Hi guys. This is my first post here. Were going to create a music-amplifier with a couple of different amplifiers: Effect, GE etc.
Our first task was to dimension this compound NPN
http://imgur.com/MQmIzIt
We've choosed the Vcc as 14,3V, because we were given the resistors and the 40mA current in Q8.
Beta for Q7 is 200, and Q8 is 20, that gives us a 4000 times current amplifier.
Va for Q7 is 45 V. Now we're going to create a small signal model for this and calculate the input resistance. Could someone help us?
Thanks in advance.

 

Update: Calculate Rpi as 4000/40x40m = 2,5kOhm, and Ro as 0,9+45/40m = 1147Ohms, but I don't know how to get the Ic or Ib. What should I do next to get the total input resistance?