BC547/548 Transistors and FM Transmitters

Discussion in 'Wireless & RF Design' started by FlashFire, May 18, 2012.

  1. FlashFire

    Thread Starter New Member

    Nov 18, 2009
    5
    2
    Hey,

    I've been trying to get a feel for building LC oscillators (Colpitts) since i have a school project which will require me to build a couple. Unfortunately the only transistors i have access to at home are some BC547/548 general purpose transistors. So i took to the web and found a number of transmitters using these transistors supposedly capable of operating around the commercial FM band.
    I've tried to simulate the oscillators from these circuits using BC547/548 models and have had trouble getting them to oscillate. Originally i was attempting to physically build an oscillator to oscillate around the 10MHz (thinking the transistor would work okay at this frequency) mark with no success, so i decided to simulate and found that it wouldn't work there either. I found replacing the BC547/548 transistor with a generic NPN spice model or even a 5GHz wideband transistor would get it to oscillate in the simulations. Another thing that made the circuits oscillate in simulations was to use a large inductor (say 100uH instead of 0.5uH, but from a practical point of view I want to keep the inductor small)

    So, just wondering, is it possible to build a colpitts oscillator using a BC547/548 at a frequency of 10MHz. Im actually using a breadboard for construction (I know parasitics), but at 10MHz i've been told i should be able to get something regardless of parasitics.
     
  2. edsontaubate

    New Member

    Nov 14, 2011
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  3. FlashFire

    Thread Starter New Member

    Nov 18, 2009
    5
    2
    Cool, like i said i've seen these before. My question pertains to building an oscillator at 10MHz, and why im unable to get oscillations when i draw the oscillators from these circuits with the same biasing conditions in a simulator.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,656
    632
    Can you post a schematic of the circuit that is giving you problems?

    Maybe the problem is with your simulator, or maybe you hooked something up wrong -notice the emitter and base on the TO-92 packaged BC547 is swapped with regard to their positions on many other small signal NPN transistors.

    Make sure that there are at least a few milliamps going through the emitter resistor.

    Also, those plug-in breadboards tend of develop intermittent contact over time; that can't help either.
     
  5. FlashFire

    Thread Starter New Member

    Nov 18, 2009
    5
    2
    Thanks very much for the reply, I changed topology to a common base colpitts oscillator (like that found on wikipedia: http://en.wikipedia.org/wiki/File:NPN_Colpitts_oscillator_collector_coil.svg) and found that it simulated quite nicely, so i went and built and it worked perfectly! So anything Sub 10Mhz seems to be within my reach at the moment.
    The main difference between this topology and the one i was trying was the presence of C3 in the schematic (I wasn't using this before). Could anyone tell me what role this capacitor plays? It looks like it might change the bias level at higher frequencies, why would i want to do this? Does it have something to do with setting a specific resistance to cancel negative input resistance from the amplifier? I also need to get an oscillator to work at 100Mhz or above for my project and finding this capacitor to be a bit of a limiting factor.
     
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