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I am trying to design a output filter, a simply one (First order) just to play around with and for some reason when I run the simulation of it its not the correct Cut off frequency which it was designed for.

There's already a High Pass Filter at the input of the Amplifier Fc = 0.320Hz.

schematic

simulate this circuit – Schematic created using CircuitLab

This make sense as 560K R and 1uF C has a Fc ~= 0.318Hz

Bode Plot of Node 1:enter image description here

Bode Plot of Node 2:

But whats happening here? Why isn't the FC ~=0.284Hz but its 581mHz? enter image description here

And if I change the Fc to something like Fc = 20Hz with RL = 8 it works perfectly enter image description here

Whats going on?

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  • \$\begingroup\$ It would be helpful if you would maintain the vertical scale in both simulations. You will find out that your results would follow a 2-nd order filter, since you have two first-order identical RC filters in series, with OPA isolating one filter from another. Some explanations can be found here, electronics-tutorials.ws/filter/second-order-filters.html \$\endgroup\$ – Ale..chenski Nov 7 '18 at 3:35
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You have two factors at play here. One is the output impedance of the amplifier, which seems to be of the order of 100-150 Ohms. This impedance gets added to the 560 Ohm load, which shifts the -3 dB point. The other factor is that you have two RC filters in series, one is 500k/1uF on input, and the other is 500R/1mF on output. Transfer function of these two sequential filters is a product of two RC functions, and phase also shifts, which again shifts the -3dB point into higher frequency area.

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  • \$\begingroup\$ I did further testing, I added a buffer between the amplifier output and the filters and still the samething. Then I even changed the topology to a Dual power supply so I can take out the HPF in the input, still the samething. I believe its due to the filter in the negative feedback network \$\endgroup\$ – Leoc Nov 7 '18 at 2:45
  • \$\begingroup\$ @Pllsz, yes, this is correct. No matter how many buffers you change, the fact is that you have two nearly identical RC filters in series. Their combined cut-off frequency gets shifted. The negative feedback is totally in different frequency area and has little impact on low frequency cutoff. \$\endgroup\$ – Ale..chenski Nov 7 '18 at 2:50
  • \$\begingroup\$ Sorry, How is the filters in series if an op amp isolates the input from the output \$\endgroup\$ – Leoc Nov 7 '18 at 2:59
  • \$\begingroup\$ @Pllsz, this is exactly the point: the OPA isolates one filter from another, providing infinite load to first filter, and providing nearly zero output impedance to drive the second RC filter, thus making them in perfect series. Without OPA you can't possible get any good results. \$\endgroup\$ – Ale..chenski Nov 7 '18 at 3:03
  • \$\begingroup\$ What would be a solution to this? \$\endgroup\$ – Leoc Nov 7 '18 at 3:04

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