Which configuration yields a high-pass filter?

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

Answer: (A)

High-pass behavior comes from a path that becomes easier for high-frequency signals to pass while pulling low-frequency components away or dropping them across the reactive element. When a capacitor sits in series between the source and the load, its impedance is Zc = 1/(jωC). At high frequencies, Zc is small, so most of the input appears across the load and the output follows the input for high frequencies. At low frequencies, Zc is large, so most of the input drops across the capacitor and the output is attenuated. That makes this series-capacitor arrangement a classic high-pass.

Similarly, placing an inductor to ground between the source and load acts as a high-pass path because the inductor’s impedance grows with frequency. At low frequencies the short to ground shunts the signal away, reducing the output, while at high frequencies the shunt becomes effectively open, letting the high-frequency content reach the load. This is another way to realize a high-pass effect without a series capacitor.

In contrast, a capacitor to ground shunts high-frequency energy to ground, producing a low-pass effect, and an inductor in series blocks high frequencies, also producing low-pass behavior. A resistor in parallel with the load by itself doesn’t establish a high-pass filter.