Operational Amplifiers Overview

Operational amplifiers are high gain voltage amplifiers. They typically have five terminals on them, though the schematic symbols sometimes leave out the power supply terminals. These are active components, which means that they do need to receive power from a power supply to function.

These devices have differential inputs and, in most configurations, you have a single output. These devices were used extensively in analogue computers, where they could perform operations including addition, subtraction, multiplication, division and other mathematical operations. They are still in common usage.

Operational amplifiers are capable of producing very high output potentials compared to the difference between the potential at the input terminals. They can multiply that difference by hundreds of thousands of times, providing very high voltage gain for the applications in which they are used.

Operational amplifiers are among the most commonly used integrated circuits. Their function gives them enormous flexibility and, because they can so drastically amplify the input they receive, they can be used in a huge range of different applications.

 

Origin of the Name and Component

Operational amplifiers were first developed for use in telecom applications. It was not until later that they began to be employed in analogue computers. The term operational amplifier does not refer to electrical operation, but to the ability of these components to be used to carry out mathematical operations.

Broken out of the integrated circuit housing, and operational amplifier would be revealed to contain a great many transistors and other components. Because these are sold as integrated circuits, however, it’s more convenient to address them as the components they are typically sold as and not to concentrate on the specifics of the internal circuitry itself.

 

Construction

Operational amplifiers are very complex devices, consisting of a number of transistors, diodes and other components all connected together to provide the function of the device. Most operational amplifiers, however, are sold as integrated circuits and are very compact devices and are quite inexpensive.

These devices consist, in most cases, of five terminals. The positive terminal is referred to as the non-inverting input. The negative terminal is referred to as the inverting input. There will be an additional pair of terminals, positive and negative, which connect to the power supply for the device. There is typically only one output on these devices.

Operational amplifier components may have a different number of connections on them, but the five connection arrangement is very common. The device can function as a comparator and can take on other roles, depending upon how the inputs and outputs are connected to the circuit.

 

Schematic Symbol

The schematic symbol for an operational amplifier can vary, but typically follows a basic design. Most schematics will show operational amplifiers represented by a triangle. On the vertical side of the triangle, the noninverting input and inverting input are indicated. Power supply terminals are indicated on the top and bottom of the symbol, with the notation Vs+ and Vs- being used to differentiate between the positive and negative terminals. The output is a single line extending from the end of the triangle, labelled Vout.

 

Source: http://en.wikipedia.org/wiki/File:Op-amp_symbol.svg

 

In some cases, the power terminals are eliminated from the schematic symbol, simply for the purpose of clarity. The standalone symbol, however, will typically include them.

 

Configurations in Circuits

An operational amplifier amplifies the difference in the voltages between its noninverting and inverting inputs. This number is referred to as the differential input voltage.

These devices can be utilized with open or closed loops. In an open loop configuration, there is no feedback loop running from the output of the device back into the input.

When set up in an open loop configuration, an operational amplifier can produce a very high output voltage. This output voltage can approach the same voltage as the power supply running to the device. In situations where the output voltage actually exceeds the power supply voltage being fed to the component, the condition is said to be saturated.

In this configuration, the operational amplifier can be utilized as a comparator.

In some applications, the operational amplifier is set up as a closed loop device. This requires that the output voltage is partially fed back into the inverting input on the operational amplifier.

This provides more predictable performance from the device. It also has the effect of reducing the amount of gain that the amplifier is capable of producing. This configuration is also utilized on analogue computers to perform certain mathematical functions.

Some operational amplifiers require a split power supply, which will be indicated by the ± symbol on the schematic. Those operational amplifiers that can operate without a split voltage power supply, however, will have one of their terminals indicated as the ground terminal rather than being indicated as the V- terminal.

The term differential used in reference to these components refers to the fact that the output voltage of the device is dependent upon the difference between the voltages on the two input terminals. In most cases, the power supply dictates what the maximum voltage of the device will be. It will typically be slightly less than the amount of power that the component is receiving.

Polarity plays a role in the output voltage, as well. The output of the amplifier can be positive or negative depending upon whether the positive input is greater or less than the negative input. If the positive input is greater than the negative input, a positive voltage will be output from the device. If the negative input is greater than the positive input, a negative voltage will be output from the device.

 

How They’re Used

Operational amplifiers were devised, as was mentioned, for use in telecom applications. The problem they were designed to solve was amplifying signals that were transmitted over long distances.

The concept behind what an operational amplifier does is relatively simple. To solve the problem of coming up with an amplifier with adequate amplification properties to accommodate loss of signal, an engineer came up with the idea of producing an amplifier that offered far more amplification than was actually needed to carry the signal. Rather than trying to boost this amplifier, the amplifier’s output is typically reduced by the addition of a closed loop.

