Op-amp Basics
Regino Meriweather
Before looking at the specifications of a real op amp, it is important to understand the ideal op amp model to establish a reference. The ideal model exists to make the math involved in design easier, but cannot exist in practice. Some of the characteristics of an ideal op amp are:
An op amp normally has a single output that can swing between the range specified in the datasheet by VOL and VOH. This range is often significantly less than the range of VSS to VDD. For example, an op amp supplied with +/- 12V may only have an output swing of +/- 10V.
This of course does not exhaust the list of op amp specifications. There are others such as Total Harmonic Distortion, Common Mode Rejection Ratio, and more which may be important for certain applications. Unfortunately Digi-Key may not have these available to filter on the website. However, manufacturers will often have their own parametric search capability that includes these specifications.
Op amps are a common component in analog systems and have a wide variety of uses. Applications can vary from simply amplifying a signal to creating voltage references or implementing filters. This article covered the basics of op amp specifications as well as some of the most common op amp circuits.
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Operational amplifiers (op-amps) are some of the most important, widely used, and versatile circuits in use today. The first op-amp used vacuum tubes and was released in 1941 by Bell Labs. The ubiquitous ua741 was released in 1968 and is considered by many to be the standard upon which others are based. It is still in production today from various manufacturers. Designed to amplify a small signal up to something useful, op-amps are applicable in an extremely wide range of projects, everything from audio circuits, to data acquisition, to signal processing. My goal is to simplify the op-amp into something easy and fun to use, highlighting the important stuff and keeping it simple.
If you could really care less about the theory behind op-amps or just don't want to read right now, skip this step. There won't be any heavy math involved, just some summarizing. I recommend you take the time at some point to read up on them though since they are so useful in so many applications. Some really good educational/instructional material is available here, under Chapter 5.
Op-amps are usually two-input, one-output devices, with additional pins for +/- voltage supplies. By looking at the difference between the two inputs, and using the +/- voltage supplies as max/min output values, the op-amp will output a voltage reference value that can be many times higher than the input. The value of amplification is called the gain and is often seen measured in decibels (dB). Regardless of what you are amplifying, be it voltage, current, or power, dividing the output by the input will give you your overall gain. Different op-amp designs have different maximum values that they can achieve for the gain, but for the vast majority of applications, you get to choose the level of gain you want to apply to the input differential. You can also choose to have the output be the inverse of the input or match the input. The inputs are labeled "inverting" and "non-inverting" and there are two equations to determine the gain value of your op-amp design, one for a non-inverting configuration and the other for an inverting configuration. Note that for the non-inverting equation, you have an additional gain of 1 that you can't avoid. If, for example, you connect the non-inverting pin to GND and the inverting pin to your signal, the output will be phase shifted by 180 deg and amplified by the gain. On a graph, it would be completely flipped upside down over the x-axis (see image 2). If you switch the inputs and connect the inverting pin to ground and the non-inverting pin to your signal, the output will look just like the input (see image 3).
Op-amps typically have an extremely high gain built in by default which you the user cannot change, and if you don't design feedback into the system, you'll saturate the op-amp very quickly and hit one of the voltage supply rails. That implies that an op-amp with no feedback will function as a comparator, meaning that if there is a difference in voltage between the two inputs (+ or -), even by the tiniest amount, the output will match the value of the corresponding supply voltage rail. In logic terms, you get a 1 or 0. This can be useful in certain applications, like generating a square wave from a sine or triangle wave, but not in all cases. Many times you want the output to be a scaled version of the input, identical except for magnitude. In order to control the gain, you must implement feedback, connecting one input or the other to the output through one or more passive components like resistors, capacitors, or inductors.
Some applications of op-amps include voltage buffers/followers, low-, high-, and band-pass filters, comparators, integrators, differentiators, peak detectors, voltage/current regulators, and analog-to-digital converters and digital-to-analog converters. I will be going over some of these uses in later steps.
