Which configuration yields a high-pass filter?

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 means the circuit passes higher frequencies while attenuating lower ones. A capacitor in series between the source and the load does exactly that: its impedance is 1/(jωC). At low frequencies this is very large, so most of Vin drops across the capacitor and the load sees little signal. At high frequencies the capacitor's impedance is small, so most of the signal gets to the load. If you instead put a capacitor to ground between the source and load, it shunts high-frequency components to ground, which reduces the output at high frequencies and acts like a low-pass path. An inductor in series grows in impedance with frequency, blocking high frequencies and also yielding a low-pass effect. A resistor in parallel with the load doesn't introduce frequency dependence on its own. Therefore, the capacitor in series between the source and the load produces the high-pass response.

High-pass behavior means the circuit passes higher frequencies while attenuating lower ones. A capacitor in series between the source and the load does exactly that: its impedance is 1/(jωC). At low frequencies this is very large, so most of Vin drops across the capacitor and the load sees little signal. At high frequencies the capacitor's impedance is small, so most of the signal gets to the load. If you instead put a capacitor to ground between the source and load, it shunts high-frequency components to ground, which reduces the output at high frequencies and acts like a low-pass path. An inductor in series grows in impedance with frequency, blocking high frequencies and also yielding a low-pass effect. A resistor in parallel with the load doesn't introduce frequency dependence on its own. Therefore, the capacitor in series between the source and the load produces the high-pass response.