Question

Solved.

Answer 1

Vpi = iin rpi
gm Vpi = iin gm rpi
using miller current theorem, or by inspection
rin = rpi + (1 + gm rpi) RE
Vpi = Vin {rpi/(rpi + (1 + gm rpi) RE)}
Vout = - gm Vpi RC = - gm Vin {rpi/(rpi + (1 + gm rpi) RE)} RC
Vout/Vin = - gm {rpi/(rpi + (1 + gm rpi) RE)} RC
Vout/Vin = - gm RC /(1 + (1/rpi + gm) RE)}
You can also solve using series feedback