# Small- signal analysis of BJT

Can you explain to me how to obtain the small signal voltages of vc(vout),vb( for the upper BJT) and ve( small-signal voltage of the common emitter ? for this small-signal equivalent circuit. I have tried to do write KCL equations but I think I am not able to find the proper relationships. sorry for image quality.

• I can't even understand the annotations. – dim Dec 12 '16 at 19:53
• What does the u mean, as in r_o = 100ku? Can you show the full schematic so we can verify if your small-signal representation is correct? – vofa Dec 12 '16 at 19:57
• sorry again for my bad hand-writing. its ohm not u. they are resistances. – utdlegend Dec 12 '16 at 20:00
• Badly drawn circuit, not even sure that small signal model makes any sense, unreadable component value, not even showing an attempt at solving the equations. I'm moving on ! – Bimpelrekkie Dec 12 '16 at 20:03
• Could you look at it now? I have added the main circuit, hope it will be help full. – utdlegend Dec 12 '16 at 20:20

This will give you $$\frac{V_o}{V_b}$$ In order to find $$\frac{V_o}{V_{in} }$$ you would need to solve for $$\frac{V_b}{V_{in}}$$ and find the product of these results. The latter can be found by using the voltage divider between the 2.2kOhm resistor and the 1890 Ohm resistor in parallel with the base resistance. Additionally, to simplify further, you can find the resistance looking down into the emitter of the PNP transistor and treat it as an AC resistance. Hopefully this helps you answer the question!