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question on RF amplifiers

Discussion in 'Electronic Basics' started by Strelnikov, Sep 8, 2006.

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  1. Strelnikov

    Strelnikov Guest

    Hi everybody,

    I am a beginner in RF.
    Looking at several linear amplifier diagrams for the UHF range, I almost
    always see two active stages coupled like this: the collector or drain of a
    previous stage drives a coil connected to the supply rail, then via a
    capacitor, from the collector the signal goes to a matching element, like a
    tank or a transformer, and then to the base or gate of the following active
    element. My question is: why the primary of an eventual impedance adapting
    trafo couldn't be connected between the first stage collector and the
    positive rail, resuming in one component the coil and the matching element?
    Thanks in advance for the answers, and/ or for suggestions.

    Mirko
     
  2. Joerg

    Joerg Guest

    Hello Mirko,


    We all were at some point :)

    I have seen them both ways. However, two things can lead a designer to
    select the ground-based approach:

    a. Reducing noise on the power rail to modulate into the signal path.

    b. Unwanted feedback and instability resulting from that. If a board has
    to be done at rock-bottom cost you may only have two layers available.
    On RF boards there will (usually) still be a nearly complete ground
    plane but the supply rail is a wide trace and some bypass capacitors.
    This may not be enough to prevent RF from the last stage to make it into
    the supply rail and from there into the tank or matching circuit of a
    younger stage if these circuits hang on the rail. This can cause
    instability or oscillation.
     
  3. PeteS

    PeteS Guest


    Indeed so.

    This is one of the reasons that much communications equipment, even
    today, uses a negative power rail: i.e. the transistor collectors
    ultimately go to ground (which is in fact, the positive side of the
    power supply) and the emitters ultimately go to the negative rail.

    This can simplify the design (depends on many things of course).

    [Slightly O.T. for the OP] Although it's possible to use a positive
    supply with P-channel or PNP devices (so the collectors could return to
    ground with a positive supply although the only difference between a
    positive supply and a negative one is which way round the output is
    connected), the physics of the various transistors makes the N-channel
    / NPN devices preferable in RF amps [in general], but that's a separate
    subject :)

    Cheers

    PeteS
     
  4. Joerg

    Joerg Guest

    Hello Pete,
    Yes, and then also you have to go with what's on the menu. Pretty much
    all the hot RF transistors are npn if you don't want to end up with hard
    to procure boutique parts.
     
  5. PeteS

    PeteS Guest

    Hi Joerg - a fine weekend to you.

    As you note, it's virtually all N-channel / NPN, but then that's
    probably because they are preferable because of their internal
    characteristics (chicken and egg problem :)

    Cheers

    PeteS
     
  6. Strelnikov

    Strelnikov Guest

    Thanks everybody for the answers. So, my pcb being a two layer, I'll have
    more than a reason to use an extra inductor and to stick the primary to the
    ground layer.

    Bye

    Mirko
     
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