Hello, I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz. When I simulated the design using AC analysis, I found out that I've got a problem with high frequencies. The problem is that there is a decrease in the output. The following picture will explain my problem. http://forum.allaboutcircuits.com/attachment.php?attachmentid=50010&stc=1&d=1356210439 PS: I used active filters. I'd appreciate some explanation of why do I have such problem.