A band pass op amp circuit isolates a specific frequency range while rejecting signals above and below it. For audio and general-purpose active band-pass filters, the safest default part numbers are the TL072 (JFET input, low noise, ~$0.60) and NE5532 (bipolar, high drive, ~$0.80). To build a stable 1kHz center frequency filter with a Q-factor of 5, use a Multiple Feedback (MFB) topology with 10nF capacitors and standard 1% metal film resistors on a ±12V dual supply.
Unlike passive LC filters that require bulky inductors, an active band pass filter relies on the high input impedance and low output impedance of an operational amplifier to shape the frequency response without loading down the preceding stage. Below is a complete bench guide to selecting, wiring, and debugging these circuits.
The Direct Answer: Default Parts and Core Topology
While you can buy dedicated switched-capacitor filter ICs, building an active filter with a standard op amp gives you exact control over the center frequency ($f_0$), bandwidth, and gain. The Texas Instruments TL072 and the NE5532 remain the undisputed workhorses for these tasks in 2026 due to their low cost, wide availability, and predictable behavior.
Standard 8-Pin Dual Op Amp Pinout
Most default op amps come in an 8-pin DIP or SOIC package containing two independent amplifiers. When wiring your band pass circuit, you will use one half of the chip. The standard pinout is:
- Pin 1: Output A
- Pin 2: Inverting Input A (-)
- Pin 3: Non-Inverting Input A (+)
- Pin 4: V- (Negative Supply / Ground in single supply)
- Pin 5: Non-Inverting Input B (+)
- Pin 6: Inverting Input B (-)
- Pin 7: Output B
- Pin 8: V+ (Positive Supply)
Selecting and Biasing the Op Amp for Active Filters
Choosing the right op amp requires looking beyond just the supply voltage. You must evaluate the Gain-Bandwidth Product (GBW) and the input bias current. If your filter's center frequency is 10kHz and you need a gain of 10, the op amp must have a GBW significantly higher than 100kHz to maintain phase margin and prevent peaking.
Safe Default Part Numbers and Ratings
| Part Number | Input Type | Max Supply (V) | GBW (MHz) | Noise (nV/√Hz) | Best Application |
|---|---|---|---|---|---|
| TL072 | JFET | ±18V (36V) | 3.0 | 18 | General audio, mid-Q filters, high-impedance sensors |
| NE5532 | Bipolar | ±22V (44V) | 10.0 | 5 | Low-noise audio, driving heavy capacitive loads |
| OPA2134 | FET | ±18V (36V) | 8.0 | 8 | Precision instrumentation, high-fidelity audio |
Operation Regions
An op amp in an active filter must remain in its linear region to function correctly. If the input signal is too large, the output rails, destroying the filter profile.
| Region | State | Typical Output Voltage (±12V Supply) | Result in Filter Circuit |
|---|---|---|---|
| Linear | Active Filtering | -10.5V to +10.5V | Correct frequency shaping and gain |
| Positive Saturation | Clipping (High) | +10.5V to +11.2V | Severe THD, loss of Q-factor, flat-topped waveforms |
| Negative Saturation | Clipping (Low) | -10.5V to -11.2V | Severe THD, recovery time delays (slew limiting) |
Complete 1kHz Multiple Feedback (MFB) Band Pass Circuit
The Multiple Feedback (MFB) topology is preferred over the Sallen-Key for band pass filters because it is less sensitive to the op amp's open-loop gain limitations and handles higher Q-factors (Q > 3) without becoming unstable. Below is a complete design for a 1kHz center frequency, Q=5, and a passband gain of 2 (6dB).
Component Values (Assuming ±12V Dual Supply)
- C1, C2: 10nF (C0G/NP0 ceramic or polypropylene film; avoid X7R for low distortion)
- R1: 39.2kΩ (1% metal film)
- R2: 1.65kΩ (1% metal film)
- R3: 158kΩ (1% metal film)
- U1: TL072 or NE5532
Wiring Steps
- Power the IC: Connect +12V to Pin 8 and -12V to Pin 4. Place a 100nF MLCC bypass capacitor directly across Pins 4 and 8, as close to the IC body as possible to suppress high-frequency supply noise.
