If you are asking what do op amps do, the direct answer is that an operational amplifier (op amp) is a high-gain, DC-coupled voltage amplifier with differential inputs and a single-ended output. It amplifies the voltage difference between its non-inverting (+) and inverting (-) pins, multiplying that tiny difference by an open-loop gain typically exceeding 100,000. In practical circuits, we use negative feedback to tame this massive gain, forcing the op amp to perform precise mathematical operations like amplification, filtering, buffering, and signal conditioning.
The Core Function and Safe Default Part Numbers
Choosing the right op amp can be paralyzing given the thousands of SKUs available from manufacturers like Texas Instruments, Analog Devices, and Microchip. For 90% of hobbyist, student, and general-purpose prototyping tasks, you only need to keep a few "safe default" part numbers in your bench drawer. The table below outlines the exact part numbers you should stock, complete with their critical datasheet ratings.
| Part Number | Channels | Supply Voltage (V) | GBW (MHz) | Slew Rate (V/µs) | Input Bias Current | Best Application |
|---|---|---|---|---|---|---|
| LM358 | Dual | 3V to 32V (Single) or ±1.5V to ±16V | 1.0 | 0.3 | ~20 nA | Low-cost DC sensor buffering, LED drivers, slow-moving signals. |
| TL072 | Dual | ±5V to ±15V (Dual supply preferred) | 3.0 | 13.0 | ~5 pA (JFET input) | Audio preamps, active filters, high-impedance sensor inputs. |
| MCP6001 | Single | 1.8V to 6.0V (Single supply) | 1.0 | 0.6 | ~1 pA (CMOS input) | 3.3V/5V microcontroller ADC buffering, battery-powered IoT. |
| OPA2134 | Dual | ±2.5V to ±18V | 8.0 | 20.0 | ~5 pA (FET input) | Precision measurement, high-end audio, low-noise instrumentation. |
Notice that the legacy LM741 is excluded from this list. While often used in university textbooks, the 741 requires high dual-supply voltages, has terrible slew rate limitations, and cannot swing its output close to either supply rail. The LM358 and MCP6001 are vastly superior modern defaults for single-supply DC work, while the TL072 remains the undisputed king of dual-supply audio prototyping.
Pinout, Symbol, and Operation Regions
To understand what op amps do in a physical circuit, you must understand their pinout and how they behave across different voltage regions. The most common physical package for bench work is the 8-pin DIP (Dual In-line Package). According to standard semiconductor conventions, the standard 8-pin single-op-amp pinout is as follows:
- Pin 1: Offset Null (used to trim DC offset to zero; often unconnected in modern CMOS parts).
- Pin 2: Inverting Input (-).
- Pin 3: Non-Inverting Input (+).
- Pin 4: V- (Negative Supply Rail or Ground in single-supply).
- Pin 5: Offset Null (or sometimes a second trim pin).
- Pin 6: Output.
- Pin 7: V+ (Positive Supply Rail).
- Pin 8: NC (No Connect) or sometimes a shutdown/enable pin on modern micro-power parts.
An op amp operates in one of three distinct regions depending on the differential voltage applied to its inputs and the constraints of its power supply rails.
| Operation Region | Vout Voltage Range | Output Current Limit | Differential Input (V+ minus V-) |
|---|---|---|---|
| Linear (Active) | -13.5V to +13.5V (Typical ±1.5V dropout) | Up to ±20mA (Continuous) | Near 0V (Microvolts, forced by feedback) |
| Positive Saturation | Clipped at ~+13.5V | Short-circuit protected (~25mA max) | > 0V (Millivolts or greater) |
| Negative Saturation | Clipped at ~-13.5V | Short-circuit protected (~25mA max) | < 0V (Millivolts or greater) |
How to Bias and Select an Op Amp for the Job
The most common mistake makers and students ask about when figuring out what op amps do is why their circuit outputs a flat, clipped line when powered by a single 5V battery. Op amps cannot output a negative voltage if their V- pin is tied to 0V (Ground). If you feed an AC audio signal centered at 0V into a single-supply op amp, the negative half of the waveform is instantly clipped.
