Hello, I am getting stuck on the determining Rth and Eth in the red section of the circuit. I tried to calculate Rth, Eth by Thevenin's Theorem but I cann't get result.I am confused if I need to replace transistor by transistor equivalent model. I have tried the following way: In the base-emiter loop of figure 1: Vcc = Ib.Rb + Vbe+ (β + 1) Re and then I have the circuit like firgure 2. Can I conclude Rth= (β + 1) Re, Eth= Vbe ? Could anyone help me?