Common Collector Transistor Question - Voltage?

Discussion in 'General Electronics Chat' started by cube01, Nov 16, 2011.

  1. cube01

    Thread Starter New Member

    Mar 10, 2011
    16
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    I've been working through the tutorials here on this site - what a great resource, and well written. My sincere thanks to the writers...

    There is one thing that is bothering me though, concerning Common Collector transistor circuits and their output voltage:

    Given the below circuit (taken from the tutorials here) I understand why there is current gain and why the voltage across Rload would be lagging behind by .6volts if there is no voltage gain from V1. What I don't understand is, why is there no voltage gain from V1? Why don't I see voltage from V1, at all, across Rload?

    My multimeter says it is all dropped across Q1, but why? Shouldn't I still see some of it across Rload? It being in series and all?



    [​IMG]


    Thanks in advance!!
    -Cube
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    Voltage gain for Common Collector is smaller than one.
    Vout = Vin - Vbe = Vin - 0.6V (for DC voltage)
    So if we say that Common Collector has no voltage gain.
    This will simply mean that voltage gain is less then one.
     
  3. jimkeith

    Active Member

    Oct 26, 2011
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    99
    While the DC 'gain' is less than unity, the incremental or AC gain is very, very close to unity.
     
  4. cube01

    Thread Starter New Member

    Mar 10, 2011
    16
    1
    I understand all that... What I don't understand is why we don't see voltage across Rload from V1.

    Since Rload is in series with V1, shouldn't we see some of V1's voltage across it?
     
  5. jimkeith

    Active Member

    Oct 26, 2011
    539
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    99% (depending upon beta or hFE) of the current through RL is the transistor collector current--the remaining 1% is the base current coming from Vin. If things are not so, you must have an error somewhere such as wrong pin-out or wrong type of transistor.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Most of the load current comes from V1, but it is not possible for the Rload voltage to rise even as high as Vin let alone rise above it, assuming that the transistor is working normally: not broken down with excessive voltage, nor leaking severely.

    The reason is that the base-emitter junction of the transistor requires forward bias for the transistor to conduct. Any tendency for the emitter voltage to rise above about Vin-0.6V (for a typical silicon transistor) would immediately* choke off the emitter current.

    Edit: *Not the most fortunate choice of words , given that transistor action involves delays - what I meant here is that reduction of the current would follow as a direct consequence. This is a form of negative feedback, and results in the emitter voltage "following" the base, but with the negative voltage drop required to turn on the base-emitter junction.
     
    Last edited: Nov 17, 2011
    cube01 likes this.
  7. cube01

    Thread Starter New Member

    Mar 10, 2011
    16
    1

    Ahhh... I see.
    Very good explanation, and answers my question perfectly... I had assumed it was some form of negative feedback.
     
  8. cube01

    Thread Starter New Member

    Mar 10, 2011
    16
    1
    To everyone else who answered - you did not address my specific question in your responses... Did I use a wrong term somewhere that caused you to misunderstand what I was asking?


    (Just for my FYI)

    Thanks again...
     
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