Merits and demerits of active vs passive filters

An active filter uses R, C and an active device (op-amp) in place of inductors; a passive filter uses only R, L and C.

Merits of active filters

1. No inductors: inductors are large, heavy, lossy (low Q) and pick up magnetic interference, especially at audio frequencies. Active filters avoid them.
2. Gain: the passband gain can be set above unity, so the filter can also amplify.
3. Isolation: the op-amp has high input and low output impedance, so stages can be cascaded without interaction; a high-order filter is designed as a cascade of independent biquads.
4. Tunability: $\omega_o$, $Q$ and gain are often adjustable by separate resistors.
5. Size and cost: small, light and cheap; can be integrated (with switched-capacitor techniques).

Demerits of active filters

1. Power supply needed: they consume power and need a DC supply.
2. Limited frequency range: the op-amp's finite gain-bandwidth and slew rate limit use to roughly below 1 MHz; passive LC filters work up to GHz.
3. Limited signal swing: output saturates near the supply rails, so they cannot handle large voltages or power.
4. Noise and offset: op-amps add noise, DC offset and distortion.
5. Higher sensitivity: many active circuits (e.g. Sallen-Key at high Q) are more sensitive to element changes than doubly terminated LC ladders.
6. Reliability: active devices may fail or drift with temperature.

Typical use: active filters are preferred for low-frequency, low-power applications (audio, instrumentation, biomedical signals), while passive filters are used at RF and where large power must be handled.