Differential Amplifier plus CB

Discussion in 'Homework Help' started by Guitarras, Dec 10, 2010.

  1. Guitarras

    Thread Starter New Member

    Dec 10, 2010
    14
    0
    Hi everyone!

    I have some questions to scale the resistances of a differential amplifier with passive load. I will present the circuit:

    [​IMG]

    The objective is to scale the circuit of the figure to obtain a collector current of 5 mA transistor Q3. My first step was to create a current source of 5mA. I used a diode and a BJT and tested in a simulator, it is okay.
    Now my question is how to scale the resistors R1 and R2.

    Things that I know:
    vb = 0
    vbe = vb-ve = 0.7 -> v =- 0.7

    (12-vc) / R1 = ic -> VC1 = 12 - ic1 * R1 ...

    Can someone tell me how I can get some value of R1 and R2 only with this information?
    I don't ask for resolutions, I really prefer a explication of the operation or a method to resolve my problem than expressions with values, etc, etc because I'm not very comfortable with this.

    I appreciate any clarification.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    The only think we need is to use Ohms law and KVC, KVL.
    For example if we assume
    Ic1 = Ic2 = 2.5mA and Ic3 = 5mA
    So form KCL we have:
    IR2 = Ic2 + Ic3 = 7.5mA
    IR1 = 2.5mA
    Ib3 = 5mA/β = 5mA/100 = 50μA
    And from KVL

    Vb3 = 0V ----> becaues R3=R4 = 12V/ (10 * Ib3) 22KΩ
    Ve3 = Vb3 + Vbe = 0.7V

    R2 =( Vcc - Ve3) / (Ic2 + Ic3) = 1.5K
    R2 = (Vcc - Ve3)/ Ic1
    4.7K
    And finally we can chose R5 for a given Vce3.
    And if we assume Vce=6V then R5=1.2KΩ
     
  3. Guitarras

    Thread Starter New Member

    Dec 10, 2010
    14
    0
    Thanks, Jony130.
     
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