this is the circuit with which I am struggling. U_CC is 30V, U_out is supposed to be roughly 15V and R3 is 2kOhm. Our Prof. also requires a high input impedance.

My thoughts:

The resistors need to be of equal size, in order to achieve roughly 15V for U(out), this rules out one possible solution (R1 = 100kOhm and R2 = 20kOhm).

That left me with a problem. Isn't the input impedance calculated, h(FE) multiplied by R3? If this is the case, R1 and R2 don't play a role in input impedance and therefore I can pick up any of the three remaining results...

So, I looked around and found a different way to calculate the input impedance = U(in)/I(in). So, I was thinking, I use the 30V and divide it by R1 (20kOhm, 40kOhm or 100kOhm), this gives me the current flow through R1. Afterwards I divide 15V by R2(20kOhm, 40kOhm or 100kOhm). Finally I subtract Current I2 from I1 and I get the current running through the base(I(B)) of the transistor. Does this calculation make sense so far?

In order to calculate the impedance I divided 15V by I(B) and got the highest input impedance for the resistor pair with 100kOhm each.

According to my Prof. the right answer is 40kOhm for each resistor.

So what is wrong with my ideas? What am I missing?

Happy new year from Germany

Tobias