(adsbygoogle = window.adsbygoogle || []).push({}); 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) ifseen 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?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Filter circuit impedance?

**Physics Forums | Science Articles, Homework Help, Discussion**