common collector power amplification

Discussion in 'General Electronics Chat' started by mentaaal, Sep 27, 2007.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Hey guys, we are currently studying the transistor in college and i was looking at the common collector mode of transistor amplification and i dont understand how there can be a current gain when the voltage gain remains roughly the same???
    I mean say for example beta for the transistor is 100.
    So the current gain for the load will be beta plus 1 (according to this website?)
    How is this possible? I mean say the signal source places 4 volts across the load. the current say is X. Then the current going through the load should be 100x + 1 right? then if thats the case surely the voltage across the load will be V=RI : V = R[100Ibase +1]I would be something like over 400 volts?
    Surely if the current going through a resistor increases the voltage across that resistor has to increase in accordance with ohms law??
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The common collector arrangement makes voltage gain impossible. The output voltage across the load is always .7 volts less than the base voltage. The load limits the current according to ohm's law - I = E/R.

    So, the way to look at it is that the base current is the emitter current divided by (beta + 1).
     
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    thanks for the reply. Ok so if there is no voltage gain or a gain of o -.7V and the current gain for the output is beta +1 then surely the input voltage dropped across the load would be alot less to facilitate the lower current going through the load. So v = ri so Vinput across the load would be (Ioutput/beta+1)(Rersistance of the load)
    If this is the case wouldnt most if the input voltage be dropped across the transistor then? And also if the output voltage magnitude is limited to something less than the input voltage wouldnt the remainder supply voltage be dropped across the transistor as well across the emitter collector junction?
     
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