Active Filters: Sallen–Key, MFB, Butterworth | OP-AMP #7

Alternatives: How to Design an Active Filter (Sallen–Key, 1 kHz Example) Op-Amp Active Filters Explained: Q, Poles and Real Values Sallen–Key vs MFB, Butterworth vs Bessel | OP-AMP #7 DESCRIPTION: Lesson 7 of the op-amp series: active filters, op-amps with resistors and capacitors, with gain, buffering and no inductors. From filter types and first-order stages to second-order sections and Q, the Sallen–Key and multiple-feedback circuits with real values, Butterworth, Chebyshev and Bessel responses, higher orders, op-amp requirements, anti-aliasing, and the state-variable filter. What you'll learn • Low-pass, high-pass, band-pass, notch; cutoff at −3 dB; 20·n dB/decade • First order: RC + buffer, fc = 1/(2πRC) ≈ 1.6 kHz (10 kΩ, 10 nF) • Why second order: complex poles and a sharp corner • H(s) = ω0² / (s² + (ω0/Q)s + ω0²) and what Q does (0.5, 0.707, 1, 2) • Sallen–Key: f0 = 1/(2π√(R1R2C1C2)), Q = ½√(C1/C2) with equal R • Design: Butterworth low-pass at 1 kHz → 10 kΩ, 22 nF, 11 nF → f0 ≈ 1.02 kHz, Q = 0.707 • High-pass Sallen–Key and the equal-component trick: Q = 1/(3 − K), K = 1.586 • Multiple feedback (MFB) low-pass and band-pass • Butterworth vs. Chebyshev vs. Bessel: magnitude and step response • 4th-order Butterworth: Q = 0.541 and 1.307, −80 dB/decade • Op-amp and parts: GBW ≥ 100·Q·f0, 1 kΩ … 100 kΩ, C0G, 1 % • The anti-aliasing filter in front of an ADC • The state-variable filter: LP, BP and HP at once Full series playlist: Op-Amps: From Zero to Circuit Design Previous: OP-AMP #6, comparators and the Schmitt trigger Next: OP-AMP #8, oscillators CHAPTERS: 0:00 Active filters 0:20 Four types 0:40 First order: RC + buffer 1:01 Why we need second order 1:21 Second order: f0 and Q 1:53 Sallen–Key 2:16 Design: Butterworth low-pass at 1 kHz 2:36 High-pass and the gain trick 2:57 MFB low-pass 3:19 MFB band-pass 3:38 Butterworth, Chebyshev, Bessel 3:58 Higher order: cascade sections 4:18 Op-amp and parts 4:38 The anti-aliasing filter 4:58 The state-variable filter 5:17 Summary and next lesson