---
title: "Merits and demerits of active vs passive filters"
subject: "Filter Design"
chapter: 6
key: "semester_5/filter_design:active-passive-merits-demerits"
source: https://www.bctengineer.com/q/semester_5/filter_design/active-passive-merits-demerits
site: BCT Engineering
---

# Merits and demerits of active vs passive filters

Filter Design · Chapter 6: Active Filters

- Asked in: 2079 Baisakh, 5 marks
- Video: [Merits and Demerits of Active Filters - Operational Amplifier and 555 Timer - Industrial Electronics](https://www.youtube.com/watch?v=bkb9bSBYPyE) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/filter_design/active-passive-merits-demerits
- Whole chapter: https://www.bctengineer.com/md/semester_5/filter_design/chapter-6

## Question

What are the merits and demerits of active filter as compared with passive filter? Explain.

## Short answer

Active filters have no bulky inductors, can provide gain, allow easy cascading due to high input and low output impedance, are tunable, small, and cheap. Demerits include needing a DC supply, limited frequency and power range, added noise and offset, higher sensitivity, and reliability issues.

## Answer

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.

| Point | Active | Passive |
|---|---|---|
| Inductors | Not needed | Needed |
| Gain | Can exceed 1 | ≤ 1 |
| Supply | Needed | Not needed |
| Frequency | Up to ~MHz | Up to GHz |
| Power handling | Low | High |
| Cascading | Easy | Loading problem |
| Noise | More | Less |

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do active and passive filters differ in construction?
   A. Active use R, C and an op-amp; passive use R, L and C
   B. Active use only inductors; passive use op-amps
   C. Active use R, L, C; passive use transistors
   D. Both use op-amps but differ in supply
   Answer: A. Active filters replace inductors with R, C and an active device; passive filters use R, L, C only. (Key point: Defining difference)

2. Which is a merit of active filters?
   A. They work at GHz frequencies
   B. They handle large power
   C. They need no supply
   D. Gain above unity and easy cascading of stages
   Answer: D. Op-amps allow gain above 1 and isolate stages, so high-order filters are cascades of biquads. (Key point: Merit: gain and isolation)

3. Why are active filters limited to roughly below 1 MHz?
   A. Finite gain-bandwidth and slew rate of the op-amp
   B. Resistors become inductive
   C. Capacitors block high frequency
   D. They have no tuning
   Answer: A. Op-amp bandwidth and slew rate limit them, whereas passive LC filters work up to GHz. (Key point: Demerit: frequency range)

4. Which is a demerit of active filters compared with passive ones?
   A. They need inductors
   B. They need a DC supply and have limited signal swing
   C. They cannot be cascaded
   D. They cannot give gain
   Answer: B. They consume power from a supply, saturate near the rails and add noise/offset. (Key point: Demerit: other limits)

## Related in Filter Design

- [Insertion gain/loss, design steps, active vs passive](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Ainsertion-loss-steps-active-passive) — Filter Design, chapter 1
- [Active vs passive; non-inverting (s+8)/(s+2)](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-passive-difference-noninverting) — Filter Design, chapter 6, asked 5×
- [Active vs passive; realize 4(s+2)/(s+1)](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-passive-noninverting-4s-plus-2) — Filter Design, chapter 6, asked 5×
- [Techniques for higher order active filters](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Ahigher-order-active-techniques) — Filter Design, chapter 8
- [Active filter advantages; bilinear zero 800, pole 400](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-advantages-bilinear-800-400) — Filter Design, chapter 6
- [Active filter advantages; Sallen-Key Butterworth at 4 kHz](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-advantages-sallen-key-4khz) — Filter Design, chapter 6

## Related in other subjects

- [Passive vs active replication; availability](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Apassive-vs-active-available) — Distributed System, chapter 7, asked 2×
- [Active vs passive; active replication working](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aactive-passive-working) — Distributed System, chapter 7
- [Forward vs reverse engineering merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aforward-reverse-engineering-merits) — Object Oriented Analysis and Design, chapter 4, asked 3×
- [Active and passive components, transconductance](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aactive-passive-component) — Basic Electronics Engineering, chapter 1