common base amplifier definition

Discussion in 'General Electronics Chat' started by boom, Oct 14, 2006.

  1. boom

    Thread Starter New Member

    Oct 14, 2006
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    hi, i noticed in chapter 4, the common base is called that because the "the signal source and the load share the base of the transistor as a common connection point", however other websites define it as a grounded base and the signal comes in on the emitter leg. just looking for some clarification, thanks.
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
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    Yes this is a question that appears to pop-up from time-to-time. The description here at AAC is correct and is applicable to all common-base configurations, i.e. it is the general case for the common-base configuration. The examples on other websites, and widely shown in textbooks, is where the base is grounded, i.e. 0V, and the input (Vin) is fed to the emitter is a specific case of the common-base configuration - what you should see is that the common-base principle applies in this example where the common point is ground. For an arbitrary case the base could be at 5V, 10V or any other voltage that the designer would require.

    Hope that clears up what is often a misconception regarding the common-base (or common-gate for MOSFETs) configurations.

    Dave
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    I second Dave's point. We make extensive use of the common base configuration as a level shifter. The base is connected to +5VDC. The emitter is connected to an 8051 quasi bidirectional port through an emitter resistor. The collector is connected to the base of a PNP switch and a bias resistor. The emitter is connected to +24 VDC and the collector goes to the load.

    The 8051 quasi bidiretional port has an easy time sinking current in the low state and a difficult time sourcing current in the high state.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    The input signal on the emmitter has a reference point - ground in most cases, but not all. The output signal also has a reference point. The base is that common reference point in a common base amplifier, whether the base is grounded or not.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    To further clarify, grounded (or common) base means that the signal source driving the emitter, the collector load, and the base are all referred to AC ground. See attachment.
     
  6. Søren

    Senior Member

    Sep 2, 2006
    472
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    Hi,


    Why do I get a "you're not logged in" screen when I try to view the attachment (and re-log in just goes to the same screen) ??
     
  7. Dave

    Retired Moderator

    Nov 17, 2003
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    Strange, I don't get that screen. Does this occur when you view any attachment here at AAC?

    Dave
     
  8. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi Dave,

    Must have been a temporarily hick-up, 'cause now it works just fine :)

    Sorry for whatever concern I may have caused.
     
  9. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Thanks for the feedback.

    Dave
     
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