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
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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
    15,652
    2,348
    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
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    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

    Well-Known Member

    Nov 29, 2011
    825
    229
    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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