To fix this, you must bias the input to a virtual ground, typically exactly halfway between V+ and V- (e.g., 2.5V on a 5V supply). This allows the AC signal to swing symmetrically above and below the 2.5V midpoint without hitting the 0V or 5V rails.
Complete Application Circuit: AC-Coupled Non-Inverting Amplifier
Below is a complete, buildable non-inverting amplifier designed to boost an AC audio signal from a microphone or guitar pickup using a single 5V supply. We are using the MCP6001 (a 5-pin SOT-23 or 8-pin DIP rail-to-rail op amp).
- Power: VCC = 5V, GND = 0V.
- Bias Network: R3 = 100kΩ (from 5V to Pin 3), R4 = 100kΩ (from Pin 3 to GND). This sets the DC bias at exactly 2.5V.
- Input Coupling: C1 = 1µF ceramic capacitor in series with the audio input to block external DC offsets.
- Feedback Network: R1 = 10kΩ (from Output Pin 6 to Inverting Pin 2), R2 = 10kΩ (from Pin 2 to GND).
- Gain Calculation: Gain = 1 + (R1 / R2) = 1 + (10k / 10k) = 2 V/V (or +6 dB).
- Output Coupling: C2 = 1µF capacitor on the output to block the 2.5V DC bias from reaching the next stage.
According to Texas Instruments' op amp design guides, adding a small feedback capacitor (e.g., 100pF) in parallel with R1 is highly recommended to roll off high-frequency noise and prevent the circuit from acting as an RF antenna, which can cause rectification and DC offset shifts at the output.
Failure Modes and Multimeter Testing
Op amps are generally robust, but they will die quickly if subjected to specific electrical abuses. Understanding how they fail is critical to debugging a dead breadboard.
Common Failure Modes
- Output Short Circuit: Accidentally wiring the output pin directly to VCC or GND. Most modern op amps have internal thermal shutdown, but prolonged shorts will melt the internal bond wires, resulting in a permanently dead output.
- Overvoltage / Latch-up: Exceeding the absolute maximum supply voltage (e.g., feeding 7V into an MCP6001 rated for 6V max) causes internal parasitic SCR structures to trigger, creating a dead short from V+ to V-. The chip will get burning hot and draw massive current.
- ESD Damage to Inputs: The input stage consists of ultra-thin gate oxides (in CMOS) or delicate base-emitter junctions (in bipolar). Touching the input pins without a grounded wrist strap can punch through the die, resulting in massive input bias currents or a permanently shorted input.
How to Test an Op Amp with a Multimeter
If you suspect a dead chip, follow this numbered diagnostic sequence using a standard digital multimeter (DMM).
- De-energize the Circuit: Remove all power. Disconnect the op amp from the breadboard if possible to prevent parallel circuit paths from skewing your readings.
- Check Power Pin Shorts (Diode Mode): Set your DMM to diode test mode. Place the red probe on V+ (Pin 7) and black on V- (Pin 4). Note the reading. Swap the probes. You should see a high reading (OL) in one direction and a standard diode drop (0.5V to 0.8V) in the other. If you read 0.00V (a dead short) in both directions, the internal silicon has melted from latch-up or overvoltage. Throw it away.
- Check Input-to-Ground Shorts (Resistance Mode): Set the DMM to measure Ohms. Measure between the Inverting Input (Pin 2) and V- (Pin 4), then Non-Inverting (Pin 3) and V-. You should read in the megaohms or OL. A reading under 50Ω indicates blown input ESD protection diodes.
- Live Virtual Ground Test (Voltage Mode): Rebuild a simple voltage follower (buffer) circuit: tie the Output (Pin 6) directly to the Inverting Input (Pin 2). Apply power. Feed a known DC voltage (e.g., 2.5V) into the Non-Inverting Input (Pin 3). Measure the Output pin with your DMM. If the output does not match the input voltage within a few millivolts, the internal differential pair is dead or the output stage is blown.
By keeping a stock of LM358s and MCP6001s, respecting the common-mode input limits, and properly biasing single-supply circuits, you can reliably harness op amps for virtually any analog signal processing task on the bench.
