Class AB Diode-biased Push-Pull Amplifier

Discussion in 'Homework Help' started by CrashingThunder, Mar 11, 2014.

  1. CrashingThunder

    Thread Starter New Member

    Mar 11, 2014
    I'm a bit confused in finding a few things in regards to the circuit below:


    I need to find the input resistance, the output resistance, and the voltage gain of the circuit. I found r_pi already, it being 49.5Ω. The β value of the top transistor is around 20 or so (not too great, but whatever). Applying the half circuit model (assuming Q2 is OFF and Q1 is ON), here's what I think R_in is:

    R_in = R1||R2||(r_pi + (β+1)(Re + R_L))

    with R1 being the 3.9k, R2 being the 6.8k, and Re being the 0.5Ω. Here's what I think R_out is:

    R_out = Re + (r_pi + (R1||R2))/(β+1)

    I'll leave things at that for now. Any help is much appreciated, but I ask that if you just provide an answer, please explain it. I'm a bit lost.

    EDIT: Also, I lied. The transistors used in the circuit are TIP41/42 BJTs, if that makes any sort of difference, or maybe in case you were curious for whatever strange reason.
    Last edited: Mar 11, 2014