Here's the circuit
If C5 is 0.1F then the group delay plot is constant in the pass band:
If C5 is 470uF then for the frequencies between 1 and 3 Hz it's not constant anymore:
Why does that happen?
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\$\begingroup\$ You reduced the RC time constant by a factor of 200. With 0.1F, extend the frequency response plot down to 0.01Hz and you'll start to see it. \$\endgroup\$– user16324Commented Nov 18, 2021 at 22:12
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1 Answer
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It's an RC high pass filter, and when modifying the capacitance, it modifies the cutoff frequency, and modifies phase response.
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\$\begingroup\$ @MissMulan: No. This circuit does have a lowpass RC filters, but not involving C5. C5 is definitely part of a high-pass filter. \$\endgroup\$ Commented Nov 18, 2021 at 22:22
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\$\begingroup\$ @MissMulan No, I don't mean low pass. \$\endgroup\$– JustmeCommented Nov 18, 2021 at 22:31
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\$\begingroup\$ @MissMulan They are looking at the first stage, which is an opamp as a follower. Not the remaining two stages. The 1st stage input uses a voltage divider to bias the input half-way between the rails and uses an AC capacitor to pass along the AC while blocking the DC. That's the purpose of that arrangement. So you may be correct to suggest it's not strictly intended as a high-pass filter. However, it also is a high-pass filter, regardless of its biasing and DC blocking functions. \$\endgroup\$– jonkCommented Nov 19, 2021 at 4:29
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\$\begingroup\$ The V/f graph is one of a low pass filter that's why I assumed (maybe falsely) it is a low pass filter. \$\endgroup\$ Commented Nov 19, 2021 at 10:04