Operational Amplifier / Op Amp Tutorial

- the operational amplifier, or op amp is an easy to use analogue circuit building block that can be used within many circuits from amplifiers to filters, and integrators to oscillators . . .

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.

A photo showing a selection of op amps in different packages - DIL and SMT

Operational amplifiers 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 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. As the output from the amplifier is dependent upon the difference in voltage between the two inputs, it is known as a differential amplifier.

The circuit symbol used to denote an operational amplifier, or op amp
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 - often the power rails are assumed to be connected. The power is generally supplied as a positive rail and also a negative rail. Often voltages of +15 V 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. While levels of gain may be too high for use on their own, the fact that negative feedback with various characteristics can be used enables them to form the basis of many circuits from very flat amplifiers, to filters, oscillators, switches, and much more.

Gain

The open loop gain of an operational amplifier is exceedingly high - this is the gain when no feedback is applied round the operational amplifier. Gain values may be of the order of 10 000 or higher.

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 enables considerable levels of feedback to be applied to enable the required performance to be achieved.

The gain of an op amp circuit is important in terms of both the open and closed loop scenarios. Read more about op-amp gain.

Bandwidth

Another important basic aspect of op amp circuits is the bandwidth. Many op amp circuits provide the high bandwidths, others may be limited to audio frequencies or less.

While the open loop gain may fall at a very low frequency - the famous 741 has an open loop break point of just 10 Hz, but with feedback controlling the overall voltage gain, much higher bandwidths are achievable. Read more about op-amp bandwidth & frequency response.

Slew rate

The slew rate of a circuit is the maximum rate of change the output can achieve. For example if a large voltage change is required, then the output of the circuit may not be able to deliver the rate of change to move from one voltage to the next in the time available.

The slew rate may be a limiting factor even if the voltage change is within the frequency bandwidth of the circuit. The limiting factor is not the frequency, but the rate of change of voltage for the output, and it obviously becomes more of an issue when for large output levels. The gain of an op amp circuit is important in terms of both the open and closed loop scenarios. Read more about op-amp slew rate.

Offset null

The offset null capability is available in many op amp chips. It is used to remove or significantly reduce the small DC offsets that occur.

The offset null is important in circuits where DC coupling is required and where the absolute DC levels are of importance. The small DC offsets that occur in op amps are amplified by the levels of gain of the circuit, and they can create problems for some DC applications. Read more about op-amp offset null.

Op-amp circuits

There is a huge variety of electronic 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.

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.

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.

By Ian Poole


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