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Operational Amplifier / Op Amp Basics

Operational amplifiers are one of the workhorses of the analogue electronics scene. Op-amps, as they are also known are widely available in the form of integrated circuits, many costing only a few cents or a few pence for the standard versions. High performance op amp integrated circuits still offer excellent value for money, but obviously cost a little more. In view of their ease of use and low cost, these integrated circuits are used in vast quantities enabling high performance electronics circuits to be developed and designed with a minimum of electronics components.

Operational amplifiers, op amps are virtually the ideal amplifier. They provide a combination of parameters that are of great use:

  1. Very high gain

  2. Very high input impedance

  3. Very low output impedance

The operational amplifier is what is known as a differential amplifier. The differential amplifier has two inputs and this enables it to be used in a wide number of circuit configurations.


Op-amp basics

The circuit symbol for an operational amplifier consists simply of a triangle as shown below. The two inputs are designated by "+" and "-" symbols, and the output of the operational amplifier is at the opposite end of the triangle. Inputs from the "+" input appear at the output in the same phase, whereas signals present at the "-" input appear at the output inverted or 180 degrees out of phase. This gives rise to the names for the inputs. The "+" input is known as the non-inverting input, while the "-" input is the inverting input of the operational amplifier. As the output from the amplifier is dependent upon the difference in voltage between the two inputs, it is known as a differential amplifier.

Operational amplifier / op-amp circuit symbol
Operational amplifier / op-amp circuit symbol

Often the power supply rails for the operational amplifier are not shown in circuit diagrams and there is no connection for a ground line. The power rails for the operational amplifier are assumed to be connected. The power for the operational amplifier is generally supplied as a positive rail and also a negative rail. Often voltages of +15V and -15 V are used, although this will vary according to the application and the actual chip used.

The gain of the operational amplifier is very high. Figures for the levels of gain provided by an operational amplifier on its own are very high. Typically they may be upwards of 10 000.

While levels of gain may be too high for use on their own, the application of feedback around the operational amplifier enables the circuit to be used in a wide variety of applications, from very flat amplifiers, to filters, oscillators, switches, and much more.


Op-amp open loop gain

The gain of an operational amplifier is exceedingly high. Normally feedback is applied around the op-amp so that the gain of the overall circuit is defined and kept to a figure which is more usable. However the very high level of gain of the op-amp enables considerable levels of feedback to be applied to enable the required performance to be achieved.

When measured the open loop gain of an operational amplifier falls very rapidly with increasing frequency. Typically an op-amp may have an open loop gain of around 100 000, but this usually starts to fall very quickly. For the famous 741 operational amplifier, it starts to fall at a frequency of only 10 Hz.


Op-amp slew rate

With very high gains the operational amplifiers have what is termed compensation capacitance to prevent oscillation. This capacitance combined with the limited drive currents mean that the output of the amplifier is only able to change at a limited rate, even when a large or rapid change occurs at the input. This maximum speed is known as the slew rate. A typical general purpose device may have a slew rate of 10 V / microsecond. This means that when a large step change is placed on the input, the device would be able to provide an output 10 volt change in one microsecond.

The figures for slew rate change are dependent upon the type of operational amplifier being used. Low power op-amps may only have a slew rate of a volt per microsecond, whereas there are fast operational amplifiers capable to providing slew rates of 1000 V / microsecond.

The slew rate can introduce distortion onto a signal by limiting the frequency of a large signal that can be accommodated. It is possible to find the maximum frequency or voltage that can be accommodated. A sine wave with a frequency of f Hertz and amplitude V volts requires an operational amplifier with a slew rate of 2 x pi x V x V volts per second.


Offset null

One of the minor problems with an operational amplifier is that they have a small offset. Normally this is small, but it is quoted in the datasheets for the particular operational amplifier in question. It is possible to null this using an external potentiometer connected to the three offset null pins.

In most applications no provision is made for the offset null. This is because it is normally not a problem and it adds further components and an adjustment. However it can be a problem where large levels of DC gain are required. Here the offset voltage is amplified by the gain and could appear as a significant voltage at the output.


Op-amp circuits

There is a huge variety of electronics circuits that can be designed using operational amplifiers. These opamp circuits range from amplifiers to multivibrators, integrators to comparators and oscillators to timers. It shows how versatile these op-amp integrated circuits are. All these different circuits tend to use the high input impedance, low output impedance and high gain of the op-amp, combined with the fact that the operational amplifier is a differential amplifier. In most circuits, feedback (usually negative feedback) is placed around the op-amp and the way this is done provides the required function. Only in very few applications is no feedback applied.


Operational amplifier packages

The packages in which electronics components are available is very important. Many electronics components are available in a wide variety of package styles, and the operational amplifier is not exception. Like many other electronics components, a vast number are used as surface mount components in mass produced electronics products. They are available in the SOIC (small outline integrated circuit) package as well as many others. Some are even available in five leaded versions of transistor packages and this makes them ideal to drop into a circuit without using up much board space. However the more traditional style of electronics component packages are also available. They are available in the DIL (dual in line) package, often as a single operational amplifier in an eight pin DIL, or duals in eight pin packages (with no offset null connections) or fourteen or sixteen pin DILs.


Summary

Operational amplifiers are in widespread use in analogue electronics design and production. These op amps provide a particularly useful combination of circuit parameters that make them an indispensable tool for the electronics design engineer. While digital electronics is growing, the use of op-amps will nevertheless remain in vast quantities as a result of their cost, performance and ease of use. These electronics components will therefore remain very cheap for many years to come.

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Key Words : Amplifier, Impedance, slew rate, SOIC,

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