Differential Amplifier plus CB

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

  1. Guitarras

    Thread Starter New Member

    Dec 10, 2010
    Hi everyone!

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


    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
    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
    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
    Thanks, Jony130.