Question about BJT CC, and CE amplifier.

Discussion in 'Homework Help' started by RyanKim, Mar 18, 2012.

  1. RyanKim

    Thread Starter Member

    Sep 18, 2011
    Hello all you wonderful people,

    I was wondering, Im working on some BJT material, specifically 2 variations that I'm having trouble being consistent with. I've attached a picture with the configuration one CC and one CE amp. I've drawn their correct AC equivalent cct's/

    My question is, in the CE configuration, with a resistor at the emitter: when we calculate Rout (Resistance seen at the output) do we not factor in the low side current side of the transistor? THat is, why are RB and rb not considered. I ask this because in the CC configuration the calculation for Rout does factor in RB and rb by adding them in series and "scaling" them by a factor for beta ac +1(or hfe + 1). These configurations are nearly identical, yet I find this calculation a little counter intuitive. Conversely, Im a little unsure why we even look at the RB and rb resistance when getting Rout for the CC BJT. Do those resistances act like more internal resistance for the current source?

    Any help would be much appreciated.

    PS the naming conventions we use seem to be inconsistent with much of the world, so if there is some confusion I apologize in advance.