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So I have been having trouble understanding the difference of Rc filters that have the capacitor in series with the resistance and Rc filters that have the capacitor parallel to the resistance . To make this more specific , why are both low pass filters? Can all filters be made in parallel and in series?

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  • \$\begingroup\$ The voltage on a capacitor can't change "suddenly". So it will block any fast (high frequency) signals. That's the intuition. The rest is a math. \$\endgroup\$
    – Eugene Sh.
    Dec 1, 2017 at 16:30
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    \$\begingroup\$ They are not both low pass filters. The upper circuit is a high-pass filter. \$\endgroup\$
    – The Photon
    Dec 1, 2017 at 16:51
  • \$\begingroup\$ Check the circuit at the two extremes. Effective resistance of a capacitor at 0 freq is infinite. Conversely, at infinite freq, the capacitor looks like a short. \$\endgroup\$
    – horta
    Dec 1, 2017 at 16:53
  • \$\begingroup\$ Convert both filters to Z impedance and you'll see that their different \$\endgroup\$
    – Voltage Spike
    Dec 1, 2017 at 16:55

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They are not both low pass filters. The upper circuit is a high-pass filter.

But the fact the capacitor is labelled \$\rm C_f\$ suggests this filter might be used in the feed back path of an op-amp circuit.

In that case, the overall op-amp circuit might very well be a low-pass filter. Because using a high-pass filter to increase the negative feedback at high frequencies will result in reducing the response of the overall op-amp circuit at those frequencies.

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