An operational amplifier, usually called an op-amp, is a small electronic component that takes the difference between two input voltages and amplifies it by an extremely large factor. It’s one of the most fundamental building blocks in electronics, found in everything from audio equipment to medical instruments to the sensor circuits inside your phone. Understanding how it works gives you the foundation for most analog circuit design.
The Five Terminals
An op-amp is a five-terminal device. Two terminals are inputs: the non-inverting input (marked with a “+”) and the inverting input (marked with a “−”). The third terminal is the output. The remaining two connect to a power supply, which determines the voltage range the output can swing between. All the input and output voltages are measured relative to a common ground reference.
On a schematic, the op-amp appears as a triangle pointing toward the output, with the two inputs on the left side and the output at the tip. The power supply connections are often drawn at the top and bottom of the triangle, though they’re sometimes left off the diagram for simplicity.
What Happens Inside
Internally, an op-amp has three stages working in sequence. The first is a differential input stage, which is a pair of matched transistors that sense the tiny voltage difference between the two inputs and convert it into a current. The second is a high-gain stage that takes that current and develops a large voltage from it. This stage also contains the internal frequency-shaping that keeps the amplifier stable. The third is an output stage, designed to deliver the amplified signal at low impedance so it can drive whatever comes next in the circuit.
You don’t need to think about these stages when using an op-amp. They’re all packed onto a single chip, often no bigger than a grain of rice. But knowing they exist helps explain why op-amps behave the way they do, particularly the tradeoffs between speed, power consumption, and precision.
The Ideal Op-Amp Model
Engineers often start by treating the op-amp as ideal. An ideal op-amp has infinite gain, infinite input impedance (meaning zero current flows into either input), and zero output impedance (meaning it can drive any load without voltage drop). These assumptions simplify circuit analysis enormously and produce answers that are remarkably close to real-world behavior.
When negative feedback is connected from the output back to the inverting input, two “golden rules” make analysis straightforward:
- Rule 1: No current flows into either input terminal.
- Rule 2: The voltage at the inverting input equals the voltage at the non-inverting input.
These two rules let you solve almost any basic op-amp circuit with simple algebra. They hold as long as the op-amp is operating within its limits and negative feedback is present.
How Real Op-Amps Differ
Real op-amps come impressively close to the ideal, but they do have limits. A typical bipolar-input op-amp might have an input impedance of 5 megaohms and an output impedance around 60 ohms. Open-loop gain (the raw amplification with no feedback) is finite, though it’s usually in the tens of thousands to millions range for DC signals.
Real op-amps also draw a small bias current at each input because the internal transistors need it to operate. This current is tiny, often in the nanoamp range, but it matters in high-precision circuits. For most everyday applications, these imperfections are small enough to ignore.
Two Core Circuit Configurations
Almost every op-amp application is built on one of two basic configurations. In the non-inverting amplifier, the input signal connects to the “+” terminal, and a pair of resistors (R1 and R2) form a feedback path from the output to the “−” terminal. The voltage gain equals 1 + (R2 / R1). If R2 is nine times R1, for example, the circuit amplifies the signal by a factor of ten.
In the inverting amplifier, the input signal feeds through a resistor to the “−” terminal, while the “+” terminal connects to ground. The gain equals the negative ratio of the feedback resistor to the input resistor. The “negative” part means the output is flipped in polarity: when the input goes positive, the output goes negative by the amplified amount. This 180-degree phase flip is useful in many signal processing applications.
Gain Bandwidth Product
Every op-amp has a parameter called gain bandwidth product (GBW), and it represents a hard tradeoff between how much you amplify and how fast your signal can be. If an op-amp has a GBW of 1 MHz, you can get a gain of 100 up to about 10 kHz, or a gain of 10 up to 100 kHz, or a gain of 1 up to the full 1 MHz. The product of gain and bandwidth is always the same constant. If you need both high gain and high frequency response, you’ll need an op-amp with a larger GBW, or you’ll need to cascade multiple stages.
Slew Rate
Slew rate measures how fast the output voltage can change, expressed in volts per microsecond. It sets a ceiling on how quickly the op-amp can respond to large, sudden input changes. A low-power device like the OPA369 slews at about 5 millivolts per microsecond, consuming less than a microamp of supply current. At the other extreme, a high-speed device like the OPA847 can slew at 850 volts per microsecond, but it draws over 18 milliamps. Choosing the right op-amp often comes down to balancing speed against power consumption for your specific application.
Common Mode Rejection
Because an op-amp amplifies only the difference between its two inputs, it naturally rejects any signal that appears equally on both inputs. This is called common mode rejection, and it’s measured in decibels. Typical values range from 70 dB to 120 dB at low frequencies, meaning the op-amp suppresses shared noise by factors of roughly 3,000 to 1,000,000. This makes op-amps excellent at extracting a small signal buried in electrical noise, which is exactly the situation you face when reading a sensor.
Output Voltage Limits
The output of an op-amp can’t swing all the way to its power supply voltages. In classic bipolar designs, the output typically falls 1 to 2 volts short of each supply rail. So an op-amp powered by ±15 V might only swing from about −13 V to +13 V. “Rail-to-rail” op-amps, built with CMOS technology, get much closer, with output swing reaching within a few millivolts of the supply. If your circuit needs the output to reach near zero volts or near the supply voltage, a rail-to-rail device is the better choice.
Where Op-Amps Show Up
Sensors are one of the most common places you’ll find op-amps at work. A temperature sensor, pressure transducer, or light detector typically produces a signal in the range of tens of millivolts. That’s far too small for a microcontroller or data converter to read accurately. An op-amp amplifies the signal, filters out noise, and matches the voltage range to what the next stage expects. In one university implementation, a reflective object sensor’s phototransistor output (a tiny current) was converted into a usable voltage using a single op-amp in a non-inverting configuration.
Beyond sensors, op-amps serve as active filters that select specific frequency ranges, as buffers that prevent one part of a circuit from loading down another, as oscillators, as voltage regulators, and as the comparison stage inside analog-to-digital converters. The name “operational” originally came from their use in analog computers, where they performed mathematical operations like addition, subtraction, integration, and differentiation. Those mathematical functions are still used constantly in modern signal processing circuits.