What is the DC output behavior of a high-pass RC filter to a constant input?

• A. Output amplifies DC input.
• B. Output is zero for DC input.
• C. Output equals input for DC input.
• D. Output is unpredictable.

Answer: (B)

The key idea is that a high-pass RC filter blocks steady voltages and only passes changing signals. With a constant input, the signal has zero frequency (DC), and the capacitor prevents a steady current from flowing through the resistor. When the input first steps, the capacitor behaves like a short and the output across the resistor jumps briefly, but as the capacitor charges, its impedance grows and current through the resistor falls to zero. In that steady state, no current flows through the resistor, so the output voltage across it is zero. In other words, a high-pass filter suppresses DC, so the DC output for a constant input is zero.