Confusuion on BIPOLAR JUNCTION TRANSISTORS Ch-4

Discussion in 'Feedback and Suggestions' started by Circuit learner, Jul 27, 2009.

  1. Circuit learner

    Thread Starter New Member

    Jul 27, 2009
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    Thank you for this nice book that intuitively teaches people about electronics. I was enjoying learning reading the Lessons In Electric Circuits -- Volume III book until I got confused in the Chapter 4 - BIPOLAR JUNCTION TRANSISTORS. For me the common collector circuit looks very similar to common emitter circuit. I think the transistor needs to be flipped vertically with emitter on top and collector below in the circuit. Am I correct? If not please clear my confusion.

    Thank you.
     
    Last edited: Jul 28, 2009
  2. Ratch

    New Member

    Mar 20, 2007
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    Circuit learner,

    What good would it do to draw the circuit upside down? In the common emitter circuit, the output is taken off the collector resistor. In the common collector, the output is taken off the emitter resistor. Two different topologies.

    Ratch
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    They also have two different amplifier properties, the common collector is also called a voltage follower, no voltage gain what so ever (though lots of current gain) and does not invert the signal.

    The common emitter has lots of voltage gain, and the output signal is inverted 180°. In the digital world this is an invertor.

    Even though the BJT is made of layers (such as a NPN), there is a definate difference in structure between the emitter and the collector.
     
  4. Circuit learner

    Thread Starter New Member

    Jul 27, 2009
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    Thank you Bill and Ratch for the explaining but I still have a confusion, why is it called common collector (C).

    In common emitter (E) we have one battery connected between B (base) and E, and another battery between C and E, so E is common.

    In common base configuration we have one battery between E and B, and another battery between C and B; thus B is common.

    But in common collector C is not connected to input battery at all. The notes says that Common collector amplifier has collector common to both input and output. In the figure at link http://www.ibiblio.org/kuphaldt/electricCircuits/Semi/SEMI_4.html#03100.png the emitter still looks common, except for a resistor after the emitter. I was expecting batteries between B and C, and between C and E. Please clarify. Thank you in advance.
     
  5. steveb

    Senior Member

    Jul 3, 2008
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    469


    The batteries are not important for the classification. The common collector has the base as the input and the emitter as the output. The common base has the emitter as the input and the collector as the output. The common emitter has the base as the input and the collector as the output. You can see the pattern. Note that the collector must be an output or a common, while the bass must be the input or the common. However, the emitter can be the input, output or common.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Once again I think we are mixing details of mechanism and operation to the detriment of understanding.

    Simply put:

    The transistor has three legs (terminals).

    Considering the signal.
    A circuit general 'device' has two terminals input and two terminals output making four

    So one of the 3 transistor terminals must do a double duty; this is the 'common' terminal.

    The common terminal plays a part in both input and output.
    The other two appear in the input or the output but not both.

    So with common emitter

    The input is between base and emitter (plus some biasing etc)
    The output is between collector and emitter. (plus some biasing etc)

    Hope this helps clear it up.
     
  7. Circuit learner

    Thread Starter New Member

    Jul 27, 2009
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    Thank you for the great explanations, makes me think differently now. So, CAN WE DESIGN JUST ONE CIRCUIT THAT ACTS AS BOTH common E as well as common C (depending on where we take the output). We give input to the base and we connect a resistor Rc in series with collector, and a resistor Re in series with emitter. Now if we take output across Rc it will be common emitter and if we take output across Re it will be common collector. Is my understanding correct? Thank you.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    NO NO NO

    Read my post again carefully.

    You should know that an input or output or any other signal or any voltage at all is between 2 points or 2 terminals or 2 anything.

    There is no such thing as an input to the collector, base or emitter, or an output from the collector, base or emitter.

    When you connect a battery, how many connections do you have to make? When you connect a loudspeaker , how many connections do you have to make?
     
  9. Ratch

    New Member

    Mar 20, 2007
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    studiot,

    I think he means one input in series with the base, and two outputs each from across the emitter resistor and collector resistor respectively. That should work if the loads are small enough so that crosstalk is not a problem.

    Ratch
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Hello Ratch,

    I know what the OP means, and how a phase-splitter works.

    No one should be trying to understand how a transistor circuit operates, unless they have a clear idea of the basics - current, voltage, resistance etc.

    An input comes along two wires and an output goes along two wires - making four in all.
    A transistor has three wires.

    So I'm inviting the OP to think about how to connect four wires to three.

    The OP refers to "the output" which in my english lessons meant there was only one output.

    Correct thinking and correct english at an early stage leads to a proper grasp of circuit properties in the long run.
     
    Last edited: Jul 29, 2009
  11. Unregistered

    Guest

    Firstly, please do not hijack another thread. Secondly, if you wish to ask a question please sign up and do so in the appropriate forum - this forum is only for feedback and suggestions relating to the site and material. Thank you - Dave
     
    Last edited by a moderator: Aug 4, 2009
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