A general purpose op amp is a high-gain, DC-coupled voltage amplifier with differential inputs and a single-ended output. While specialized op amps exist for ultra-low noise, high-speed, or high-voltage applications, the general purpose op amp remains the workhorse of the electronics bench. They are forgiving, inexpensive, and capable of handling 90% of everyday signal conditioning, filtering, and buffering tasks.
The direct answer to "which one should I use?" depends on your supply voltage. For single-supply 5V to 12V sensor circuits, the LM358 is the undisputed default. For dual-supply ±12V audio or precision instrumentation, reach for the TL072. If you are interfacing directly with 3.3V microcontrollers like an ESP32, use a rail-to-rail CMOS part like the MCP6002.
The Safe Defaults: Part Numbers and Ratings
Walking into a component distributor or searching online yields thousands of op amp variants. To avoid analysis paralysis, stock your bench with these four proven defaults. Prices reflect typical 2026 retail costs for single units in DIP-8 or SOIC-8 packages.
| Part Number | Topology | Supply Range | Key Characteristic | Typical Cost |
|---|---|---|---|---|
| LM358 | Bipolar | 3V to 32V (Single) or ±1.5V to ±16V (Dual) | Includes ground in common-mode range; cheap | $0.15 |
| TL072 | JFET | ±5V to ±15V (Dual) | Low noise, high slew rate, high input impedance | $0.45 |
| NE5532 | Bipolar | ±3V to ±20V (Dual) | Classic audio standard, high output drive (38mA) | $0.60 |
| MCP6002 | CMOS | 1.8V to 6V (Single/Dual) | Rail-to-rail I/O, ideal for 3.3V MCU ADC buffering | $0.35 |
Pinout, Symbol, and Biasing Basics
The standard symbol for an op amp is a triangle pointing to the right. The non-inverting input (+) is on top, the inverting input (-) is on the bottom, and the output is at the apex. Power rails (V+ and V-) are typically omitted from the schematic symbol to reduce clutter, but they are physically required for the silicon to function.
For dual op amps in an 8-pin DIP package (like the LM358 or TL072), the industry-standard pinout is:
- Pin 1: Output A
- Pin 2: Inverting Input A (-)
- Pin 3: Non-Inverting Input A (+)
- Pin 4: V- (Negative Supply or Ground)
- Pin 5: Non-Inverting Input B (+)
- Pin 6: Inverting Input B (-)
- Pin 7: Output B
- Pin 8: V+ (Positive Supply)
How to Bias and Select for the Job
Biasing an op amp means setting its DC operating point so the AC signal can swing without clipping. If you use a dual supply (e.g., ±12V), the bias point is naturally 0V (ground). The input signal is centered around 0V, and the output can swing symmetrically positive and negative.
If you are using a single supply (e.g., a 9V battery), the op amp cannot output a negative voltage. You must create an artificial "virtual ground" at half the supply voltage (4.5V). You do this by wiring a voltage divider (two equal resistors, say 10kΩ each) from V+ to Ground, and buffering the midpoint with a spare channel of the same op amp or a large bypass capacitor (10µF). This biases the input and output at 4.5V, allowing a 2V peak-to-peak AC signal to swing between 3.5V and 5.5V without hitting the rails.
Operation Regions and Electrical Limits
Understanding where an op amp operates linearly versus where it saturates is critical for troubleshooting. According to Texas Instruments' LM358 datasheet, the output stage has physical transistor voltage drops that prevent it from reaching the exact supply rails.
| Operation Region | Condition | Typical Output Voltage (LM358 on 9V Supply) | Behavior |
|---|---|---|---|
| Linear | V+ > V- (by microvolts) | 1.5V to 7.5V | Output follows gain equation; negative feedback is active. |
| Positive Saturation | V+ >> V- | ~7.5V (V+ minus 1.5V drop) | Output clips at the upper limit; internal PNP transistors saturated. |
| Negative Saturation | V- >> V+ | ~0.02V (near ground) | Output clips at the lower limit; internal NPN transistors saturated. |
Note that the LM358 can pull its output down to within 20mV of the negative rail (ground), but it falls about 1.5V short of the positive rail. If your circuit requires the output to hit exactly 9.0V, you must either increase your supply voltage to 11V or switch to a rail-to-rail output (RRO) op amp like the MCP6002.
Practical Application: Non-Inverting Amplifier Circuit
Let's build a standard non-inverting amplifier to boost a 0.5V DC sensor signal to 5.5V for an Arduino analog input. We will use an LM358 powered by a 9V battery.
