An op amp inverting amplifier flips the phase of an input signal by 180 degrees and scales its amplitude by a precise ratio determined by two external resistors. The closed-loop voltage gain is defined by the formula Av = -Rf / Rin, where Rf is the feedback resistor and Rin is the input resistor. Unlike open-loop op amp configurations that saturate instantly, the inverting topology uses negative feedback to force the inverting input pin to a 'virtual ground,' yielding a highly predictable, stable output.
This guide skips the abstract semiconductor physics and goes straight to the workbench. We will cover how to select the right IC, bias it for single or dual supplies, build a complete audio preamp circuit, and troubleshoot failures with a digital multimeter (DMM).
Symbol, Pinout, and Safe Default Part Numbers
The standard schematic symbol for an operational amplifier is a triangle with five primary nodes: the non-inverting input (+), the inverting input (-), the output, and the positive/negative power supply rails (V+ and V-). In a standard 8-pin DIP (Dual In-line Package) layout, the pins are assigned as follows:
- Pin 1 & 5: Offset Null (used to trim DC offset to zero on precision models)
- Pin 2: Inverting Input (IN-)
- Pin 3: Non-inverting Input (IN+)
- Pin 4: V- (Negative Supply or Ground)
- Pin 6: Output (OUT)
- Pin 7: V+ (Positive Supply)
- Pin 8: NC (No Connect) or Compensation
While the legacy LM741 is still taught in textbooks, it is noisy, requires high supply voltages, and suffers from output crossover distortion. For modern prototyping and production, use these safe default part numbers based on your application:
| Part Number | Supply Voltage Range | Slew Rate | GBP (Gain-Bandwidth) | Best Use Case |
|---|---|---|---|---|
| LM358 (TI) | 3V to 32V (Single/Dual) | 0.3 V/µs | 1 MHz | General purpose, single-supply DC sensing, low cost |
| TL072 (TI) | ±5V to ±18V (Dual) | 13 V/µs | 3 MHz | JFET input, general audio, high-impedance sensors |
| NE5532 (ON Semi) | ±3V to ±20V (Dual) | 9 V/µs | 10 MHz | Low-noise bipolar audio preamps and mixing consoles |
| OPA2134 (TI) | ±2.5V to ±18V (Dual) | 20 V/µs | 8 MHz | Precision FET, high-end audio, active filters |
Source: Texas Instruments Op Amp Overview
Biasing and Selecting the Right Op Amp for the Job
The most common mistake when building an op amp inverting amplifier is ignoring the power supply topology. Op amps do not inherently know what 'ground' is; they only see the voltage difference between their V+ and V- pins, and they can only output voltages that fall between those two rails.
Dual Supply Biasing (The Easy Route)
If you power the op amp with a symmetric dual supply (e.g., +12V, GND, -12V), you tie the non-inverting input (Pin 3) directly to the 0V ground. The output can swing both positive and negative, perfectly accommodating AC signals like audio or sensor waveforms that cross zero.
Single Supply Biasing (The Virtual Ground Trap)
If you are running off a single 5V or 12V battery, tying Pin 3 to 0V means the op amp cannot output negative voltages. If you feed an AC audio signal into the inverting input, the negative half of the wave will be brutally clipped at 0V.
Complete Application Circuit: Audio Preamp with 10x Gain
Let's build a practical inverting amplifier that takes a line-level audio signal and amplifies it by a factor of 10 (a gain of -10, meaning 180° phase shift). We will use the TL072 for its high input impedance and low noise profile.
Component List:
- IC: TL072CP (8-pin DIP)
- Rin (Input Resistor): 10kΩ (1% metal film)
- Rf (Feedback Resistor): 100kΩ (1% metal film)
- Rcomp (Compensation Resistor): 9.1kΩ (or closest 1% value to Rin || Rf)
- C1, C2 (Decoupling Caps): 100nF (0.1µF) MLCC, X7R dielectric
- Cin (Input Coupling Cap): 1µF C0G/NP0 film or ceramic
Wiring Steps
- Power & Decoupling: Connect +12V to Pin 7 and -12V to Pin 4. Place C1 between Pin 7 and GND, and C2 between Pin 4 and GND. Keep these capacitors within 5mm of the IC pins to prevent high-frequency oscillation.
- Input Network: Connect your audio signal source to one leg of Cin. Connect the other leg of Cin to Pin 2 (IN-) through the 10kΩ Rin resistor.
- Feedback Loop: Connect the 100kΩ Rf resistor between Pin 2 (IN-) and Pin 6 (OUT). This establishes the negative feedback path and sets the gain to
-100k / 10k = -10. - Non-Inverting Bias: Connect Pin 3 (IN+) to the 0V system ground through the 9.1kΩ Rcomp resistor. This minimizes DC output offset errors caused by input bias currents.
- Output: Route Pin 6 to your load or next circuit stage.
