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The operational amplifier was first used in the 's as the basis of analogue computers, and much development took place to design accurate gun aiming systems during the Second World War. Since integrated circuits were unknown at the time this was before the invention of the transistor , the earliest versions were made using valves. The basic concept is to have an amplifier with differential inputs, thanks to Alan Blumlein who patented the circuit we now call a 'long tailed pair' in The ultimate goal was a circuit whose operation is controlled only by the external feedback components.

By rearrangement of the feedback circuit, different 'operations' could be performed.


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Typically, these early opamps could add and subtract, and these are essential functions to this day even in audio. For more information on the early history of opamps, see References. With the advent of the IC and mass production techniques, the opamp became very popular and remains so - with considerable justification. This article will concentrate mainly on audio including hi-fi applications, but there are some configurations that are just so wonderful that I cannot resist the temptation to include them. For the most part, any of the configurations shown can use the simplest and cheapest opamp you can get especially for testing , unless extremely wide bandwidth or low noise is a prime consideration.

For any of the test circuits this is not an issue. I also suggest that you build up the Opamp Design and Test Board Project 41 , which is ideal for the experimenter. Most of the circuits shown can be built using this test board, and will function perfectly, although there will be limitations as to bandwidth and noise because of the LM dual opamps recommended for the project.

This recommendation is for a purpose - if fast opamps are used, many circuits will oscillate because of long tracks and wires from inputs and outputs. To understand this article, you need to know Ohm's law and its derivatives. Ohm's law is fundamental to electronics, and with little more it is possible to derive most of the other resistance based formulae. Ohm's law states that a potential of 1 Volt through a resistance of 1 Ohm will cause a current of 1 Ampere to flow. This is expressed as:. Later on, we will also use the formulae for inductive reactance and capacitive reactance, as well as calculating frequency response and some filter design.

These will be presented as needed. Many people are 'scared' off electronics because they think that high-level maths knowledge is necessary, but for basic circuitry this is not the case at all. In all cases, I try to keep formulae to the minimum required for a good understanding. The ESP site doesn't show detailed and complex maths functions unless they are absolutely essential to understand what's going on.

You will see references to 'an instantaneous level of 'x' volts AC'. At any point in time, an AC voltage has an instantaneous voltage - this is the voltage that is present at that moment, and for analysis can be treated as DC.

This is valid only when we consider this 'DC' level as a transient thing, since many of the circuits do not operate down to DC at all many others do, but this is beside the point. There are two Rules, and although real life is never like theory I could fill the page with suitable examples, but shall refrain , they describe the operation of all opamp circuits very accurately:. Needless to say, this requires some explanation. So let's look at Rule 1.

Design criteria for low distortion in feedback op amp circuits

Any change of voltage on either terminal is reflected by a change in the output that causes more or less current to flow in the feedback circuit to restore equilibrium. If this is unclear to you, see the further explanations below - but remember the 1 st Rule! While it sounds simplistic, it actually describes the linear operation so well that you will rarely need to concern yourself at least during circuit analysis with the minor deviations that inevitably occur due to limited gain, input offset voltages, etc.

These are important, but they don't help with understanding what the device is trying to achieve. If the -in terminal is more positive, the output will swing negative. There is almost no opamp circuit that you cannot understand once these Rules are firmly established in your thinking. Even circuits that use external transistors in strange ways will obey the Rules. An opamp that does not perform as above is being used outside of its normal operating parameters, and the results will be unpredictable and almost always unsatisfactory. It is often explained that an opamp reacts only to the difference between the two inputs, and not to their common voltage common mode voltage is any voltage that appears on both inputs when the circuit is in equilibrium.

While essentially true, this doesn't have the absolute clarity of 'The Rules', nor does it help general understanding. Before we cover the circuits themselves, we need to look at some of the parameters you will come across, how to apply power and bypass the supply rails and so on. There are many parameters that you will see in data sheets, and these are covered in more detail a little later.

There is no point doing it now, as the importance will be lost until you know more about the opamp itself. Many of the quads use the same pinouts as well, and this has enabled people to swap opamps for 'better' ones for a very long time.

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The available styles and dimensions are available in the datasheet for the opamp you want to use. However - Don't count on complete standardisation! There are some variations, and although uncommon, they do exist. I shall not be concerned with any of the different devices - only the common pinout versions will be shown. Figure 1 - Common Opamp Pinouts. Figure 1 shows the standard connections for single, dual and quad opamps, but be aware that the remaining pins on the common single devices can occasionally have uses other than those shown.

