EC21 · Sem 5 · Filter design

Active band-pass filter (multiple feedback)

Aim: Design a band-pass filter with one op-amp and measure its centre frequency.

🧪 Live simulation🧊 Real-time 3D bench⚠ Simulated values, not physical measurements

🔬 Virtual lab

Starting the lab…

📘 Theory

The multiple-feedback topology uses two capacitors and three resistors around an inverting op-amp to give a second-order band-pass response with gain at the centre frequency.

f₀ = 1/(2πC·√(R3·(R1‖R2)))

🧰 Apparatus (virtual)

Op-amp
10 kΩ / 1 kΩ / 100 kΩ
two 10 nF capacitors

📝 Procedure

Run the AC sweep
Read the centre frequency and peak gain
Measure the bandwidth and Q
Change one resistor and see the centre shift

Observations and result (kept on this device)

🎤 Viva questions

Where are band-pass filters used

Equalisers, tone detection, radio IF stages.

What does Q control

The sharpness of the peak.

🛡️ Safety

Virtual lab. With real RF equipment, respect exposure limits, never transmit without a licence, and keep antennas away from people.