low-pass vs high-pass filter RC fundamentals

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mspagon

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In a very simple RC circuit where there is an AC power source (AC), resistor (R), capacitor (C)...

Fundamentally, why is a series circuit with the configuration AC->R->C with the output taken across the capacitor a Low-pass filter

WHEREAS

a series circuit with the configuration AC ->C->R with the output taken across the resistor a High-pass filter?





I started playing with LTSPICE (https://www.linear.com/designtools/software/), which is AWESOME btw... and I want to start learning circuit design with a basic research approach starting with RC filters.

I'm going to spend a week on my own discovering the fundamentals and then come back to view the answers this wonderful community has left me!
 

Mspagon, the best method to find an answer to your question is to find the transfer function for both cases.
You have to do nothing else than to apply the classical voltage divider formula for the parts R and X=1/jwC.

Then, apply the formulas to see what happens if the frequency w=2*Pi*f is rising starting at 0 Hz.
I am sure you will see what happens.
When you want to go into details it is advisable to split the complex transfer function into two parts: Magnitude and phase.
 

Impedance of capacitor is 1/jwC . At zero frequency , impedance is infinite and at infinite frequency impedance is 0. So capacitor acts as a open circuit at zero frequency and short circuit at infinite frequency. So AC->R->C acts as a low pass filter and AC ->C->R acts as high pass filter.
 

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