📖 How to Use
🎯 Getting Started
This tool designs active filters using op-amps to create precise frequency response:
- Low-Pass Filter: Passes low frequencies, attenuates high frequencies (audio smoothing, anti-aliasing)
- High-Pass Filter: Passes high frequencies, blocks low frequencies (DC blocking, AC coupling)
- Band-Pass Filter: Passes a specific frequency range (radio tuning, audio EQ)
Why Active Filters? Op-amp-based filters provide sharper roll-off, gain control, and no loading effects compared to passive RC filters.
📊 Filter Order & Roll-Off
The filter order determines how sharply frequencies are attenuated:
- 1st Order: -20 dB/decade (-6 dB/octave) - Simple RC with buffer
- 2nd Order: -40 dB/decade (-12 dB/octave) - Sallen-Key topology (industry standard)
- 3rd Order: -60 dB/decade (-18 dB/octave) - Cascaded stages
- 4th Order: -80 dB/decade (-24 dB/octave) - Two 2nd-order stages
Higher order = sharper transition between passband and stopband.
🔧 Sallen-Key Topology
The Sallen-Key is the industry-standard 2nd-order active filter:
- Single op-amp per 2nd-order stage
- Unity gain or positive gain configurations
- Equal component values (R₁ = R₂, C₁ = C₂) for Butterworth response
- Critical frequency: fc = 1 / (2π R C)
Butterworth Response: Maximally flat passband (no ripple), -3 dB at cutoff frequency.
🎚️ Q Factor (Quality Factor)
For 2nd-order and higher filters, Q controls the peaking at the cutoff frequency:
- Butterworth: Q = 0.707 (maximally flat, no overshoot)
- Bessel: Q = 0.577 (linear phase, best step response)
- Chebyshev: Q > 1 (sharper roll-off, passband ripple)
This tool uses Butterworth (Q = 0.707) for optimal frequency response.
💡 Design Tips
Component Selection:
- Start with standard resistor values (1kΩ, 10kΩ, 100kΩ)
- Choose capacitors from E12 or E24 series
- For low noise, use smaller resistors (1kΩ - 10kΩ)
- For high impedance, use larger resistors (100kΩ - 1MΩ)
Op-Amp Selection:
- Ensure op-amp GBW ≥ 100× cutoff frequency
- For audio: TL072, LM358, NE5532
- For precision: OPA2134, OPA2604
📐 Unit Prefixes
Use standard metric prefixes:
- Frequency: 1k, 1kHz, 10.5k, 440, 100Hz
- Resistance: 10k, 1kΩ, 100, 4.7M
- Capacitance: 1u, 100nF, 10p, 100n
Supported: p (pico), n (nano), u/µ (micro), m (milli), k (kilo), M (mega)