colpitts oscillator

Discussion in 'General Electronics Chat' started by mik3ca, Feb 5, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    It seems that my regen detector is almost the same as this one:

    [​IMG]

    My Re is instead a 0.1uH inductor
    My transistor is NPN, and the battery polarity is switched (to satify an NPN transistor)
    A 0.15uH inductor is connected between Collector and the Cc/Rc junction.
    I omitted T1
    I omitted C1
    and I have a capacitor between the Cc/Rc junction and the NPN base.

    I have one question about this detector. Why is a capacitor C1 connected to the output of Cc instead of to the input of Cc?

    Am I supposed to treat coupling capacitors as filters?
     
  2. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    actually, my circuit is more like the left-most one in this image:

    [​IMG]

    except the polarity is reversed, I'm using an NPN, and the emitter is next to the Q1 label. and the 100nF capacitor connected to the base, and C4 is grounded, and instead of a transformer, I use an inductor, and my antenna is connected to the NPN's base.

    everything else is the same.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    It's not a detector, it's a Colpitts oscillator. Looks like something out of the NEETS modules. Your description of differences between your circuit and the one pictured tell me that they are two different animals. If you omit C1 and T1, you no longer have an oscillator, especially not a Colpitts. The tank is a defining structure of an oscillator. The capacitive divider is the defining structure of a Colpitts oscillator.

    Coupling capacitors simply block DC. In a loose sense, you could say they were high-pass filters, but I've never heard them described as such. Treat them as DC blocking caps. That's all they are.

    In the picture shown, Cc couples the amplifier portion of the oscillator to the tuned tank portion of the oscillator. C1 is part of the tank, not part of the amp.
     
  4. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    an article that I found at:

    http://www.eix.co.uk/Articles/Radio/Welcome.htm

    indicated to me that I was on the right track so far. My second stage amplifer uses the fig. 3b model (see my last post above), and according to the article I should use fig. 3c model.

    For some reason, I don't think figure 3c will work so well, but I guess I will have to try it out.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Mike,

    Why don't you download one or more of the SPICE simulators I recommended to you yesterday, and start trying out some of these ideas of yours?

    Start with a known-working example schematic, get the model working as it's shown, and then make modifications to it, and test your changes in software to see what your ideas of changes to the working examples do?

    For some unknown reason, you seem to blatantly disregard good advice that's been given in good spirit by people with quite a bit more experience than yourself. For example, in your "My radio is ALMOST complete" thread, located here: http://forum.allaboutcircuits.com/showthread.php?t=9214
    your schematic, help.gif located here: http://forum.allaboutcircuits.com/attachment.php?attachmentid=2051&d=1202008728
    shows no indication of implementing ANY recommendations that either Audioguru or myself had made in a prior thread about exactly the same type of audio amplifier, on which the two of us individually spent some time on.

    You seem to like to make changes. You need to start using tools like I suggested that will show you how your changes will affect existing circuits. You are extremely fortunate to live in a day and age where such things are available.

    Meanwhile, the suggested changes you apparently completely ignored will cause your output signal to be painfully distorted. If you model that portion of the circuit using a SPICE simulation, you will discover that I'm giving you factual information.

    Additonally, you should have used the 4401/4403 or 3904/3906 transistors instead of the 2222/2307s, as the hFE is more favorable.

    If you aren't willing to implement the suggestions offerred, what's the point in asking questions on here?
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    How many threads for the same radio?
    In the recent thread about this British super-regen receiver's audio transistor, I remembered his changes to another receiver and commented that his tank in the emitter destroys the gain and the missing parts at the emitter eliminates the quenching oscillator that makes automatic sensitivity adjustment.

    Now he is talking about trying figure 3c but doesn't realize that the transistor is not a common-collector NPN, it is a common-emitter PNP. Now he says that the power amplifier is his second stage but his schematic in the other thread shows that it is his only stage.

    This is a colpitts oscillator with the tank at the collector but it doen't have quenching oscillator parts at the emitter like in the two articles about a super-regen receiver.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This thread was inadvertently closed for a bit, which caused the OP and myself to carry on via PMs. Just tidying up loose ends here ;)

     
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