I need to design a second order active filter low-pass Rauch structure of Bessel. Design data: f-cut = 15 Hz; C1 = 50 pF. More info in img
I believe what is happening is that you are using the value 1 for A0. This is not surprising as the notes are incorrect. This circuit actually has negative gain because it's an inverting amplifier. The formula should be A0=-R2/R1. With the given information that R1=R2=R3=R, then the gain is actually A0=-R/R=-1. Now using this value, you could find the minimum value C2 must be using the last equation. Note that it may help to choose value for C2 then find value for R. You could also start with a value for R and then find value for C2. I'm not sure on how practical the values for resistors and C2 need to be for your professor to be satisfied but it may help to find reasonable values for them and not extremely high values like 100M ohm resistor or 1F capacitor. With this information you can solve the rest.