# Why are op amps used so frequently in analog electronics?

I've read in several books and papers the observation: "Op amps are the bread-and-butter of analog electronics", or "... op amps are the most commonly encountered building block in analog circuits ..." and to that effect.

Although my experience is not broad enough either to concur or to refute that claim, it's certainly borne out in the circuits I have seen.

It makes me think I'm missing something fundamental, to explain why a component like this would be perhaps something like a "for" loop in programming or something, a fundamental pattern, that once available, finds pervasive application.

What is it about the fundamental nature of analog electronics that makes an op amp the fulfillment of such a basic and versatile pattern?

• They're easy enough to use that mostly digital designers, like myself, can successfully use them as building blocks connected to either the analog inputs and digital (or analog) outputs of a microcontroller in an embedded design. – tcrosley Aug 1 '15 at 5:27
• Of course, in the "good old days" it was transistors that was the bread and butter of electronics... but yes, it's much simpler to use an op-amp than having to design an amplifier using transistors to do the same job. – Baard Kopperud Aug 1 '15 at 10:14
• Because the design with OP-AMP is easier than using discrete components, and almost impossible to get wrong. The effect of this is that finding discrete elements (JFET, BJT etc.) has become more and more difficult, outside very specialized specs (HF, ULNA, high power, whatever). So designing with discrete components has become more difficult. Loop from start ;-). – Rmano Aug 1 '15 at 12:37
• Just to recap the core info you can get from the all the good answers below: an opamp is almost an ideal differential voltage amplifier and is extremely versatile and cheap. Some jellybean opamps cost less than some optimized low power BJTs or FETs! – Lorenzo Donati Aug 1 '15 at 12:43
• I suppose it's analogous to why MCUs are used so often instead of just discrete logic gates. – DKNguyen Mar 3 at 20:10

## 9 Answers

Op amps are pretty close to being ideal differential amplifiers. So the real question is, what's so great about amplifiers? There are (at least!) three answers.

First, the obvious -- amplifiers let you change the amplitude of a signal. If you have a small signal (say, from a transducer), an amplifier lets you raise its voltage to a useful level. Amplifiers can also reduce the amplitude of a signal, which could be useful to fit it into the range of an ADC, for example.

Amplifiers can also buffer a signal. They present a high impedance on the input side and a low impedance on the output side. This allows a weak source signal to be delivered to a heavy load.

Finally, negative feedback allows amplifiers to filter a signal. So-called active filters (which use amplifiers) are much more flexible and powerful than passive filters (which use only resistors, capacitors, and inductors). I should also mention oscillators, which are made using amplifiers with filtered positive feedback.

Amplitude control, buffering, and filtering are three of the most common things you can do to analog signals. More generally, amplifiers can be used to implement many kinds of transfer functions, which are the basic mathematical descriptions of signal processing tasks. Thus, amplifiers are all over the place.

Why op amps in particular? As I said, op amps are essentially high-quality amplifiers. Their key characteristics are:

• Very high differential gain (sometimes as high as 1,000,000!)
• Very high input impedance (teraohms at low-frequency for FET-input op amps)
• Very high common-mode rejection ratio (typically >1000)

These characteristics mean that the behavior of the amplifier is almost entirely determined by the feedback circuit. Feedback is done with passive components like resistors, which are much better-behaved than transistors. Try simulating a simple common emitter amplifier across voltage and temperature -- it's not great.

With modern improvements in integrated circuits, op amps are cheap, high-performance, and readily available. Unless you need extreme performance (high power, very high frequency) there's not much reason to go with discrete transistor amplifiers anymore.

• It also allows to build stuff like comparators, schmitt triggers, integrators, differentiators, filters … – Michael Aug 1 '15 at 12:59
• There are an awful lot of good answers to this question here, I encourage folks who find it on search to read them all; it's not a single-right-answer question it seems :) It's hard to pick between them, but I'm deferring to the wisdom of crowds here and accepting this answer as it's received more than twice the "is-useful" votes of the runner-up. Thanks to all who responded, I learned an awful lot from studying your answers :) – scanny Aug 2 '15 at 23:31

An op amp is three 5 basic tools in one (if not more).

• First a comparison device, like an if else statement (if a > b, output = a, else b).

• Second a buffer (in = 1, out = 1, refreshed).

