differential amplifer help with bjts

Discussion in 'Homework Help' started by whatever125, Dec 9, 2012.

  1. whatever125

    Thread Starter New Member

    Dec 9, 2012
    Need help making a differential amplifier using bjt's. Vcc is +15 V and Vee is -15 V. The gain for the first stage needs to be 54.75 V/v with a bias current source of 2 mA. The second stage needs to have a gain of 39 V/v with a bias current of 4 mA.

    I'm having troubles getting the right resistors values. Help me please!

    This is done on pspice.
  2. #12


    Nov 30, 2010
    I actually don't know the answer, and it is forbidden for me to give you the answers, but I can steer your thinking.

    Considering that the emitters have a constant current limit, the only way Q4 can change the current through R3 is to steal it from Q3. That is the only way to get a voltage change across R2 and R3. So, if the base of Q4 becomes 1 millivolt more positive, Collector Q4 must become less positive. Meanwhile, collector Q3 must become more positive and these combined make up the voltage gain.

    I hope this is enough to get you started.