Op-amps also come in many, many different design options, so choosing the right one can be difficult. Should you use an OP37 or LM741? You decide you want really high speed, so you choose the OP37. But which version? The OP37A, C, E, F, G, N, NT, GT, or GR? Will you need more than one in your design? If so, should you use singles, duals, or quads? Of course each one has it's own datasheet, so it can be difficult to do comparisons easily. Just to give you an idea, I've included an Excel spreadsheet with just a few parameters listed to show the wide range of ICs available. It is not an exhaustive listing of all specs, just some basic data.
By comparing some of the data, we can see that the 741 op-amp is not very high speed (low slew rate), nor does it have a high gain-bandwidth product (GBP). The OP37 however has a much (much, much) higher slew rate and GBP, so it can be used over a much wider range of frequencies than can the 741. The other ICs all fall somewhere in the spectrum of speed vs reliability vs... whatever else you want to compare. Each one has it's own application, and it's up to you to decide how you want to use it. For most applications though, pretty much any op-amp will work. If you are designing something that is on the extreme end (e.g. high speed, high voltage, high gain), look through the datasheets to find the one that best suits your needs. As mentioned, I will be showing some simple op-amp circuits that can be built with any of these chips, but there will be some points where I point out the strengths/weaknesses of certain chips. For more information about op-amps, see this website.
These tools can be expensive and take up a lot of space, so I recommend the Digilent Analog Discovery or the Electronics Explorer Board, both of which contain all three in one simple, easy to use package. They both require the free Waveforms software. I will be using the Discovery, so all scope images will be screen shots from that. Also, the Discovery can supply +/- 5V at about 150mA, so all Vs connections will be those respective values. The EE board can supply +/- 9V at up to +/- 2A if you need more power output.
The last image is of a 741 op-amp pin-out diagram, which is the chip I will be using. Double check the pin-out diagram for the op-amp you want to use, especially multiple op-amp packages. Positive voltage from your power supply connects to pin 7 and the negative to pin 4. Pin 2 is the inverting input and pin 3 is the non-inverting input. Pin 6 is the output. Pins 1 and 5 are the offset null pins, which are rarely used and so will not be covered in depth here as most op-amps don't even have them, especially in larger dual and quad packages. Pin 8 is not connected.
One of the most basic uses for op-amps is the voltage follower or buffer (image 1). Ideal op-amps have the characteristic that they have infinite input impedance, so if there is a point in your circuit where you can't draw too much current from the previous portion of the circuit but you still need to use the voltage level as it is, you can add a voltage follower/buffer in between. This will buffer the previous part of your design from too much current draw while allowing the output voltage to exactly follow the input.
Amplifiers are another basic function of op-amps. First we look at the inverting configuration in image 1. This is easier to use since the equation simply inverts the input and scales it by the gain factor as determined by the resistor ratio (R2/R1). Technically the gain is considered to be negative for an inverting amplifier, but most applications will not be dependent on the phase of the input signal, so inverting it won't affect the outcome, and thus the negative sign can be ignored.
Build: Connect the power pins as before, + to pin 7 and - to pin 4. R2 goes across the IC between pins 2 and 6. One end of R1 goes to pin 2 while the other end is where the input signal connects. Pin 3 is connected to ground. (image 2) Use a small signal as your input for the circuit as the gain here is 10X. From the o-scope image you can see that the input (red) is about 200mV, while the output is 2V, which is what we want (image 3).
Next is the non-inverting configuration (image 4). The gain is still dependent on the ratio of the resistors, but with an extra 1 thrown in: (1 + (R2/R1)). The output phase matches the input phase, but the gain is slightly higher. The extra 1 becomes more and more negligible as the ratio (R2/R1) increases, but as a personal preference, I only use this circuit if I absolutely need the signal phases to match.
Build: Same power connections as before, but this time we simply switch where the input and ground connections go. Ground goes to the resistor tied to pin 2 and the input goes directly to pin 3 (image 5). Image 6 shows the o-scope data, and we can see that the phases now match, but the output (blue) is slightly higher than it was before because of that extra 1 we get from the gain equation.
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