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transistors - emitters connected, caps & resistors ??

Discussion in 'Electronic Basics' started by andrew_h, Feb 13, 2006.

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  1. andrew_h

    andrew_h Guest

    Hi All,

    I've been investing how this remote my father made back in the late
    1970's works - am trying to get a picture up of the circuit board etc.
    etc, and am slowly piecing together concepts. It is a RF transmitter
    operating on 27.445 Mhz. The idea was the remote could turn on two
    mains lights in the house before coming inside (i.e. coming at night
    time etc.). It may be a run-of-the-mill thing, I dont know - but I'm
    learning about electronics now too so its good to build up basic
    concepts, especially with transistors to amplify signals, crystals etc.

    In the meantime though - there are four transistors. Three are C828 NPN
    - from what I read lower power, general purpose transistors, and one
    A564 PNP - which is connected to the inductor.

    Anyway my question relates to the three C828 transistors. On the first,
    a resistor is connected to the base, and one to the connector
    (obviously to establish correct bias?). With the second and third, the
    collector's and bases both have resistors biasing them, but also
    capacitors from the collector (some small pF ones and one 150Z one,
    ceramic caps). Lastly, all the emitters go into the next (i.e. copper
    tracks link one to the other, to the other).

    Questions - 1) Where amplification is needed such as this situation
    (i.e. an RF transmitter), does the base and collector always need to be
    biased with resistors?
    2) What is the purpose of the capacitors - do they filter the
    collector-emitter current in some way ????
    3) Are the emitters connected so that essentially, the last one would
    have a current equal to all the amplification (i.e. all the
    amplification of the other added to the last one)?

    Sorry if I sound newbie - am learning fast!

  2. You might be looking at a bandpass filter to eliminate spurs from getting
    too large as to cause problems with the FCC
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