The gain equation for a non-inverting amplifier is: Gain = 1 + (Rf / Rin).
For a gain of 11, we need Rf = 10kΩ and Rin = 1kΩ.
Component List and Wiring Steps
- Power the IC: Connect 9V to Pin 8 (V+) and GND to Pin 4 (V-). Place a 100nF ceramic decoupling capacitor directly across Pins 4 and 8 to prevent high-frequency oscillation.
- Wire the Input: Connect your 0.5V sensor signal to Pin 3 (Non-Inverting Input).
- Set the Feedback Network: Connect a 1kΩ resistor (Rin) from Pin 2 (Inverting Input) to GND.
- Close the Loop: Connect a 10kΩ resistor (Rf) from Pin 2 (Inverting Input) to Pin 1 (Output).
- Verify: Measure the voltage at Pin 1 with a multimeter. It should read 5.5V (0.5V × 11). If it reads 7.5V, your input signal is too high and the op amp has hit positive saturation.
Failure Modes and Multimeter Diagnostics
Op amps rarely fail spontaneously. As detailed in All About Circuits' semiconductor guides, failure is almost always caused by external abuse: overvoltage on the inputs, shorting the output to a low-impedance rail, or exceeding the maximum supply voltage.
How to Test an Op Amp with a Digital Multimeter
You cannot test an op amp's amplification capabilities with a basic DMM, but you can test for catastrophic internal shorts using the diode-test function.
- De-energize the Circuit: Remove all power from the breadboard or PCB. Disconnect the op amp from the circuit if possible, or ensure no parallel low-resistance paths exist.
- Set DMM to Diode Mode: This applies a small test current and measures the forward voltage drop.
- Test Power Rails: Place the red probe on V+ (Pin 8) and black on V- (Pin 4). You should read an open circuit (OL) or a very high number. If it reads near 0.00V, the internal power bus is shorted. The IC is dead.
- Test Output to Rails: Place the red probe on the Output (Pin 1) and black on V- (Pin 4). A healthy bipolar op amp (like the LM358) will show a diode drop of roughly 0.6V to 0.8V due to the internal output protection diodes. If it reads 0.00V, the output transistor has shorted to ground.
- Test Inputs to Rails: Check between the input pins and the power rails. You should see high impedance (OL). A low resistance reading indicates blown input differential pairs, usually caused by exceeding the common-mode voltage limit.
Frequently Asked Questions
Can I use a general purpose op amp as a comparator?
Technically yes, but practically it is a bad idea. A general purpose op amp like the LM358 is designed to operate in a closed loop with negative feedback. When used open-loop as a comparator, the internal compensation capacitor causes a slow slew rate, meaning the output might take several microseconds to transition between logic levels. Furthermore, many general purpose op amps lack overdrive recovery, meaning if the input voltage goes too far beyond the rails, the output can "lock up" or invert. For comparing voltages, always use a dedicated comparator IC like the LM393.
Why is my general purpose op amp output stuck at the positive rail?
If your output is pinned to the positive supply voltage (minus the saturation drop), the op amp is in positive saturation. This happens when the voltage at the non-inverting input (+) is significantly higher than the inverting input (-). Check your feedback network; if the feedback resistor (Rf) is broken or disconnected, the op amp loses negative feedback and acts as an open-loop comparator, slamming the output to the rail. Also, verify that your input signal isn't exceeding the common-mode input range specified in the datasheet.
What is the difference between a general purpose op amp and an instrumentation amplifier?
A general purpose op amp has a single differential input stage. An instrumentation amplifier (like the INA128) is a specialized integrated circuit built using three internal op amps. It provides extremely high input impedance on both inputs, excellent common-mode rejection ratio (CMRR), and allows you to set the gain with a single external resistor. Use a general purpose op amp for basic buffering and filtering; use an instrumentation amplifier when measuring tiny millivolt signals from strain gauges or thermocouples in noisy environments.
How do I power a general purpose op amp from an ESP32 or Arduino?
Microcontrollers typically output 3.3V (ESP32) or 5V (Arduino Uno). Standard bipolar op amps like the TL072 or NE5532 require a minimum of 5V to 10V total supply and cannot read signals near the ground rail. To interface with an ESP32, power the op amp from the 3.3V pin and use a CMOS rail-to-rail op amp like the MCP6002. If you must use an LM358 with an Arduino, power the LM358 from the Arduino's 5V pin, but remember that the LM358's output will max out around 3.5V (5V minus the 1.5V headroom), which is perfectly safe for feeding back into a 5V Arduino analog input.