- Wire the Input Network: Connect your AC input signal to R2 (1.65kΩ). Connect the other end of R2 to the inverting input (Pin 2). Also connect C1 (10nF) from the input signal to Pin 2.
- Wire the Feedback Loop: Connect R1 (39.2kΩ) between Pin 2 (Inverting) and Pin 1 (Output). In parallel with R1, connect C2 (10nF).
- Wire the Ground Reference: Connect R3 (158kΩ) from Pin 2 (Inverting) to circuit ground. Connect Pin 3 (Non-Inverting) directly to circuit ground.
- Verify and Test: Inject a 1Vpp sine wave at 1kHz. Measure the output at Pin 1 with an oscilloscope. You should see a 2Vpp sine wave. Sweep the frequency down to 500Hz and up to 2kHz; the amplitude should drop by -3dB (to ~1.4Vpp) at the bandwidth edges.
Failure Modes and Multimeter Troubleshooting
When an active band pass filter fails on the bench, it usually manifests as no output, a stuck DC voltage at the rail, or high-frequency oscillation. Here is how to isolate the fault using a standard digital multimeter (DMM).
Step-by-Step Diagnostic Path
- Check Power Rails First: Set your DMM to DC Voltage. Measure Pin 8 to Ground (expect +12V) and Pin 4 to Ground (expect -12V). If either is missing, check your power supply and breadboard contacts before blaming the IC.
- Check the DC Output Offset: Measure DC voltage from Pin 1 (Output) to Ground. With the input grounded, this should read very close to 0V (typically ±5mV to ±20mV). If it reads +11V or -11V, the op amp is saturated. This usually means a missing ground connection on Pin 3 or a broken feedback trace on R1/C2.
- Test for Internal Short Circuits: Power down the circuit entirely. Set your DMM to Diode Test mode. Place the red probe on Pin 4 (V-) and touch the black probe to Pins 1, 2, and 3. You should read a forward diode drop (0.5V to 0.7V). If you read 0.00V (short), the op amp's internal ESD protection diodes or output transistors have blown, often caused by capacitive load oscillation or a soldering short. Replace the IC.
- Verify Passive Components: If the IC tests fine but the filter response is skewed, desolder one leg of R2 and R3 and measure their resistance. A 1.65kΩ resistor that has drifted to 2kΩ will shift your center frequency and destroy the Q-factor.
Band Pass Op Amp Frequently Asked Questions
How do I calculate the Q factor for a band pass op amp?
The Q (Quality) factor defines the selectivity of the filter. It is calculated as the center frequency ($f_0$) divided by the -3dB bandwidth ($BW$). For example, if your filter is centered at 1000Hz and the amplitude drops by 3dB at 900Hz and 1100Hz, the bandwidth is 200Hz. Therefore, $Q = 1000 / 200 = 5$. In an MFB circuit, Q is primarily set by the ratio of the feedback resistor (R3) to the input resistor (R2). Higher Q values require tighter resistor tolerances (1% or 0.1%) to prevent the circuit from turning into an oscillator.
Can I use a single-supply op amp for an active band pass filter?
Yes, but it requires careful biasing. Standard dual-supply circuits assume the input and output swing above and below 0V. On a single supply (e.g., 0V to +5V), the signal cannot go negative. You must bias the non-inverting input (Pin 3) to half the supply voltage (2.5V) using a buffered voltage divider. This shifts the entire AC signal up so it swings between 1.5V and 3.5V instead of -1V and +1V. You must then use AC coupling capacitors at the input and output to block this 2.5V DC offset from affecting adjacent stages. For single-supply 5V designs, the MCP6002 or TLC27M2 are better choices than the TL072, as they feature rail-to-rail input/output capabilities.
Why is my band pass op amp oscillating at high frequencies?
High-frequency oscillation (often in the 1MHz to 5MHz range, invisible to basic multimeters but obvious on an oscilloscope as a thick, fuzzy trace) is almost always caused by poor PCB layout or capacitive loading. If you have a long coaxial cable or a large capacitor connected directly to the op amp's output pin, the capacitive load introduces a pole in the feedback loop, destroying the phase margin. To fix this, insert a small series resistor (typically 22Ω to 47Ω) directly at the op amp's output pin, before the feedback network taps off and before the load connects. Additionally, ensure your power supply bypass capacitors are placed within 2mm of the IC's V+ and V- pins.
