Which statement accurately distinguishes passive filters from active filters, with an example of each?

• A. Passive filter uses active components to realize a filter; Example: RC low-pass; Active filter uses inductors only
• B. Passive filter uses only passive components; Example passive RC low-pass; Active filter uses op-amps; Example active Sallen-Key low-pass
• C. Passive filter always provides amplification; Active filter always attenuates
• D. Passive filter cannot realize a high-pass; Active filter can only realize low-pass

Answer: (B)

The key distinction is whether a filter uses only passive components or includes an active, powered element. A passive filter contains only resistors, capacitors, and inductors; it does not add energy to the signal, so it can shape the spectrum but cannot provide gain greater than one. A classic example is a simple RC low-pass filter, where a resistor and a capacitor form a network that attenuates higher frequencies without amplification.

An active filter, on the other hand, includes a powered element such as an operational amplifier. This lets the circuit provide gain and better control over the filter characteristics, including isolation between stages and higher quality factor without needing inductors. A common example is a Sallen-Key low-pass filter, which uses an op-amp to realize a second-order low-pass response with buffering and adjustable gain.

Why the other ideas don’t fit: a filter that relies on active components is not merely “using resistors and capacitors”; it involves powered devices that can supply gain. Conversely, passive filters do not inherently provide amplification, so saying they always amplify or that active filters are limited to low-pass doesn’t match how these topologies are used. And indeed passive networks can realize high-pass responses with appropriate RC arrangements, while active filters can realize high-pass, low-pass, band-pass, and notch configurations.