1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution Hi all, i have some doubts on how to find the impedances for active filter circuits. Let's start with a simple low pass filter : http://img85.imageshack.us/img85/6484/52244578oo4.th.jpg [Broken] The input impedance should be R + Xc; and the output impedance should be ZL || (RF+R1) once i short the input. However, a same filter but at 2nd order configuration is confusing me. Assuming the circuit diagram is given as below : http://img85.imageshack.us/img85/4529/lp2ia8.th.jpg [Broken] Will be output impedance be solely ZL || (RF+R1) if seen from port A-B ? Or will it have to include the feedback capacitor and the input side with the source short-circuited as well? As for the input impedance, is this derivation method along the right line? applying KCL at red node V1 Ii = I1 + I2 (Vi-V1)/R1 = (V1-Vo)/XC2 + (V1-0)/(R2+XC2) where vo=KVx ; K is the DC voltage gain of the amplifier. Vx = (V1*XC1)/(R2+XC1) After that, from definition of Zi = Vi/Ii; try to arrange the equation to fit Vi/Ii?