Operation Regions and Expected Voltages
An op amp does not behave linearly at all times. Understanding the operation regions is critical for debugging clipped signals or locked outputs. The table below assumes a standard ±12V dual supply using a TL072.
| Operation Region | Condition | Typical Output Voltage | Output Current Limit |
|---|---|---|---|
| Linear (Active) | V- is held at virtual ground via feedback | Av * Vin (e.g., -10V for 1V in) | Up to ±30mA (short-circuit protected) |
| Positive Saturation | Input demands voltage > V+ headroom | +10.5V (approx. 1.5V below V+ rail) | Limited by internal protection |
| Negative Saturation | Input demands voltage < V- headroom | -10.5V (approx. 1.5V above V- rail) | Limited by internal protection |
| Common-Mode Violation | Input pins exceed V+ - 1.5V or V- + 1.5V | Unpredictable (phase reversal or latch-up) | N/A (Device malfunction) |
Source: Analog Devices Operational Amplifier Design Center
Troubleshooting: How It Fails and Multimeter Testing
Op amps rarely fail gracefully. They usually succumb to electrostatic discharge (ESD) on the input pins, thermal runaway from driving heavy capacitive loads, or latch-up from exceeding the common-mode input range. Here is how to test a suspect inverting amplifier using a standard DMM.
Step 1: The Power-Off Diode Test (Checking for Blown Output Stages)
Remove all power from the circuit. Set your DMM to Diode Test mode. Place the red probe on the V+ pin (Pin 7) and the black probe on the Output pin (Pin 6). You should read a forward voltage drop of roughly 0.6V to 0.8V (the internal protection diode). Reverse the probes (black on V-, red on OUT) and check for a similar reading. If the meter reads 0.00V (a dead short), the internal output transistor is blown. Desolder and replace the IC.
Step 2: The Power-On Virtual Ground Check
Apply power. Set the DMM to DC Volts. Measure the voltage at Pin 2 (IN-). Because of negative feedback, this pin should read exactly the same voltage as Pin 3 (IN+). In a dual-supply circuit, both should read 0.00V (±2mV). If Pin 2 is sitting at +11V while Pin 3 is at 0V, your feedback loop is broken. Check Rf for an open circuit or a cold solder joint.
Step 3: Hunting for High-Frequency Oscillation
If your DMM reads a chaotic, fluctuating DC voltage at the output, or if the IC is physically hot to the touch with no load attached, the circuit is likely oscillating at MHz frequencies. A DMM cannot accurately read this. You must use an oscilloscope to verify. If oscillation is present, add a small 10pF to 47pF ceramic capacitor in parallel with Rf to roll off high-frequency gain, or ensure your decoupling capacitors are physically touching the IC pins.
Frequently Asked Questions
Why is my op amp inverting amplifier outputting a clipped or flat signal?
Clipping almost always points to a power supply headroom issue or a common-mode violation. First, check your supply rails with a DMM. If you are using a single 5V supply and attempting to amplify a ±1V AC signal without a Vcc/2 virtual ground bias, the negative half of the wave will hit the 0V floor and clip. Second, ensure your input signal isn't exceeding the op amp's common-mode voltage range. For an LM358 on a 5V supply, the inputs cannot go higher than roughly 3.5V; exceeding this causes phase reversal, where the output violently slams to the opposite rail.
How do I calculate the input impedance of an op amp inverting amplifier?
Unlike the non-inverting configuration (which boasts near-infinite input impedance), the input impedance of an inverting amplifier is simply the value of the input resistor, Rin. Because the inverting pin is held at a virtual ground, the source 'sees' only Rin to ground. If you need a 10kΩ input impedance to match a specific audio source, you must set Rin to 10kΩ. If you also need a gain of 100, Rf must be 1MΩ. Be aware that using extremely high-value feedback resistors (like 10MΩ) introduces thermal noise and makes the circuit highly susceptible to stray parasitic capacitance.
Can I use an op amp inverting amplifier as an active low-pass filter?
Yes, this is one of the most common applications of the topology. By placing a capacitor in parallel with the feedback resistor (Rf), you create a first-order active low-pass filter. The DC gain remains -Rf / Rin, but as frequency increases, the capacitor's impedance drops, reducing the overall gain. The cutoff frequency (-3dB point) is calculated using the standard RC formula: fc = 1 / (2 * π * Rf * Cf). For audio anti-aliasing, use a C0G/NP0 dielectric capacitor for Cf; X7R ceramics exhibit piezoelectric microphonics and voltage coefficient distortion that will degrade audio fidelity.
What happens if I swap the inverting and non-inverting input pins?
If you accidentally wire your input signal to Pin 3 (IN+) and route your feedback network to Pin 2 (IN-), you have created a positive feedback loop (a Schmitt trigger or comparator configuration). Instead of a clean, scaled replica of your input signal, the output will instantly latch to either the positive or negative supply rail the moment the input crosses the reference threshold. The op amp will not be damaged by this, but the circuit will fail to amplify linearly. Simply swap the input and feedback connections to restore negative feedback.