• Third an amplifier, like a multiplier (in = 1, out = 10).

• Fourth, a phase shift/delay (in = x, out = x + 1).

• Fifth, an inverter (in = x, out = 1/x).

They tend to be very versatile, and able to adapt to many circuits as needed.

Fundamentally, as a signal gets processed through analog discrete elements, its amplitude—its voltage—drops. An op amp can buffer and boost an analog signal, ensuring it is readable or useful at the end.

Incidentally, a for loop would be a counter. A decade counter works like a for (i = 0, i < 10, i++) loop.

• And it's also very good at recursion. – Ignacio Vazquez-Abrams Aug 1 '15 at 4:21
• @IgnacioVazquez-Abrams please explain how its good at recursion? – muhammad muheeb Aug 1 '15 at 9:14
• I understood it as kind of a joke, and a good one too :) Recursion takes the result of a function and applies that same function to it, and then again (a certain number of times). So like f(f(f(f(x)))). If the op amp input is the function argument x, and the op amp output the function result, the negative feedback "recursively" applies the op-amp (gain) function to the output. – scanny Aug 1 '15 at 18:17
• love those formulas / math equivalent, it makes me understand each term quickly. – tigrou Aug 3 '15 at 9:17
• Might be a bit confusing. "Inverter" is, I think, usually going to be taken to refer to additive inversion, i.e. the classic inverting amplifier configuration. But here you describe it using the multiplicative sense. Although you could use op amps to implement 1/x, it's not trivial, nor would that be more common than any of the other textbook "operations" which op amps can be configured to perform (e.g. differentiation/integration). – Marshall Eubanks Aug 4 '16 at 21:17

Some of the key benefits of an op-amp are

high input impedance: Due to its high input impedance, an op-amp doesn't unduly load the preceding circuit. An op-amp itself might have input impedance in the 10's or 100's of gigohms. An op-amp feedback circuit will likely have a lower input impedance, but the high input impedance of the op-amp allows this to be entirely set by the other components.

low output impedance: Due to its low output impedance, an op-amp circuit can generally drive another op-amp circuit (or an ADC or ...) without the load affecting its behavior.

high gain: The high gain of the op-amp allows it to be used in a negative feedback circuit such that the behavior of the circuit is dominated by the feedback elements rather than by the op-amp. This means

1. Often only a few precision components are required in the feedback circuit to achieve precision performance from the overall circuit.

2. Since the behavior of the circuit is controlled by the feedback circuit, the op-amp can be used with numerous different feedback elements to achieve different functions like amplification, differentiation, integration, logarithmic amplification, etc. (This may be the key reason that op-amps have such "pervasive application").

• Note you're describing a general purpose op-amp. Specialized op-amps usually excel in one part (low-noise, high-gain, low-power consumption, etc.) while not necessarily adhering to 'default' op-amp rules (I've seen op-amps with a output impedance of several kΩ). – Mast Aug 1 '15 at 13:22

I think the real answer is much simpler than those provided by others (although they are indeed true) - op-amps simply allow you to build all the "legos" you need for a more advanced circuit, see https://en.wikipedia.org/wiki/Operational_amplifier#Applications for more details. With op-amp you can get (non-exhaustive list!):

• a voltage/current buffer,
• a comparator (even with hysteresis),
• an active amplifier (both inverting and non-inverting),
• ideal diode,
• active filter (including integrator/differentiator apps),
• active rectifier,
• active math blocks (e.g. sum, diff, ply, div),
• a wave synth (square, tri, saw, even VCO),
• DAC & ADC,
• impedance converter,
• gyrator,
• ... and many others.

That's more than everything you will probably need for essential analog processing - and some of those things are neat for digital processing too. As such, op-amps are both the bread and butter here.

Also, you can easily get e.g. 2 or 4 of them in one small package with common voltage supply lines, and their operations characteristics (nearing those of ideal component for many practical applications, and quite well matched for op-amps inside a single package too) allow using them without much of the trouble needed for discrete (diode/BJT/FET) analog circuits (e.g. biasing, voltage drop, temperature compensation etc.) - allowing you to design more simple, streamlined and maintainable circuits, with less parts and easier troubleshooting.

To pick out one particular electronic component and call that the "bread and butter" is silly, as is all these "most important" kind of statements. For example, count resistors in analog circuits, and I'm sure you'll find they outnumber opamps by a wide margin.