In effect, the operational amplifier can put out far more amplification than is required, but it is more convenient to reduce the amplification by the introduction of a closed loop into the circuit than it is to try to continually amplify a weaker signal.

These inverting amplifiers typically take the input signal, run it through a resistor and then into the negative input on the operational amplifier. The signal from the output is then rerouted through a second resistor and into the negative input on the operational amplifier once more.

These different arrangements of input and output can also be used to give the amplifier its operational capabilities, including differentiators, integrators, and other circuit designs that allow the mathematical operations for which this component is named to be carried out. These different arrangements are designed by adding capacitors and resistors to the circuit, using feedback and other techniques to facilitate completing very complex mathematical operations with an analogue device.

Understanding Ideal vs. Real Operational Amplifiers and Specifications

Ideal operational amplifiers are often used in calculations. In reality, however, operational amplifiers are subject to limitations. Ideal operational amplifiers have specific characteristics that are not mirrored in real life.

An ideal operational amplifier would have infinite gain potential in an open loop configuration. In actual operation, infinite gain is impossible, but the gain can be very large. The limitation on the open loop gain is dictated by the amount of power the device is actually receiving. In most cases, this will be around 1 V less than the amount of power the component is receiving.

Another characteristic of an ideal operational amplifier is infinite input impotence. This refers to the resistance to current of the two input terminals being infinite, which would prevent any current at all from flowing through the device. In real life applications, however, every operational amplifier does have some current leakage through the inputs. This usually amounts to a very small amount of current, however.

Ideal operational amplifiers are also typically described as having zero resistance on their output voltage. This is also not reflective of real-world situations, where there is always some resistance in the terminal.

Bandwidth is utilized in AC circuits and, in the specifications for an operational amplifier, it refers to the frequencies that the operational amplifier can actually work with. In ideal operational amplifiers, this is considered to be infinite. In real-world applications, however, there is typically a ceiling on how many megahertz the device can actually handle and perform reliably.

Offset voltage refers to the output of the component when an equal amount of voltage is applied to both inputs. In an ideal component, this would be zero. An actual operational amplifier, however, always puts out a slight amount of voltage. This is the case even if both terminals are connected to ground.

Another important specification on these components is the typical slew rate. When the component reaches its slew rate, the output signal stays at a fixed rate of change. Increasing the input signal will result in no further change in the output signal. This figure is given in voltage over time, ranging from less than 1 V per microsecond to 13,000 V per microsecond.

 

Other Considerations

As is the case with all integrated circuits, there are some very basic and common considerations that need to be taken into account with operational amplifiers. The performance characteristics of an operational  amplifier can change, depending upon increases or decreases in temperature. Components are sold with minimum and maximum operating temperatures to ensure that the operating specifications can be relied upon in the conditions in which the amplifier is used.

All amplifiers also produce noise. This noise is simply voltage that escapes from the device when there isn’t any signal being input to it; the offset voltage discussed above. This may not be a consideration in some circuits, but if an amplifier is being used to produce a great deal of gain in a signal or if that signal happens to be at a very high bandwidth, noise could become a significant factor in how the circuit operates.

These components come in both through hole and surface mount designs and in pin counts of as low as the basic five all the way up to pin counts higher than 40. Operational amplifiers can be purchased with anywhere from 1 to 4 different channels and with output types that includes CMOS, Current, Differential, NPN, Rail to Rail and others.

The low cost of these components means that they quite often are sold in high quantities.

Component suppliers have many different options in terms of types, including broadband, audio, bipolar, CMOS, feedback, crossover and many other types of amplifiers. Amplifiers that meet exacting specifications can also be purchased, such as very low noise amplifiers, high fidelity amplifiers and so forth, allowing for predictable performance in very demanding applications.

 

Purchasing

These components are generally sold in enough quantity that supplying a workshop is not an expensive investment. The capacity of these amplifiers to so greatly increase a signal makes them very useful components in industrial electronics, high end audio applications and many other applications.

These components are available in very small sizes, as well, making them suitable for very compact builds. There are also highly specialized models of these amplifiers that cost considerably more than their less sophisticated counterparts and that are usually available in smaller quantities. These components are available alone and, in some cases, they are actually included within other integrated circuits.

While these are available in very precise specifications, they are also very durable components, are not particularly vulnerable to heat and, when combined, can be made into more durable versions of amplifiers that can be used with very high voltages, such as in an instrumentation amplifier.

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