Join Barron Stone for an in-depth discussion in this video What is an operational amplifier?, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] When you need to amplify a signal, or buffer a signal, filter a signal, or simply add two signals together, look no further than the operational amplifier, or op-amp for short. An operational amplifier is an electronic component that acts as a voltage amplifier, with an incredibly high amount of gain. It's DC coupled, which means it can be used to amplify both AC and DC signal components, it has two differential input terminals, and has a single-ended output.
Op-amps have become popular in modern electronics because they're fairly cheap, easy to use, and they can implement a wide range of operations for processing electrical signals. If you look at the internal circuitry of an operational amplifier, you'll see a complex network of components. For example, the op-amp schematic shown here contains 20 individual transistors, that are interconnected to form three separate amplifier stages. The thing that makes op-amps so great, is that to use them, I don't need to understand how all of this complex, under-the-hood circuitry actually works.
I can treat that complex op-amp circuit as a single component, and I just need to remember a few simple rules to use it. The schematic for an operational amplifier looks like a sideways pointing triangle, with five terminals. The two terminals on the left are differential inputs, the terminal on the top with the plus sign is called the non-inverting, or positive terminal, and the terminal on the bottom with the minus sign is the inverting, negative terminal.
As a differential amplifier, the input signal to the op-amp is the difference in voltage between those two terminals, subtracting the inverting input voltage from the non-inverting input voltage. The impedance of the input terminals on an op-amp is designed to be so high that, for most practical purposes, I can basically treat the op-amp as if it has an infinitely high input impedance. So, when I'm designing an op-amp circuit, I can pretend like no current will flow into either input terminal.
That approximation keeps things simple, and is often referred to as one of the golden rules of op-amps. The output terminal on the right side is single-ended, meaning the output voltage will be referenced to the circuit's common ground. The output terminal is designed to have a very low output impedance, so it can provide lots of current to whatever load is connected to it, to produce the desired output voltage. The last two terminals on the top and bottom of the op-amp symbol, are connected to the positive and negative voltage lines that provide power to the op-amp, which are called the power rails.
An op-amp cannot produce an output voltage that is greater than the positive rail voltage or less than the negative rail voltage, so the range between those two supply voltages needs to be big enough to encompass the full range of the expected output signal. Different types of op-amps will be able to handle different supply voltage ranges and will have their own requirements for the relationship between the output signals they can produce, and the supply voltage levels. So, be sure to check the data sheet for that, when you're choosing which type of op-amp to use for a certain circuit.
In schematics, it's common to hide the power rails to reduce clutter in the drawing, but, even if they're not shown, you always need to connect the power supply rails. Op-amps are an active component that require an external source of power to function. For prototyping circuits on a breadboard, I usually use op-amps that come in an 8-pin, plastic dual in-line package, or PDIP, form factor, which is a rectangular package with two parallel rows of four pins.
Those pins are spaced apart just right, so that the component can straddle the drop that runs down the center of the breadboard with one row of pins on each side. I typically use a black wire to connect the negative supply rail to the op-amp, and a red wire to connect the positive supply rail. The pins on the PDIP are numbered counterclockwise around the package, and the surface of the package will always have some sort of dot or orientation marking to indicate which side is which, so you can find pin #1.
Some op-amp models, like the 741 op-amp, only contain a simple amplifier within an 8-pin PDIP package. But other models, like the 358 op-amp, can have two separate amplifiers packed into a single package. The location of the input, output, and power supply pins will vary for different types of op-amps, so always check the data sheet to make sure you're connecting the part correctly. It's easy to destroy an op-amp by accidentally connecting the wrong pin to power, and unfortunately, an op-amp usually doesn't show any visible signs that it's been destroyed.
To check whether an op-amp has been broken, you'll have to measure the output signal with an oscilloscope, and decide if it looks like it's supposed to. Since it functions as a differential amplifier, the output voltage from the op-amp will be equal to the difference of the non-inverting input voltage, minus the inverting input voltage, times a gain factor known as the open-loop gain, abbreviated here as AOL. Op-amps are designed to have an incredibly large open-loop gain, usually over 100,000, which makes them incredibly sensitive to small differences between their input terminals.
For example, if the inverting input terminal was at 1 volt, and the non-inverting input terminal was at 1.001 volts, the difference between those two terminals is just 1 millivolt, but, when the op-amp scales that with an open-loop gain of 100,000, it turns that tiny 1 millivolt differential input into 100 volt output signal. Now, as I mentioned earlier, the op-amp can't generate output voltages that are greater than, or less than its power supply rail voltages, so, if I had my op-amp powered by plus and minus 12 volt power sources, that 100 volt output signal would get clipped off.
At the very most, it can only be 12 volts, and depending on the actual capabilities of the op-amp I was using, it would probably be even slightly less than 12 volts. If I swap those two input signals, so that the inverting input voltage was slightly higher than the non-inverting input, then the op-amp would see that difference as -1 millivolt, and its output would saturate in the other direction, at -12 volts. When the op-amp is used in this way, it's called the open-loop configuration.
The op-amp simply scales the input by its enormous gain factor, and as you can see, it doesn't take much input to saturate the op-amp's output. That open-loop behavior can be useful for some applications, like using an op-amp as a comparator, which I'll cover later in this course, however, to keep those wild voltage swings under control, it's much more common to use a circuit configuration that provides an external path from the op-amp's output, back to its input terminals, which creates a closed-loop configuration that provides feedback.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals