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Common Base Biasing

Discussion in 'Electronic Design' started by Warren, Apr 4, 2011.

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  1. Warren

    Warren Guest

    I am curious about common base configuration biasing. When
    using a _split_ supply, how are the emitter and collector
    resistors chosen?

    | |
    | | RC
    || | ||
    -----||-------|----- --------------||-----
    || | ^ / ||
    | ---
    .-. |
    RE | | |
    | | ===
    '-' GND

    1) I assume that you start with a desired Ic current.

    2) Given that the base is DC grounded (in this example at
    least), then it follows that the top of RE is -0.6V.

    3) So I assume that you compute RE = (VEE-0.6) / Ic.

    It appears that the collector can only swing between near
    ground and +VCC, so I would assume you target VC at VCC / 2.

    4) Hence Rc = (VCC / 2) / Ic.

    Is that the overall procedure?

    I know you can also arrange it so the base is only AC
    grounded, which then allows greater flexibility for choosing
    input/output impedances, not to mention output swing. But you
    often see this configuration listed as an example.
  2. Warren

    Warren Guest

    Phil Hobbs expounded in
    Thanks Phil. I think I'm finally getting a handle on this,
    after all these years.

    Reading Ch 2 of the Art of Electronics made the BJT biasing of
    emitter follower, common emitter (and implicitly) common
    collector configuration biasing easy. I only wish I had known
    that 30+ years ago.

    Everything else I've ever read seemed to make it so
    complicated with h-parameters etc. Finally some common sense
    prevails for a simple guy like me.

  3. Jamie

    Jamie Guest

    That looks good, I always look at that type of config as voltage
    divide with a little loss to it. They are good how ever at
    converting and semi isolation of signals.

  4. josephkk

    josephkk Guest

    Yikes. Trying to learn how to bias a transistor with the h-paramter black
    box model? Crikey, that was made artificially difficult. Must have been
    the only thing the prof understood.

    I learned bjt bias with the simplified-t model, then moved to the
    hybrid-pi model.

    The two-port h-parameter and a, y, and z equivalents are to be used at the
    next higher level, for chains of amplifier / filter stages.
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