Which configuration yields a low-pass filter?

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)

Low-pass behavior means slow signals pass with little attenuation while fast components are reduced. A capacitor to ground placed between the source and the load does exactly that: at low frequencies the capacitor looks almost open, so the load sees nearly the full input. As frequency rises, the capacitor’s impedance drops and it shunts more of the signal to ground, pulling high-frequency components away from the load and lowering the output. The same idea can be realized with an inductor in series between the source and load: it presents a small impedance at low frequencies so the slow signal gets through, but its impedance grows with frequency, attenuating the signal before it reaches the load. The other setups don’t produce a low-pass response. A capacitor in series blocks DC and passes higher frequencies, which is a high-pass effect. An inductor to ground shunts low frequencies to ground, which also emphasizes higher frequencies. A resistor in parallel with the load isn’t frequency-dependent, so it doesn’t create a true low-pass filter.

Low-pass behavior means slow signals pass with little attenuation while fast components are reduced. A capacitor to ground placed between the source and the load does exactly that: at low frequencies the capacitor looks almost open, so the load sees nearly the full input. As frequency rises, the capacitor’s impedance drops and it shunts more of the signal to ground, pulling high-frequency components away from the load and lowering the output. The same idea can be realized with an inductor in series between the source and load: it presents a small impedance at low frequencies so the slow signal gets through, but its impedance grows with frequency, attenuating the signal before it reaches the load.

The other setups don’t produce a low-pass response. A capacitor in series blocks DC and passes higher frequencies, which is a high-pass effect. An inductor to ground shunts low frequencies to ground, which also emphasizes higher frequencies. A resistor in parallel with the load isn’t frequency-dependent, so it doesn’t create a true low-pass filter.