Side-by-side comparison
| Parameter | Active | Passive Filter |
|---|---|---|
| Uses active devices | No; only R, L, C | Yes; op-amp (e.g., LM741, TL072) or transistor |
| Gain | Always ≤ 1 (insertion loss) | Can provide gain > 1 |
| Input/output impedance | Affected by source and load; loading changes response | High input impedance, low output impedance; no loading |
| Frequency range | DC to GHz (RF filters use LC) | Typically DC to ~100 kHz; limited by op-amp GBW |
| Inductors required | Yes, for LC filters (bulky at low frequencies) | No inductors; only R and C with op-amp |
| Power requirement | None; purely passive | Requires DC supply (±5 V to ±15 V for op-amp) |
| Noise addition | Adds no active noise | Op-amp adds input-referred noise (~10 nV/√Hz for TL072) |
| Order achievable easily | High order possible with LC ladder networks | 2nd order per stage; cascaded for higher order |
| Temperature stability | Good (metal film resistors, NPO capacitors) | Op-amp offset drift adds error at DC |
| Typical application | RF bandpass filters, EMI filters, power line filters | Audio EQ, sensor signal conditioning, anti-aliasing below 100 kHz |
Key differences
A passive filter introduces insertion loss — a first-order RC filter at its cutoff frequency attenuates by 3 dB and the attenuation worsens as load impedance drops. An active filter using a TL072 Sallen-Key circuit maintains its −3 dB point regardless of load and can simultaneously provide up to +6 dB of gain in a second-order Butterworth stage. However, op-amp bandwidth limits active filters: a TL072 with 3 MHz GBW cannot realise accurate filters above ~100 kHz. Passive LC filters remain the only option at RF frequencies — a 2.4 GHz bandpass filter in a Wi-Fi front-end is always passive.
When to use Active
Use a passive filter when the circuit operates at RF frequencies or when no power supply is available — for example, an LC bandpass filter centred at 455 kHz for the IF stage of an AM radio receiver.
When to use Passive Filter
Use an active filter when operating below 100 kHz and gain or load independence is needed — for example, a Sallen-Key low-pass anti-aliasing filter at 1 kHz using a TL072 before the ADC input of a data acquisition system.
Recommendation
Below 100 kHz and wherever a power supply exists, choose the active filter — it eliminates inductors, buffers the load, and can provide gain in the same stage. Above 100 kHz or at RF, choose the passive LC filter; no op-amp is fast enough to help there.
Exam tip: Examiners ask you to compare the Q factor achievable in passive versus active filters and to draw the Sallen-Key low-pass topology — know the component placement and gain formula Av = 1 + Rf/Rg for the non-inverting amplifier stage.
Interview tip: Interviewers ask why inductors are avoided in active filter design at audio frequencies and expect you to state that a 1 H inductor at 50 Hz would be physically huge and lossy, making op-amp RC filters the only practical choice.