Colpitts Oscillator

Discussion in 'The Projects Forum' started by kare, Mar 7, 2011.

  1. kare

    kare Thread Starter New Member

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    Hi guys,

    I'm currently constructing a Colpitts oscillator based on NPN 2N2222. Here I use the NI MultiSIM 9.. It works perfectly, however, I get nothing when construct actual circuit (on breadboard)

    In the real circuit, the inductor is replaced by a coil inductor of 18SWG(1.25mm) diameter coiled to the value as in simulation. I've also tried other circuit samples on the internet, but still no prevail.

    Is the coil diameter too thick?

    Can you all please help me out?

    Thanks a lot. Appreciated.

    [​IMG]
    Last edited: Mar 7, 2011
  2. kare

    kare Thread Starter New Member

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    anyone please?

    thank you
  3. hobbyist

    hobbyist Well-Known Member

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    Hi,

    Your oscilator is hooked up in a class C operation, makes it more difficult to get started unless the components are nearly exact specifications,

    try hooking it up in a class A configuration, where the coil and parrallel capacitors are at the collector of the transistor and the output is taken from the emitter.
  4. BLOBY

    BLOBY Member

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    Kare, Im having a similar problem..(very similar actually).. see my thread, it might help me and you together.. http://forum.allaboutcircuits.com/showthread.php?t=51213

    Me too building the circuit in breadboard..

    Sorry if im breaking the forum rules by posting links and all to advertise my post... I just thought our problems are similar
  5. CDRIVE

    CDRIVE Senior Member

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    Where do you see class C? That transistor is configured as an Emitter Follower and R1 & R2 provides forward bias.
  6. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    There is no problem comparing notes, it is encouraged. Where it can go wrong is to move into someone elses thread and use it as your own.

    I suspect you guys are doing what I did about 38 years ago, and trying to build various oscillators to see how they work. I was deep in the country, and aimed for the AM radio band. My Dad's favorite phrase when his TV reception was trashed was "Turn it off". Worked most of the time too.

    I used a block of wood with terminals like these screwed into it. The coil was generally from a toilet roll, the wiring very crude. I had a scope then too (you can buy them cheap if you shop around), the waveforms were definitely not sine waves, it had lots of harmonics. Probably why the TV reception was trashed.

    [​IMG]

    Just be aware if you are radiating the FCC may come a knocking. Keep the duration of the on time short.

    Attached Files:

  7. CDRIVE

    CDRIVE Senior Member

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    Bill, your photo of the terminals brought back a lot of memories. They can still be had but have you priced them lately? You would think the terminals were made of solid gold!
  8. hobbyist

    hobbyist Well-Known Member

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    Yeh, I know I thought the same thing, when I looked at his circuit, it is forward biased, but it has the same configuration, that my NRI electronics book had, when they drew the circuit and gave a very quick side note about it, and so I was just repeating, what I read, about that config.
  9. Adjuster

    Adjuster Well-Known Member

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    There are many ways in which your practical circuit may differ from the simulation. The first thing to do is to check, carefully, that you do not have any wiring mistakes. This includes checking that the correct value components have been fitted everywhere.

    Next, check that there is a reasonable voltage (about half supply) at the transistor base, with 0.65V or so less at the emitter. If this is not so, you need to find out why before going any further.

    If the DC conditions are good, you need to think about the AC circuit. You don't seem to have a capacitor across the supply, so adding a capacitor of a few hundred nano-farads from Q1 collector to ground might help.

    Next think about the parts for your tuned circuit. Are you sure your home-made coil has the right inductance? What about loss? Thick wire should not be a problem, but other things might - like winding it on the wrong kind of core. As a child, I once tried to wind a radio tuning coil on a solid steel core. It did not work - can you guess why?

    Lossy tuning capacitors can also cause problems. For instance, some physically small ceramic capacitors with relatively big values (like 10nF and 100nF) use lossy dielectric materials. They are OK for coupling and decoupling applications, but not much good for tuned circuits.
  10. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    http://www.danssmallpartsandkits.net/

    About 3/4 down from the top.

    [​IMG]

    This is one very odd site. All orders via snail mail, using checks or money orders, and due to our wonderful homeland security regs requiring weights for all parts he only ships within the United States.

    I ordered his grab bags, and did not regret it, though one of the bags of transistors out of the 1000 were dudes. Definitely a good site for a beginner to get parts cheap.

    Obviously this is where I got the original picture for the show and tell.

    Attached Files:

  11. CDRIVE

    CDRIVE Senior Member

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    When I opened the page and started scrolling down I thought.. "Oh crap, it's going to take me forever to find them". That's before I realized I was in a 'Candy Store' that I didn't want to rush through. :)

    Thanks for the link. It's now bookmarked
  12. DickCappels

    DickCappels Well-Known Member

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    Your real life components are not as nearly ideal as those in the SPICE simulator. All of your real world components have losses.

    You will probably enjoy success if you make your inductor larger or the capacitors on the emitter smaller. I would start by increasing the inductor by a factor of 20 (100 uH) and decreasing the capacitors on the emitter by a factor of 20 (470 pf). Then it would have a chance of oscillating.
  13. Adjuster

    Adjuster Well-Known Member

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    If the OP does that he may find it easier to get a satisfactory coil and to avoid using unsuitable capacitor dielectrics.

    However, at present the transistor seems to be biased for about 10mA Ic with a base bias chain current of about 1mA. The bias chain has an equivalent parallel impedance of about 3.5kΩ, which would very severely damp a tuned circuit with 100μH inductance and 235pF total capacitance (I get Q≈5). The circuit probably would oscillate, but the frequency stability would be poor.

    It would be advisable to use say 10 times bigger resistances (and so reduce the collector current to about 1mA) if using these LC values.
  14. kare

    kare Thread Starter New Member

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    thx everyone for the input,

    adjuster, the coil were air-cored hand made to specific and proper measurement. I've tried both cylinder and flat planar coil, but still no prevail. as the rest, I will look into it.

    once again, thank you all a lot. :)
  15. CDRIVE

    CDRIVE Senior Member

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    I've not been able to successfully spice the Common Collector circuit you posted in post 1 using a BJT. Using a MPF102 (in Common Drain configuration) I've had success though.

    The attached circuit is connected as a Common Emitter which spiced well.

    Attached Files:

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