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Common Base RF amp - CE feedback capacitor?

Discussion in 'Electronics Homework Help' started by Merlin3189, Sep 4, 2014.

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  1. Merlin3189


    Aug 4, 2011
    In my wanderings around the web I've just come across the TalkingElectronics site. In its vast compendium I latched on to the 27MHz section with descriptions of the circuits used in toy remote controls. I was intrigued by the connection of the receiver antenna to the collector of the Common Base RF amplifier. (See the first receiver circuit at Transmitters/27MHzLinks-1.html)
    There is lots of explanation and multiple variations shown, but unfortunately I seem to be on a different wavelength to the author and could not understand the circuit.
    It looks like a CB amp, but with 100% positive feedback (the 39pF cap) making it an oscillator (Colpitts?) Even as an amplifier, it is strange to feed the signal into the collector, (though one can see why they might want to do this: later on they show ccts where the same stage is used as both receive amp and transmit output in a walkie-talkie.)

    So I'm wondering if anyone here can shed some light on this cct, in particular the role of the 39pF capacitor from C to E and how its value is determined?
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I guess you're referring to this circuit:


    I'm afraid I can't answer your question. I'm just posting here to save others the trouble of finding the schematic.
    Merlin3189 likes this.
  3. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    This looks like a common base Colpitts and the 39pF is what provides the positive feedback for oscillation. The value, um well probably a formula somewhere I'll see if I can find one. But I guess as long as you made it say 50-100 times lower impedance than the base terminal so it can turn the transistor off, then it should oscillate.
  4. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Firstly you have to realise the circuit is actually an oscillator and it fills the surroundings with a very low field of signal.
    Because the circuit is already oscillating, it is very easy to "upset" it. This means it is very sensitive to detecting another signal of the same frequency.
    All the component-values have been chosen to make the circuit oscillate with as large an amplitude as possible using the smallest current possible.
    That's the main purpose of the 70t choke. It keeps the emitter away from the 0v rail and allows the transistor to "jump up and down" and produce a large amplitude on its collector.
    When the signal on the collector is increasing, it is transferred to the emitter via the 39p and this has the effect of turning the transistor OFF more and allowing the energy from the inductor on the collector to produce the largest waveform possible because the transistor is effectively being removed from the circuit and is not putting a load on the inductor.
    However the emitter put a very low impedance effect on the bottom lead of the 39p and the amplitude of the signal at this point is very small. Most of the signal gets lost across the 39p. This means the 39p put a fairly heavy load on the signal being produced by the top inductor and will have an effect on its amplitude.
    This means we have two opposing decisions to make for the value of the 39p. It has to be as large as possible to transfer the signal from the collector to the emitter and yet as small as possible to have the least effect on the amplitude of the signal on the collector.
    It's that easy.
    The only way to select the value is by trial and error.
    Last edited: Sep 5, 2014
  5. Merlin3189


    Aug 4, 2011
    Thanks for your comments. And may I say you've got a very interesting site. I've only just discovered it and I can see I'll be spending some time exploring & reading there.

    I think the answer to my questions about this circuit is to build it and have a play. When I look at it, it just doesn't slot neatly into any of the circuits I understand (or think I understand.) From your comments it sounds like it's operating a bit like a GDO and the LF variations in bias, caused by the changes in oscillation amplitude, give the output signal. I also thought it might be a Superregen, but I couldn't see how the quenching would work.
    But what's in a name? Transistors don't know what we call the circuit they're in: they just do what transistors do. This circuit obviously does what it's supposed to, so all the questions can be answered by measuring things and changing things to see what difference it makes.

    Perhaps this can be my first project on Electronicspoint! (Unless I spot something even more interesting amongst your many circuits.)

    Regards. Don.
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