    61
    0
    Ok, so I should put a decoupling cap on the collector. I also noticed the biasing resistors are of different values. That is where I'm struggling to comprehend the math and concept. This is an emitter follower topology right?
     
  6. SLK001

    Senior Member

    Nov 29, 2011
    1,029
    358
    Probably too much gain (of the transistor) at the desired frequency. IIRC, a resistor from the emitter to C7-C8 junction will lower the amount of positive feedback.
     
  7. Bill B

    Thread Starter Active Member

    Nov 29, 2009
    61
    0
    Ok, I'll give that a try. Thanks.
     
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