Positive Feedback Enhancement

Important: Positive Feedback Enhancement is a feature applicable to second and third order MFB's only

When operating near the limitations of the op amp with a high Q band pass or notch filter, the degradation of the filter performance may be minimized with the use of positive feedback enhancement elements. This done by adding a positive feedback path to the filter with the resisters labeled R and KR in the diagram below. The amount of feedback is determined by the enhancement ratio K. K is the value entered into the Active Control Panel Opamp section. Each biquad stage in the filter may have the enhancement ratio set independently by clicking on the op amp.

Positive Feedback Enhancement

 

The unenhanced and enhanced band pass filters with Qs of 10 and finite gain opamps are shown below to illustrate. The unenhanced filter suffers a degradation of 7 dB at the center frequency. An enhancement ratio of 4 reduces the degradation to only 2 dB. The ideal filter trace is shown in dark blue.

Negative SAB With No Positive Feedback Enhancement

 

Negative SAB With Enhancement Ratio of Four