Cascade BJT AMP two stages - HELP

Discussion in 'The Projects Forum' started by darryl capulla, Dec 16, 2014.

  1. darryl capulla

    Thread Starter New Member

    Dec 14, 2014
    Hello Engineers,
    I just want to Design a Cascade BJT Amplifier Two Stages,
    but my problem is, Im only given two values
    Vin = 50mVp @ 25Khz
    Vo= 3Vp

    Vcc = Any Value

    Amplifier should work 25kHz

    Any suggestion how to find the values or of resistor and capacitors??
    Please help me.. :(:(
  2. darryl capulla

    Thread Starter New Member

    Dec 14, 2014
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    There are no hard and fast rules for determining the values, but I'd use the following steps for both stages, as a first approximation. Re and/or Rc might then need tweaking to get the exact voltage gain you need:-

    1) Choose Vcc. Since you need Vo=3Vpk, make Vcc >> 3V. I'd choose 9V for example.
    2) Choose the collector current Ic. As you are not constrained re the load the amp drives, I'd go for ~2mA (arbitrary).
    3) Calculate the Rc value such that the collector current drops half the supply voltage. This will bias the collector appropriately.
    4) Choose Re as about 20% of the Rc value.
    5) Calculate the Re voltage drop and add 0.7V to get the base voltage Vb.
    6) Calculate R1 and R3 as about 10 x Rc.
    7) Calculate R2 and R4 to give the calculated Vb.
    8) Calculate the total gain as 3V/50mV = 60 and assign a gain A of square root 60 = ~8 to each stage.
    8) Replace each Re by two resistors Rea,Reb in series, where Rea = Rc/A and Reb = Re-Rea.
    9) Connect Ce across Reb only. Calculate Ce to have a reactance of < Reb/100 at 25kHz.
    10) Calculate Cb to have a reactance of < Rea at 25kHz.
    BobTPH and darryl capulla like this.
  4. #12


    Nov 30, 2010
    Remove Ce1 and Ce2. They make the gain unpredictable in that configuration.
  5. darryl capulla

    Thread Starter New Member

    Dec 14, 2014
    Thanks alot Sir!!! :):):)