I'm working through a tutorial on filters at https://www.electronicstutorials.ws/filter/filter_5.html (yeah, why not ask them? Because I get such good answers here, and I cannot add attachments to questions there!). They have an active lowpass filter example exercise  an RC filter amplified by a noninverting opAmp  which I thought I cracked. I used LTSpice to simulate the circuit, the Bode plot came out like I thought (and they said) it should. Then they "simplified" the circuit, moving the capacitor into the opAmp feedback loop. The idea is the Gain will become frequency dependent and provide the filtering attenuation. My Bode plot kind of works fine around the cutoff frequency, but at modest higher frequencies things go wrong (10KHz, so I'm assuming the choice of opAmp is not critical). I'm trying to understand what is going on.
I tried the circuit with about3 different opAmp models that I thought should work, all gave similar results. So the crux of my question is "should the second "simplified" circuit be behaving like the first one, and if not, what have I not understood, or what is causing the difference in these Bode plots?"
Thanks for any insights.
Peter
I tried the circuit with about3 different opAmp models that I thought should work, all gave similar results. So the crux of my question is "should the second "simplified" circuit be behaving like the first one, and if not, what have I not understood, or what is causing the difference in these Bode plots?"
Thanks for any insights.
Peter
Attachments

56.4 KB Views: 22

52 KB Views: 23

2.6 KB Views: 12