The Low Pass Inverting Op Amp: Quick Answer & Default Picks

A low pass inverting op amp combines two functions in a single stage: it amplifies (and inverts the phase of) an input signal while simultaneously attenuating high-frequency noise above a specific cutoff threshold. You achieve this by placing a resistor in the input path, and a parallel resistor-capacitor (RC) network in the feedback loop.

If you are at the bench right now and need to build one, here is the direct answer for which chip to grab. As of 2026, these remain the definitive safe defaults based on your application:

  • General Audio & Instrumentation (Dual Supply): Texas Instruments TL072 (~$0.85). JFET inputs, low noise (18 nV/√Hz), 3 MHz Gain Bandwidth Product (GBP). Requires ±5V to ±15V.
  • Battery-Powered & Single Supply: Microchip MCP6002 (~$0.45). CMOS, rail-to-rail I/O, 1 MHz GBP. Operates from 1.8V to 6.0V single supply.
  • High-End Audiophile / Studio Gear: Texas Instruments OPA2134 (~$4.50). Burr-Brown audio lineage, ultra-low THD (0.00008%), 8 MHz GBP. Requires ±2.5V to ±18V.
Bench Tip: Never use a generic LM358 for audio-frequency active filters unless you are strictly budget-constrained. The LM358 suffers from severe crossover distortion near the zero-crossing point, which will ruin the signal-to-noise ratio of your low-pass filter.

Pinout, Symbol, and Core Operation Regions

Most hobbyist and prototyping op-amps come in an 8-pin DIP (Dual In-line Package) or SOIC footprint. While the internal schematic symbol shows a triangle with an inverting (-) and non-inverting (+) input, the physical silicon requires power and specific pin routing.

Standard 8-Pin DIP Pinout (e.g., TL072, LM358, MCP6002)

  • Pin 1: Output A
  • Pin 2: Inverting Input A (-)
  • Pin 3: Non-Inverting Input A (+)
  • Pin 4: V- (Negative Supply or GND for single-supply)
  • Pin 5: Non-Inverting Input B (+)
  • Pin 6: Inverting Input B (-)
  • Pin 7: Output B
  • Pin 8: V+ (Positive Supply)

Operation Regions Table

Understanding which region the op-amp is operating in is critical for troubleshooting. The feedback capacitor in a low-pass filter alters the phase margin, meaning improper compensation can accidentally push the op-amp into saturation or oscillation.

Operation Region Voltage Condition (Vout) Input Differential (V+ minus V-) Typical Current Draw
Linear (Active) Between (V-) + 1.5V and (V+) - 1.5V ~0V (Virtual Short, < 2mV) Quiescent (e.g., 1.4 mA for TL072)
Positive Saturation Pegged near V+ rail (V+ - 1.5V) V- is significantly lower than V+ Max output current (e.g., 20-40 mA)
Negative Saturation Pegged near V- rail (V- + 1.5V) V- is significantly higher than V+ Max output current (sinking)

Complete Application Circuit: 1 kHz Audio Filter

Let's design a practical, copy-pasteable circuit: an inverting low-pass filter with a cutoff frequency ($f_c$) of ~1 kHz and a passband gain of -10 (20 dB). This is ideal for stripping high-frequency PWM noise or RF interference from an audio or sensor signal.

The Math and Component Selection

The passband gain is set by $A_v = -R_2 / R_1$. The cutoff frequency is set by the feedback components: $f_c = 1 / (2 \pi R_2 C_1)$.

  1. Choose Input Resistor ($R_1$): 10 kΩ. This sets our input impedance.
  2. Choose Feedback Resistor ($R_2$): For a gain of -10, $R_2$ must be 100 kΩ.
  3. Calculate Feedback Capacitor ($C_1$): $C_1 = 1 / (2 \pi \times 100,000 \times 1000) = 1.59 \text{ nF}$.
  4. Select Standard Value: Use a 1.5 nF capacitor. This shifts the actual cutoff to ~1,061 Hz, which is perfectly acceptable for a 1 kHz target.
Critical Dielectric Rule: For $C_1$, you must use a C0G (NP0) ceramic capacitor or a polypropylene film capacitor. Do not use X7R or Y5V ceramics. X7R capacitors exhibit severe voltage coefficient (capacitance drops as voltage increases) and microphonics, which will cause your cutoff frequency to wander and introduce harmonic distortion into the audio band.

