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difference between CE and CC amplifiers

Discussion in 'Electronic Basics' started by Eric Lada, Feb 4, 2006.

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  1. Eric Lada

    Eric Lada Guest


    I've been told that common emitter and Common collector and Common
    Base amplifiers are so named because the inputs and outputs either share
    a common emitter, common collector, or base. I see where this is the
    case with the CE, and CB amplifiers, but the CC amp to me just seems to
    be a CE amp with the load attached to the emitter. So wouldn't it still
    have the emitter in common with the input and output. Could someone
    please tell me what I am not understanding?

    Thanks for any light you can shed on this subject.
  2. Ralph Mowery

    Ralph Mowery Guest

    If you look closer at he circuits you should see the common element is
    connected (AC or signal)
    to the ground or common of the input and output sides. While the cc and ce
    look similar, the coupling capacitor for the output is moved and also the
    capacitor for the emitter in the CE circuit is removed and placed from the
    collector to the ground.

    Each circuit has differant ammounts of current and voltage gain. Also the
    impedance for the inputs and outputs are differant.
  3. Guest

    If you draw your cc circuit with a pnp transistor, ie. with the
    collector connected to the ground, it will then "look" common. In fact
    as far as the ac signal is concerened all the power rails are common.
  4. Pooh Bear

    Pooh Bear Guest

    Common collector is more usually called emitter follower. That is a more
    helpful name IMHO.

    Your idea about the terminology is mistaken. For 'common'
    emitter/collector/base it may prove helpful to think *earthed/grounded*
    emitter/collector/base. Where 'earthed/grounded' means the ac signal is
    'earthed or grounded'. For example in a typical common-collector ( emitter
    follower ) configuration the collector is connected to the supply rail which
    is an ac ground.

    It's the common terminal that *doesn't* have a 'signal' connection on it. It
    is in a sesne 'common' because both input and output use ground as 'the
    other side of the signal'.

  5. These are good points and bring up to mind a confusion of my own, some
    time ago. And anything connected to any voltage rail is _supposed_ to
    not have any "signal" on it. If it has a signal there, the voltage
    rail isn't doing its job!

    I had not allowed myself to "see" a +5V rail as being similar to the
    "ground" or 0V rail. To me at that time, they were very different
    things, separated at least by a "battery," which was some complex
    thing but certainly not "nothing" in my mind. Even the fact that I
    used Thevenin equivalents to simplify circuits didn't clue me in,
    though it should have.

    The text that changed my mind (after some thinking) said that the
    battery has an effective zero impedance and that in this sense there
    is a "dead short" between 0V and +5V, for example, via the battery,
    with the important detail being the voltage change. This started me
    thinking more closely about exactly what the phrase, "voltage source,"
    meant and to start clearing up my internal ideas of both "voltage
    source" and "current source." I'd read texts saying this and that,
    but in my confusion I hadn't bothered to think more closely about
    them, and just left myself in a general state of conflation and

    Just to point out a circumstance which will "test" internal ideas
    about what all this means is a cascode BJT pair. The node between the
    two BJTs (the one connecting the collector of one to the emitter of
    the other) is held (at least, in terms of the signal) at a relatively
    stable voltage even though one does not actually see a voltage rail

    : ++V
    : |
    : |
    : \
    : / R1
    : \
    : /
    : |
    : +----> signal_out
    : |
    : Q2 | Q2 operates as a 'grounded'
    : |/c base, passing along the
    : +V-----| signal-generated current
    : |>e from emitter to collector
    : |
    : | <--- relatively stable
    : | voltage point located
    : Q1 | here has a voltage of
    : |/c about (+V - Vbe)
    : signal_in >----|
    : |>e
    : | because of the above stable
    : | voltage above, Q1 operates
    : \ as an emitter follower
    : / R2 (a good voltage controlled
    : \ current sink, here, because
    : / it's collector is 'clamped')
    : |
    : |
    : gnd

    I'm just a hobbyist and still learning slowly, though. So keep that
    in mind.

    By the way, when I was learning to draft electronic circuits in a
    course I took on the Tektronix campus, I was taught to organize the
    schematics so that "signal" flowed "left to right" on the sheet, that
    hole flow (or reversed electron flow) should always cascade from the
    top of the sheet to the bottom, and to _not_ bus voltage rails around
    (the argument here is that busing voltage rails _confuses the eye_ and
    makes it seem as though those wires are somehow important to
    understanding the schematic.) So one of the helpful things for me was
    to take schematics found in magazines, which rarely followed any of
    these rules if ever and are horribly confusing to read, I think, and
    to rewrite them into a form with the above rules applied.

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