The additional connections available are most commonly:. You may at times see these connections used in unconventional ways. Either way, I shall not be delving into these aspects of the design process. No active circuit works without power, so this has to be the first step. Most opamps will operate with a maximum of 36V between the supply terminals. Some opamps are rated for higher voltages, and others for less, so consult the spec sheet from the manufacturer.

A dual supply is not required, but it does simplify the design and is recommended for most applications. A dual supply has the advantage that all inputs and outputs are earth ground referenced. This can eliminate a great many capacitors from a complex design, and is the most common way to power most opamp circuits. Note that from a commercial perspective, elimination or reduction of capacitors is done for economic reasons rather than any great desire to 'simplify' the signal path.

Since the pinouts are nearly always the same, Figure 1 will be applicable in most cases, but as I said earlier "Don't count on it! When in doubt, get the specification sheet from the manufacturer. When not in doubt, get the specification sheet anyway. Most specification sheets give the test conditions for this measurement, and this should be consulted if an unusual design is contemplated.

Mostly it can be ignored. Bypassing Although most opamps have a very good PSRR, this cannot compensate the IC for power supply lead or track inductance, and this can cause serious misbehaviour of the opamp in use.

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It is always recommended that the supply be bypassed with capacitors - with special attention needed with high speed opamps. Bypassing should always use capacitors with good high frequency performance, and multilayer aka monolithic ceramics are the best in this regard. It is common for designs to use electrolytic capacitors, themselves bypassed by low value nF capacitors. This ensures that all trace inductance is properly 'neutralised', and helps to prevent oscillation.

When this occurs with a high speed HS opamp, it will commonly be in the MHz region, and may be extremely hard to see on basic oscilloscopes.


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  • A sure sign of oscillation is inexplicable distortion, that mysteriously disappears or appears when you touch the opamp or a component in its immediate vicinity. Figure 2 - Bypassing The Opamp Supplies. Even with HS opamps, electrolytic capacitors are usually not needed for each device generally needed only on each board , but the use of ceramic bypass caps between the supply pins of each device is highly recommended. Figure 2 shows a common method of bypassing power supplies for opamp circuits 'A' , but there are others. In some cases, the supplies may not be bypassed to earth ground , but just to each other.

    This has the advantage of not coupling supply noise into the earth ground system 'B'. The approach I usually take with PCB designs is shown in 'C', with a pair of electros at the point where the DC is connected to the PCB, and a bypass cap between the supplies of each opamp or opamp package. These claims are often made by frauds and charlatans, then perpetuated by unwitting hobbyists and others who don't know enough to be able to perform detailed analysis.

    Claims like this should be completely ignored - they have no basis in fact whatsoever, and indeed, quite the reverse may be true in each case. Note that bypassing alone is not sufficient to ensure stability under all conditions. Poor PCB layout can create problems too, and it's often necessary to take extra precautions with the layout to avoid issues that can be extremely difficult to track down. This is doubly true for inexperienced designers who are unaware of the general 'risk factors'. You will know that you have a layout or bypassing problem if a slow opamp works fine, but a faster one oscillates or causes severe ringing on transient signals including squarewaves.

    A common error is to omit an output resistor typically ohms to isolate the opamp's output from capacitive loads such as coaxial cables including standard RCA interconnects. An ideal opamp has an infinitely high input impedance, and therefore needs no bias current. It is also capable of infinite gain without feedback, so there are no errors between the two inputs i.

    The ideal opamp also has infinite bandwidth, no internal delay, and zero ohms output impedance. It is capable of supplying as much current as the load can draw, without the voltage being reduced at all. The ideal opamp does not exist.

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    Although it does not exist, the ideal opamp is the common model for nearly all opamp circuits, and few errors are encountered in practice as a result of designing for the ideal, and actually using a real non-ideal device. The tolerance of even the best resistors will ultimately limit the accuracy of any opamp circuit at low frequencies where gain is highest. This does not mean that any opamp can be used in any circuit - the designer is expected to be able to determine the optimum device for the task. During the design phase, one of the tasks of the designer is to set up the reference, which is simply a connection that's common to both the input and the output.

    It only has to be within the bounds set by the power supplies and the device itself. Depending on the design, it could be some other voltage - the opamp doesn't care as long as it's used within datasheet specifications. The primary practical limitations of real-world opamps are as follows:. There are others, such as input offset voltage and current, but we shall not concern ourselves with these parameters just yet. Power opamps IC power amplifiers may be capable of up to 10A, but these are outside the scope of this section of the article.

    The use of ideal opamps is assumed for much of the following, but all are designed to function properly with real world devices. In practice the difference between an ideal opamp and the real thing are so small as to be ignored, but with one major exception - bandwidth. This is the one area where most opamps show their limitations, but once properly understood, it is quite easy to maintain a more than adequate frequency response from even basic opamps.