your "Common Base" amplifier page is wrong

Discussion in 'Feedback and Suggestions' started by Viper, Sep 11, 2006.

  1. Viper

    Thread Starter New Member

    Sep 11, 2006
    1
    0
    this page:
    The common-base amplifier

    is wrong.

    That is a common emmitter (well, common emmiter with a degeneration resistor) amplifier.

    In particular, this statement : "It is called the common-base configuration because (DC power source aside), the signal source and the load share the base of the transistor as a common connection point" is very incorrect.

    A common base amplifier has the input to the amplifier going into the emiiter, and the output comes out from the collector.

    The base is "common" to ground - usually at some DC voltage with a bypass cap to ground so that it's an AC ground.

    Wikipedia's Common Base for reference
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Does indeed look like some of the illustrations are off. I'd never noticed that before. Signal generator needs moving.

    The statement ""It is called the common-base configuration because (DC power source aside), the signal source and the load share the base of the transistor as a common connection point" is accurate, if cumbersome. Input is applied across emmitter and base. Output is taken across collector and base. Base is shared by source and load, whether base is grounded or not.
     
  3. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    I'm with thingmaker3 on this one, the picture is a little clumsy (particularly considering the traditional circuit for a common-base configuration). However, it certainly isn't a common-emitter configuration even if there is ambiguity in the diagram.

    I would be interested in hearing other peoples comments regarding its clarity, and I will get Dennis involved to see what he thinks about this particular page.

    Dave
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It appears to be correctly connected in a common-base configuration as far as I can tell.

    If it were a common-emitter configuration, the negative terminal of the collector's DC power source would not be referenced to the base.

    hgmjr
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    We all know that electrons could care less how schematics are drawn. The diagrams look fine to me. I would suggest to Viper, that before you throw stones at someone else's work you be very sure that you are standing on solid ground.

    BTW we use a common base configuration with the base connected to +5V as a level shifter. It works great.
     
  6. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    If I recall, the 741 employs a similar set-up to the common-base configuration shown in the above link to provide this level-shifting for the intermediate stage.

    However, my recallection of analogue circuit design is somewhat hazy in recent times!

    Dave
     
  7. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    416
    11
    The circuit, while technically correct, is usually drawn horizontlly. The small signal input in the spice circuit, even though in series with the emitter-base circuit, is usually moved within the loop closer to the emitter. These changes would have no effect on the funciton of the circuit, but would surely improve the clarity.

    Since I am now working on completing the Semiconductor volume, it will be easy to clarify the schematic diagrams. The bad news is that completing the Semi volume will take a couple years. So, unless you need this change in the master copy righ away, look for it in the next big release.
     
  8. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Hi Dcrunkilton,

    Thanks for your comments regarding the common-base configuration circuit.

    I think the important point to make here is that the circuit as draw is the general case of the common-base configuration, however it is typical of textbooks and notes to draw the circuit with the base grounded and the input to the emitter, which is a special/particular case of the common-base configuration. We have recently had someone ask this very question in the forums. Maybe a sub note on this point in the on-line textbook should be sufficient.

    I am not of the opinion that we should be changing this circuit until at least the updated version is created.

    Dave
     
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