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Active Filters Workshop
Learn2026-01-14

Active Filters Workshop

#electronics#filters#active

Overview

Practical active filter design using op‑amps: Sallen‑Key, multiple feedback, and how component tolerances affect response and Q.

Prerequisites

  • Basic filter theory and op‑amp familiarity

Learning objectives

  • Design 2nd‑order active filters for specified cutoff and Q
  • Select component values and understand tolerance impacts
  • Measure and tune filter response on the bench

Hands-On Mini Task

  1. Design a unity‑gain Sallen‑Key low‑pass with fc = 1 kHz and Q = 0.707. Build and measure its magnitude and phase response.
  2. Swap component tolerances (e.g., ±5% → ±1%) in simulation and observe the effect on fc and Q.

Expected result: achievable fc within tolerance limits and observable Q variations with component tolerances.

Theory and component effects

  • Q factor determines peaking and selectivity; active filter Q influenced by op‑amp bandwidth and component tolerances.
  • Real op‑amps have finite slew-rate and GBW — check that op‑amp can support the chosen fc and Q without distortion.

Worked example — Sallen‑Key unity gain low‑pass

  1. Choose R and C values for fc = 1 kHz and Q = 0.707 using standard Sallen‑Key equations. Simulate and build the circuit; measure the -3 dB point and passband ripple.
  2. If the op‑amp output is distorted at higher amplitudes, check slew-rate limitations.

Troubleshooting

  • If Q is higher than expected, component tolerances or op‑amp phase shift near bandwidth limits may be the cause; lower Q in design or choose tighter-tolerance parts.

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