transfer function op amp feedback circuit
The transfer function of such a system with feedback can be found using the general formula

Vout/Vin=H(forward)/(1-loop gain).

In you case:

Vout/Vref=G(s)(1+Z2/Z1)/[1-G(s)(-Z2/Z1)*H(s)]

If H(s) is not existent set H(s)=1.