Sallen-Key Filter
A Sallen-Key filter is a popular type of active analog filter made using:
- An operational amplifier (op-amp)
- Resistors
- Capacitors
It is widely used to build:
- Low-pass filters
- High-pass filters
- Band-pass filters
The circuit was developed by R. P. Sallen and E. L. Key.
Main Idea
A Sallen-Key filter combines:
- An RC network (frequency selection)
- With an op-amp buffer/amplifier
This gives:
- Better filtering
- Stable response
- Higher gain
- Less loading effect
Most Common Type: Low-Pass Sallen-Key Filter
It allows:
- Low frequencies to pass
- High frequencies to be blocked
Cutoff Frequency
The cutoff frequency is:
fc = 1 / (2ฯ √(R1 × R2 × C1 × C2))
Where:
- R1, R2 = resistors
- C1, C2 = capacitors
- fc = cutoff frequency
Types of Sallen-Key Filters
| Type | Function |
|---|---|
| Low-pass | Pass low frequencies |
| High-pass | Pass high frequencies |
| Band-pass | Pass middle frequencies |
| Band-stop | Reject certain frequencies |
Advantages
- Simple design
- Uses few components
- Good frequency stability
- Easy to tune
- Works well with op-amps
Disadvantages
- Not ideal for very high frequencies
- Op-amp bandwidth limits performance
- Sensitive to component tolerances
Applications
Used in:
- Audio systems
- Equalizers
- Sensor circuits
- ADC anti-aliasing filters
- Communication systems
Example applications:
- Removing noise from audio signals
- Smoothing sensor outputs
Why It’s Popular
Compared to passive RC filters:
- Gives sharper response
- Can provide gain
- Prevents signal loss due to loading
Example
Suppose:
- R1 = R2 = 10kฮฉ
- C1 = C2 = 0.01ยตF
Then:
fc ≈ 1.59 kHz
This means frequencies above about 1.59 kHz start getting attenuated.