If you are building analog front-ends, sensor conditioning boards, or audio preamps, you need reliable operational amplifier defaults. For 90% of general-purpose, single-supply bench work, the LM358 is your safe default. For dual-supply audio and low-noise applications, the TL072 is the standard. If you need a comparator, skip the op amp and use an LM393.
This guide skips the abstract semiconductor physics and goes straight to the workbench. We will cover how to read the pinout, bias the IC for single or dual supplies, build a complete non-inverting amplifier with exact component values, and troubleshoot a dead chip with a multimeter.
Anatomy and Pinout of a Standard Op Amp
The universal symbol for an operational amplifier is a triangle pointing right. It has five fundamental nodes:
- Non-inverting input (+): A signal here appears at the output with the same polarity.
- Inverting input (-): A signal here appears at the output inverted (180° out of phase).
- Output: The amplified voltage difference between the two inputs.
- V+ (VCC): Positive power supply rail.
- V- (VEE/GND): Negative power supply rail (or ground in single-supply setups).
Most hobbyist and prototyping op amps come in an 8-pin DIP (Dual In-line Package) containing two independent amplifiers (a "dual" op amp). According to the Texas Instruments LM358 datasheet, the standard pinout for an 8-pin dual op amp is:
- Pin 1: Output A
- Pin 2: Inverting Input A (-)
- Pin 3: Non-inverting Input A (+)
- Pin 4: V- (Ground or negative rail)
- Pin 5: Non-inverting Input B (+)
- Pin 6: Inverting Input B (-)
- Pin 7: Output B
- Pin 8: V+ (Positive rail)
Selecting and Biasing Op Amp Circuits for the Job
Selecting the right IC depends on your power supply topology and signal requirements. Biasing an op amp correctly is where most DIY builds fail. Op amps amplify the difference between their inputs, but they cannot output a voltage beyond their supply rails. If you are running a single 12V supply (VCC = 12V, V- = 0V) and try to amplify an AC audio signal that swings below 0V, the negative half of the waveform will be brutally clipped.
The Single-Supply Virtual Ground
To pass AC signals on a single supply, you must bias the non-inverting input to a "virtual ground" at exactly half your supply voltage (VCC/2). You create this by using a voltage divider: two 10kΩ resistors in series from VCC to GND, with the midpoint fed to the non-inverting input. Place a 10µF ceramic or electrolytic capacitor from that midpoint to ground to filter out power supply noise.
Safe Default Part Numbers and Ratings
Do not blindly grab any op amp. Here are the bench defaults with their critical ratings:
| Part Number | Supply Voltage | Slew Rate | Input Bias Current | Best Application | Approx. Cost |
|---|---|---|---|---|---|
| LM358 | 3V to 32V (Single/Dual) | 0.3 V/µs | 20 nA | General purpose, DC sensor amps, single-supply | $0.15 |
| TL072 | ±5V to ±15V (Dual) | 13 V/µs | 5 pA | Audio preamps, active filters, low-noise | $0.40 |
| OPA2134 | ±2.5V to ±18V (Dual) | 10 V/µs | 5 pA | High-fidelity audio, precision instrumentation | $3.50 |
| MCP6002 | 1.8V to 6V (Single) | 0.6 V/µs | 1 pA | 3.3V microcontroller ADC buffering, battery powered | $0.50 |
| LM393 | 2V to 36V (Single/Dual) | N/A (Comparator) | 25 nA | Voltage comparators, open-collector logic interfacing | $0.20 |
Operation Regions and a Complete Non-Inverting Circuit
An op amp operates in three distinct regions depending on the feedback network and input voltage. Understanding these regions is critical for predicting circuit behavior, as outlined in Analog Devices' Op Amp Basics.
| Operation Region | Condition | Typical Output Voltage (12V Supply) | Current Behavior |
|---|---|---|---|
| Linear (Active) | Negative feedback is present; inputs are within common-mode range. | Between ~1.5V and ~10.5V | Output sources/sinks current up to IC limit (e.g., 20mA for LM358). |
| Positive Saturation | V(+) > V(-) by a margin exceeding the linear gain limit. | ~10.5V (VCC - 1.5V drop) | Internal PNP/NPN transistors fully saturated; output impedance drops. |
| Negative Saturation | V(-) > V(+) by a margin exceeding the linear gain limit. | ~0.2V (VEE + 0.2V drop) | Output transistor pulls hard to ground; cannot reach true 0V on LM358. |
Complete Application Circuit: Non-Inverting Amplifier
Let's build a DC-coupled non-inverting amplifier to boost a 0.5V sensor signal to 5.5V for an Arduino ADC. We will use an LM358 on a single 12V supply.