Bill of Materials (BOM)

  • U1: TL072CP (8-pin DIP)
  • R1: 10 kΩ metal film resistor (1/4W, 1%)
  • R2: 100 kΩ metal film resistor (1/4W, 1%)
  • C1: 1.5 nF C0G/NP0 ceramic capacitor (50V+ rating)
  • C2, C3: 100 nF (0.1 µF) X7R ceramic bypass capacitors
  • Power Supply: Dual ±12V or ±15V bench supply

Numbered Wiring Steps

  1. Insert the TL072 across the center trench of the breadboard. Connect Pin 8 to +12V and Pin 4 to -12V.
  2. Place the 100 nF bypass capacitors as close to the IC as possible: one between Pin 8 and ground, one between Pin 4 and ground.
  3. Connect the non-inverting input (Pin 3) directly to the breadboard ground rail. (This establishes 0V bias for dual-supply operation).
  4. Insert R1 (10 kΩ). Connect one end to your input signal source, and the other end to the inverting input (Pin 2).
  5. Insert R2 (100 kΩ) and C1 (1.5 nF) in parallel. Connect one end of this parallel pair to Pin 2, and the other end to the output (Pin 1).
  6. Route your output signal from Pin 1 to your load or oscilloscope probe.

How to Bias, Select, and Scale the Design

Choosing the right op-amp and biasing it correctly prevents 90% of active filter headaches. Use this decision tree to lock in your part number and biasing network.

Application Scenario Power Supply Available Biasing Requirement Concrete Part Pick
Audio mixing / Studio EQ Dual (±12V to ±15V) Non-inverting pin to GND. AC couple input/output. OPA2134 or TL072
Microcontroller ADC Anti-Aliasing Single (3.3V or 5V) Non-inverting pin to Vcc/2 via voltage divider. DC coupled. MCP6002 or LMV321
Motor Control / PWM Filtering Single (12V to 24V) Non-inverting pin to Vcc/2. High voltage tolerance needed. LM358 (if low freq) or TL082
High-Speed Data (>100 kHz cutoff) Dual (±5V) Non-inverting pin to GND. Keep PCB traces ultra-short. AD8055 (Current Feedback) or OPA656

Single-Supply Biasing Rules

If you are forced to use a single supply (e.g., a 9V battery or a 5V USB rail), the op-amp cannot output negative voltages. If you feed an AC audio signal into a single-supply inverting filter biased at 0V, the negative half of the waveform will be hard-clipped at ground.

The Fix: Create an "artificial ground" at Vcc/2. Use two equal resistors (e.g., 10 kΩ each) in series from V+ to GND. Tie the non-inverting input (Pin 3) to this midpoint. Your output will now idle at 2.5V (on a 5V supply) instead of 0V, allowing the AC signal to swing symmetrically. You must then add a DC-blocking capacitor (e.g., 10 µF film) in series with your final output to remove the 2.5V DC offset before it hits your load.

Failure Modes and Multimeter Testing

Active filters fail in predictable ways. The most common culprits are latch-up (caused by input voltages exceeding the supply rails, permanently shorting the internal silicon) and capacitor dielectric breakdown (shorting the feedback loop, turning the filter into a useless unity-gain buffer).

How to Test an Op-Amp with a Digital Multimeter (DMM)

You do not need an oscilloscope to verify if the silicon is dead. Grab your DMM and follow this sequence:

  1. The Diode Test (Power Off): Remove power from the breadboard. Set your DMM to diode test mode. Place the red probe on the V- pin (Pin 4) and the black probe on the In+ (Pin 3) and In- (Pin 2) pins. You should read a forward voltage drop of roughly 0.6V to 0.8V (the internal ESD protection diodes). If you read 0.0V (short) or OL (open), the input stage is destroyed.
  2. The Supply Short Check (Power Off): Set DMM to continuity/resistance. Measure between V+ (Pin 8) and V- (Pin 4). You should read high resistance (typically >100 kΩ). A dead short here means the IC has suffered catastrophic thermal failure.
  3. The Virtual Short Verification (Power On): Re-apply power and feed a valid mid-band signal (e.g., 100 Hz sine wave). Set the DMM to DC Voltage. Measure the voltage at Pin 3, then Pin 2. Because of negative feedback, the op-amp will force these two pins to be at the exact same voltage (the "virtual short"). If the voltage difference between Pin 2 and Pin 3 is greater than 5 mV while the output is railed at V+ or V-, the feedback loop is broken, the capacitor is shorted, or the op-amp output stage is blown.
Troubleshooting Edge Case: If your low-pass filter is oscillating (outputting a high-frequency sine wave with no input), your feedback capacitor $C_1$ might be too small, or the physical layout is introducing stray capacitance. Add a small 10 pF to 22 pF capacitor directly across the feedback resistor $R_2$ to compensate for phase margin loss, or move the components closer to the IC pins. For deeper topology math, refer to the Texas Instruments Op Amp Handbook or the Analog Devices Active Filter guide.

When designing a low pass inverting op amp, do not overcomplicate the silicon choice. For 95% of bench prototyping, audio filtering, and sensor conditioning tasks, wire up a TL072 on a dual ±12V supply, use a 1.5 nF C0G capacitor in the feedback loop, and verify your virtual short with a multimeter. It is a robust, predictable, and highly effective circuit topology that belongs in every maker's repertoire.