Also, things change. There was a time when vacuum tubes were the layman's silly "most important" or "bread and butter" component of analog electronics, then the transistor.

You never need to use a opamp, but it can be the most efficient way to implement a circuit to a particular spec. After all, opamps are made from transistors, so it is possible to use a bunch of transistors (with a few other components) instead.

The attraction of opamps is that they embody a common and easily utilized building block. With the magic of integrated circuits, these building blocks can be the size and cost of single transistors sometimes. Any one opamp may be overkill for any one particular application, but the great leverage of mass produced integrated circuits allows them to be cheap and small enough so that it's usually cheaper and smaller to use a whole opamp when only a few of its transistors would actually be needed.

To use your analogy with a FOR loop in a programming language, you don't actually need to use this construct. You could initialize, increment, and check a variable yourself with explicit code. Sometimes you do that when you want to do special things and the canned FOR construct is too rigid. However, most of the time its more convenient and less error prone to use the FOR construct for loops. Just as with opamps, you may not use all the features of this canned high level construct in each case, but its simplicity makes it worth it anyway. For example, most languages allow the increment to be something other than 1, but you probably only use that rarely.

Unlike with the FOR construct, there is no compiler that optimizes a opamp in a discrete circuit to just the features that you require in that instance. However, the huge advantage of volume integrated circuit production reduces those features to way less than the equivalent of a few extra instructions in a FOR loop. Think of opamps more as being a full featured FOR loop implemented in the instruction set, which takes the same instructions to execute whether all its features are used or not, and less instructions than you would have to use otherwise, even for the simple cases.

Opamps are a bunch of transistors packed up to present a "nice" building block, and made available for the cost of just one or a few of those transistors. This not only saves time in design to deal with all the biasing of the transistors and the like, but manufacturing techniques can be used to guarantee good matching between the transistors and that allow for measuring and trimming parameters closer to ideal. For example, you can make a differential front end with two transistors, but getting the input offset voltage down to just a few mV is not trivial.

All of engineering is based on using available building blocks at some point, and opamps are a useful building block for analog circuits. This is really no different that using transistors. A lot of processing went into refining the silicon, doping it, cutting it, packaging it, and testing it that we somewhat take for granted as a discrete transistor. Opamps are more integrated than individual transistors, but are still fairly "low" level in the scheme of things.

Back to the software analogy, this is the same as using existing subroutines to get on with writing the code for your particular app. In the case of OS calls, you don't have a choice to use them. That would be like refining your own silicon. Opamps are more like convenient calls that you could write yourself, but doing so would be silly in most cases. For example, you've probably had to convert a integer to a ASCII decimal string many times, but how many of those times did you write your own code for that? You probably used runtime library calls for that, or even called those implicitly thru higher level constructs available in your language (like printf in C).

The ideal opamp has infinite input impedance, 0 offset, 0 output impedance, infinite bandwidth, and costs \$0. No opamp is ideal, and these and other parameters have different relative importance in different designs. This is why there are so many opamps. Each is optimized for a different set of tradeoffs. For example, you sometimes hear that the LM324 is a "crappy" opamp. This is not true at all. It's a superlative opamp when price is a high priority. When a few mV offset, 1 MHz gain*bandwidth, etc, are all good enough, everything else is just overpriced junk.

• "opamps are made from transistors, so it is possible to use a bunch of transistors (with a few other components) instead" doesn't follow. A bunch of discrete transistors have orders of magnitude more parasitic inductance, resistance, and capacitance, as well as longer traces and more coupling to the surroundings than the transistors inside the opamp, meaning that the build-your-own opamp has much worse frequency limit and noise performance than the IC version. The software equivalent would be duplicating the logic of library functions in an interpreted environment. – Ben Voigt Aug 1 '15 at 21:50
• "The software equivalent would be duplicating the logic of library functions in an interpreted environment" . Nice analogy. Except it is actually a bit too benevolent on the roll-your-own-op-amp: for the reasons you stated. The roll your own electrical characteristics may well cause it not to function in the manner desired (wrong result..) - vs the iterative approach to programming is hypothetically simply slowing it down. – javadba Aug 3 '15 at 6:22

Regarding your comment "It makes me think I'm missing something fundamental, to explain why a component like this would be perhaps something like a "for" loop":

You might be looking for an analagous concept in electronics to the concept of Turing Complete found in computer science or to the concept of Functional Completeness found in boolean algebra (and hence digital logic).

