Feedback Capacitor Low-Pass Filter Pitfalls

Adding a capacitor in parallel with the feedback resistor of an op amp is an easy way of accomplishing low- pass filtering. This technique works quite well in an inverting  amplifier (see the curves below) but not necessarily in a non-inverting amplifier. If the NI amplifier has high gain, the filtering is not bad– but inferior to the inverting case.  As the NI amplifier gain is reduced, the filter effectiveness suffers. In fact, in a gain of +2V/V, there is only 6dB of stopband attenuation. In a voltage-follower   (gain of +1V/V), there is no low- pass filtering at all! In each amplifier, the value of R2 was stepped logarithmically from 100 ohms to 100k.\e(x,2) (Circuit is created by Neil P. Albaugh, TI-Tucson )

Circuit for Demonstration of Pitfalls related with the Feedback Capacitor in Low-pass Filters 

Feedback Capacitors Filter Pitfalls-blog3

Demonstration of Pitfalls related with the Feedback Capacitor in Low-pass Filters circuits

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Michael Koltai