Which configuration yields a low-pass filter?

• A. A capacitor to ground between the source and the load (or an inductor in series between the source and the load).
• B. A capacitor in series between the source and the load.
• C. An inductor to ground between the source and the load.
• D. A resistor in parallel with the load.

Answer: (A)

A low-pass filter lets through the slow changes (low frequency) while attenuating the fast changes (high frequency). A simple way to realize this with a single reactive element is to place it in a way that its impedance changes with frequency and interacts with the path from source to load.

Placing a capacitor to ground between the source and the load creates a path that high-frequency components can take to ground more easily, because the capacitor’s impedance drops as frequency rises. At low frequencies, the capacitor looks like a large impedance, so the load sees almost the full input. At high frequencies, the capacitor shunts more of the signal to ground, reducing the load voltage. This is the hallmark of a low-pass response.

An inductor in series between the source and the load works the same way from a different angle: its impedance grows with frequency, so low-frequency signals pass with little drop, but high-frequency components see a larger drop across the inductor, leaving less at the load.

The other configurations don’t produce the same frequency-selective behavior. A capacitor in series blocks low frequencies, acting as a high-pass. An inductor to ground provides a path to ground that mainly affects the low-frequency content, which doesn’t give the desired low-pass behavior. A resistor in parallel with the load changes the load impedance but does not create a true frequency-dependent filtering.