Component List & Values:
- U1: LM358 (8-pin DIP)
- R1 (Feedback Resistor): 10kΩ (Connects from Output Pin 1 to Inverting Input Pin 2)
- R2 (Ground Resistor): 1kΩ (Connects from Inverting Input Pin 2 to GND)
- C1 (Bypass Capacitor): 100nF ceramic (Connects directly across VCC Pin 8 and GND Pin 4)
Wiring Steps:
- Connect Pin 8 to +12V and Pin 4 to GND. Solder or place C1 as close to these pins as physically possible to prevent high-frequency oscillation.
- Feed your 0.5V sensor signal into the Non-inverting Input (Pin 3).
- Wire R1 between Pin 1 (Output) and Pin 2 (Inverting).
- Wire R2 between Pin 2 (Inverting) and GND.
The Math: The gain of a non-inverting amplifier is Gain = 1 + (R1 / R2). Here, 1 + (10,000 / 1,000) = 11. An input of 0.5V yields exactly 5.5V at Pin 1. Because the LM358 is not a "rail-to-rail" output op amp, its maximum swing on a 12V supply is about 10.5V. Our 5.5V target sits safely in the middle of the linear region.
Failure Modes: How to Test an Op Amp with a Multimeter
Op amps typically fail in three ways: the output shorts internally to a supply rail (often due to driving a capacitive load without a series resistor), the input stage blows from ESD or overvoltage, or the silicon simply overheats and cracks. Here is how to test a suspected dead op amp using a standard digital multimeter (DMM).
- Visual Inspection: Look for a bulging epoxy package, scorch marks on the PCB around the IC, or a melted plastic smell. If the chip has a visible divot blown out of it, throw it away.
- In-Circuit Short Check (Resistance Mode): With power OFF, measure resistance between VCC (Pin 8) and GND (Pin 4). A healthy op amp will read in the hundreds of kilo-ohms or higher. If it reads < 50Ω, the IC is internally shorted (or a parallel component is shorted).
- Diode Test (Junction Check): Set your DMM to Diode mode. Place the red probe on GND (Pin 4) and touch the black probe to the Output (Pin 1) and Inverting/Non-inverting inputs. You are testing the internal ESD protection diodes. You should read a standard silicon junction drop (typically 0.5V to 0.7V). A reading of 0.00V indicates a shorted junction; an "OL" (open loop) indicates a blown internal bond wire.
- Live DC Offset Test: Power the circuit ON. Ground both inputs (Pins 2 and 3) together. Measure the DC voltage at the Output (Pin 1). In a properly functioning op amp with negative feedback, the output should read very close to 0V (or VCC/2 if biased). If the output is pinned hard to VCC or GND despite grounded inputs and intact feedback resistors, the internal differential pair has failed.
Frequently Asked Questions About Op Amp Circuits
Why is my op amp circuit output stuck at the positive rail?
If your output is pegged to the positive supply rail (saturation), you have lost negative feedback, or you have violated the common-mode input range. First, check your feedback resistor (R1 in our example) with a multimeter; if it is open or a cold solder joint exists, the op amp runs at its massive open-loop gain (100,000x or more) and instantly saturates. Second, check your input voltage. The standard LM358 cannot handle input voltages higher than VCC - 1.5V. If you feed 11V into the input of an LM358 running on 12V, the internal input transistors starve, and the output will unpredictably latch to the positive rail. If you need to sense voltages near the positive rail, switch to a rail-to-rail input op amp like the MCP6002.
Can I use an op amp as a comparator?
You can, but you usually shouldn't. While both devices compare two voltages, op amps are designed to operate in the linear region with negative feedback. When an op amp is driven into hard saturation (acting as a comparator), the internal transistor junctions flood with charge carriers. When the input polarity reverses, the op amp suffers from "overload recovery time"—it can take several microseconds to pull out of saturation and respond. A dedicated comparator like the LM393 is designed to switch in nanoseconds and features an open-collector output that easily interfaces with 5V microcontrollers or 12V relays. Use op amps for analog math and signal conditioning; use comparators for threshold switching.
What is the difference between single-supply and dual-supply op amps?
Electrically, an op amp does not know what "ground" is; it only sees the voltage difference between its V+ and V- pins. A "dual-supply" op amp like the TL072 is typically rated for ±15V (meaning 30V total between Pin 8 and Pin 4), and is optimized for circuits where the signal swings above and below a true 0V ground. A "single-supply" op amp like the LM358 is specifically designed so that its input common-mode range includes the V- pin (ground), and its output can swing to within millivolts of ground. If you are running off a 9V battery or a 5V USB supply, you must use a single-supply rated op amp and bias your AC signals to the mid-rail voltage, as detailed in All About Circuits' semiconductor tutorials.
