Which statement best describes a first-order RC low-pass filter with a resistor in series and a capacitor to ground?

• A. Passes high frequencies; output equals the input at DC.
• B. Passes low frequencies; output equals the input at DC.
• C. Has the same response as a high-pass filter.
• D. Attenuates all frequencies equally.

Answer: (B)

A series resistor with a capacitor to ground at the output forms a single-pole RC low-pass. It lets slow signals pass while attenuating fast changes. At DC (zero frequency), the capacitor behaves as an open circuit, so no current flows through the resistor and the output just follows the input. That means the output equals the input at DC, and low frequencies are passed with little attenuation. As frequency increases, the capacitor’s impedance drops, shunting more of the signal to ground and reducing the output.

So the statement that best describes this circuit is that it passes low frequencies and the output equals the input at DC. The other descriptions don’t fit because high frequencies are attenuated (not passed), it’s not like a high-pass filter, and the attenuation isn’t the same across all frequencies.