A Derivation w.r.t AC Signal

Discussion in 'Homework Help' started by imfaizzi, Oct 30, 2008.

  1. imfaizzi

    Thread Starter Member

    Oct 11, 2008
    20
    0
    hi,

    can anyone gimme the Solved derivation (or its link/pdf/anything) for the following equation.

    Code ( (Unknown Language)):
    1.  
    2. [B]__
    3. [SIZE=4]R[/SIZE]E = [U]25mV
    4. [/U][SIZE=4]    I[SIZE=2]E[/SIZE][/SIZE][/B][SIZE=4][SIZE=2]
    5. [/SIZE][/SIZE]


    Also, what is RE and IE anyway?
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Re is the impedance looking into the emitter of the transistor and Ie is the DC bias current flowing through the emitter.
     
  3. imfaizzi

    Thread Starter Member

    Oct 11, 2008
    20
    0
    so how's it can be derived?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Use the diode equation

    I=Io*((expV/VT)-1)

    and find dV/dI to get Re

    dI/dV=(Io*exp(V/VT))/VT

    but Io*exp(V/VT)=I+Io

    thus dI/dV=(Io+I)/VT, because Io is very small we ignore it and we get

    dI/dV=I/VT

    but Re=dV/dI=VT/I=25mV/I, because VT equals about 25mV at room temperature.
     
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