\small u_3 = \dfrac {\left( 1 + s \, C_1 \, R_1 \right) \, s \, C_2 \, R_2 + 1 + s \, C_1 \left( R_1 + R_2 \right)} {s^2 \, C_1 \, C_2 \, R_1 \, R_2} \ u_1