The common-collector amplifier - CORRECTION

Discussion in 'Feedback and Suggestions' started by mrchen, Oct 6, 2013.

  1. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    A correction in:
    http://www.allaboutcircuits.com/vol_3/chpt_4/6.html

    Incorrect phrase:
    Common collector: Input is applied to base and collector. Output is from emitter-collector circuit.

    Correct phrase:
    Common collector: Input is applied to base and EMITTER. Output is from emitter-collector circuit.

    That's IMHO, of course.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Where are you talking about, this paragraph?

     
  3. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    No, not this one. Just search the page for the quoted phrase.

    But, incidentally, the picture with the oscilloscopes is bad as well. The (black) negative lead of an oscilloscope is connected the ground/earth. By connecting the scopes as in the picture you would SHORT the left hand ( base bias ) battery. See this video for details:

    http://www.youtube.com/watch?v=xaELqAo4kkQ
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    That's actually a touchy (not the right term, but I don't have a better one handy) point. Arguably the point of those scopes is just to illustrate a concept and, as such, they are not real scopes with the common limitations of real scopes. Plus, not all real scopes have single-ended inputs (but most do and the ones that don't are quite expensive and quite proud about it).

    Also, the "scopes" that are used in the GUI interfaces for simulation software are never (that I am aware of) single-ended, which is reasonable given what their purpose is.

    Having said all that, it's a point that needs to be made (repeatedly) because people routinely have no idea that not only are the grounds of their actual scope probes tied together (they tend to think of them as if they were like multiple voltmeters), but that they are typically tied to the chassis ground and hence to the ground wire of the AC supply cable. As a consequence, they interact with any other equipment that is mains powered. Thus, in most circumstances, you should let your oscilloscope probes define you one ground point of the circuit.
     
  5. LvW

    Active Member

    Jun 13, 2013
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    Applying a common-collector configuration I never would connect the input signal between base and emitter (by the way: "between" is more accurat than simply "and"). Rather, it is applied between base and ground!
     
  6. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    The phrase "Common collector: Input is applied to base and collector" in the original article was an attempt to explain the term "Common collector". The expression "between base and ground" is more accurate indeed but it does not explain the term "Common collector".

    BTW, when you quote a message and edit that quote, it's good style to make it clear that the quote was modified. Otherwise the readers might be misled.
     
  7. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    The scope probe feature of the immensely popular simulator LTSpice uses the circuit ground as reference, which is a general safe practice with oscilloscopes.
     
  8. LvW

    Active Member

    Jun 13, 2013
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    mrchen, was there a misunderstanding?
    I took the quote from the second part of your post - and your suggestion was "...between base and EMITTER" .

    Do you still recommend this change?
    (By the way, the only quote editing was an underlining, nothing else)
     
  9. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    I can only recommend removing the incorrect phrase. The article needs to provide a good explaination for the term "common collector". Unfortunately I cannot offer a good explanation. Hopefully someone else will.

    I don't see it that way. Just compare the OP and the quote. But let's move on, it's not paramount.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    If you look at the small-signal equivalent circuit, the collector is tied to common. Hence, common-collector. This is often described as "signal ground" or "AC ground".
     
  11. LvW

    Active Member

    Jun 13, 2013
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    OK - I follow your advice: Here comes again the quote from your first posting:

    (Start of quote)
    Correct phrase:
    Common collector: Input is applied to base and EMITTER. Output is from emitter-collector circuit.
    (End of quote).

    Do you see any difference to the quote I have given in my post #5 ?
     
  12. LvW

    Active Member

    Jun 13, 2013
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    But you have offered, indeed, something which you call a "correct phrase"!
    And for my opinion it was not a "correct" phrase because the signal is to be applied between base and ground. And - as stated recently by WBahn - the collector is ac-wise connected to signal ground.
     
  13. mrchen

    Thread Starter New Member

    Oct 6, 2013
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    This is an interesting angle, please can you elaborate with a novice audience in mind so your explanation can be incorporated into the article.
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    Oh, I don't think there's too much interest in incorporating my explanations into the E-book. My explanations on most things come from a significantly different perspective than the one they are trying to promote. Not saying better or worse, but different.
     
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