# Homework Help: How does a bandstop filter work?

1. Apr 11, 2012

### liam1992

I was wondering how a bandstop filter works when it is connected in parallel as in the picture.

I know that the low pass filter eliminates higher frequencies and the high pass filter eliminates the lower frequencies. I understand how they form a bandpass filter when connected in series but I can't seem to figure out how they create a bandstop filter in parallel.

2. Apr 11, 2012

### Staff: Mentor

I think you mean "I was wondering how a bandstop filter works when a LP filter is connected in parallel with a HP filter as in the picture."

You could think of each as a frequency-selective resistor, so it's just a pair of resistors in parallel. The low frequencies find an easy path through "Resistor" A, and the high frequencies find an easy path via "Resistor" B. Midband frequencies find both paths difficult so not much of them pass through.

Of course, for this to work you take care in designing the filters' passbands, because you'll be needing a band of frequencies where both filters attenuate.

https://www.physicsforums.com/images/icons/icon2.gif [Broken] So does this mean we do, or we do not, require for the passbands of the LP and the HP filter to share some overlap?

Note, this explanation is a general explanation, and is not exactly how your circuit is arranged, but you should be able to see the difference.

P.S. what color do you see that first line in my post? I specified light brown, but I see green here.

Last edited by a moderator: May 5, 2017