# output resistance of discrete amplifier

I have the three-stage amplifier below. I am interested in finding the output resistance of it. Just wanna check if am doing it correctly? Thanks

simulate this circuit – Schematic created using CircuitLab

Analysis: $$i_o=i_2-i_1\\i_1=i_{b3}+g_{m3}v_{\pi 3}\\i_o=i_2-i_{b3}-g_{m3}v_{\pi 3}\\i_o=\frac{v_o}{2.4k\Omega }-\frac{\left(v_{\pi 3}-v_o\right)}{r_{\pi 3}+5.6k\Omega }-g_{m3}v_{\pi 3}\\v_{\pi }=-v_o\\i_o=\frac{v_o}{2.4k\Omega \:}-\frac{\left(-v_o-v_o\right)}{r_{\pi \:3}+5.6k\Omega \:}-g_{m3}\left(-v_o\right)\\i_o=\frac{v_o}{2.4k\Omega \:}+\frac{\left(2v_o\right)}{r_{\pi \:3}+5.6k\Omega \:}+g_{m3}\left(v_o\right)\\g_{m3}=77.6m\left(\frac{A}{V}\right)\\R_o=\frac{v_o}{i_o}=12.7\Omega$$

• How are you doing it? What load current is planned? Aug 11 '18 at 18:04
• You need to add a large capacitor in series with the V3 voltage source.
– G36
Aug 11 '18 at 18:05
• try 10mF at first
– G36
Aug 11 '18 at 18:08
• Show us your work
– G36
Aug 11 '18 at 18:19
• The voltage divider equation is the answer vpi3 = -Vo x rpi3/(Rc2 + rpi3)
– G36
Aug 11 '18 at 19:36

$Zout = \dfrac{R_{C2}} {h_{FE3}}||R_{E3}$ For AC coupled output to a load , looking back into Zout, a negative voltage now sources current from the external cap swing sinks current and if this exceeds I(Re3) for