I far as I know, there is no "completeness" concept in analog circuits where all circuits can be derived from a set of basic building blocks...

There are some rules about analog circuits that you'll encounter when studying Systems Theory and in particular Linear-Time Invariant Systems.

I hope this helps, but it may not be what you're looking for.

• You hit right on the underlying "nagging feeling" I get, something perhaps like "In the signal domain, every circuit stage can be viewed as an amplifier (even if it's a resistor). The fundamental general-purpose active amplifier is the op amp ...". I just made that up, but yes, exactly, that sort of thing, like Turing completeness :) – scanny Aug 1 '15 at 17:52
• Analog circuits are typically represented as Systems in the S-domain en.wikipedia.org/wiki/Laplace_transform or the Fourier Domain en.wikipedia.org/wiki/Fourier_series. The mathematical description of a system can be described as a "transfer function" in either of these domains (there are a few other domains as well). In one sense, an op-amp can physically implement a wide range of "Transfer" functions. For more info, see: en.wikipedia.org/wiki/Transfer_function – LoveToCode Aug 1 '15 at 20:20
• The linearity aspect is really crucial IMO. – leftaroundabout Aug 2 '15 at 12:14

There are many cases, in both analog and digital electronics, where it's possible to define (but not build) an ideal component, and then design a circuit which will meet requirements if built with components that are within a certain tolerance of ideal. Reasoning about designs with components that have simplified ideal behaviors is often easier than reasoning about designs using real-world components with more complicated real-world behaviors.

In many cases it will be possible model a design using real-world components, assign allowable tolerances to the signals at each stage in a design, and then show that real-world components, when given any combination of inputs that are within the tolerance specified for those signals, will produce outputs which are within the tolerance specified for those signals. In cases where this is possible, such assignment of tolerance values will often avoid the need for more detailed analysis.

One of the reasons that op amps are so popular is that there is in some sense one clear "ideal behavior" for an op amp, and it's easy to characterize certain deviations from that behavior. If a differential amplifier is supposed to have a 10:1 differential-input gain, one must deal with the possibility that a real-world part might have a gain which is greater than the ideal or less than the ideal. Since an ideal op amp's gain is infinite, however, real-world op amps intended for amplification will generally have a lower gain [some devices, especially those intended for use as comparators, may have hysteresis which could be seen as gain beyond that of an ideal op amp]. Reasoning about real-world devices which can only deviate from the ideal in one direction is often easier than reasoning about devices which can deviate in two.

Isolation, impedance matching, scaling, level conversion, sourcing large amounts of current compared to digital components and signal generation are common applications for op-amps.

Study the basic configurations of op-amps to see why they are so popular in analog design, particularly in the role of oscillator and in signal conditioning.

Years ago, I used the inverting op-amp with gain to create an RS-232/MIL-188C converter to recuse some data from an old AT&T Model 40 Teletype using a 386 based PC running a custom QuickBasic 4.0 program.

They are indispensable as the input isolation and scaling front end for digital signal processing and can perform nifty tasks such as conversion from voltage to current and or frequency and back.

I think that the statement "bread and butter" sounds complementary to the role,the opamp can be a very good extension of circuits,where each circuit has a specialty.

For example it is used as Integrator and Differentiators in the field of Control and Regulation,which are otherwise better known as High pass and Low Pass filters.

Also It can be put in stable oscillations,as their output is largely amplified by the gain of the amplifier,just using a small input signal you can set the opamp in oscillation using positive feedback,best example are Schmitt Triggers,which then can be used in noise cancellation.Hence they form circuits like Bistable and MonoStable Osciilators which further give them a complementary role in the 555 timers.

Comparator makes the use of its common voltage mode, actually the opamp has a cascaded differential amplifier followed by a current-mirror active load,at its input which gives it specialty to be used as comparator which can compare the inputs.Most of its application are based on this property,the Dual rail supply drives the circuit immediately near to the opposite voltages.

As Current limiters in circuits where capacitors are used ,to prevent them from slow discharging they are being isolated by these opamps by their high input impedance,so that they maintain their charge,which give them nice complementary role in high speed Switch and Hold circuits