Operational Amplifier (Op-Amp)

- a tutorial, overview or summary of the basics of the operational amplifier or op-amp, widely used in analogue electronics.

Integrated circuits, ICs have made a huge impact on the electronics scene. Within the analogue electronics arena, none has made more difference than the operational amplifier, or op-amp. The op-amp enables a very high performance amplifier to be made with the addition of just a handful of other components. The operational amplifier can also form the basis of a host of other circuits ranging from filters to timers, and oscillators to comparators and astables. As such the operational amplifier is one of the most versatile building blocks available to the analogue engineer and hobbyist.

Although the term operational amplifier has now become totally integrated into today's electronics terminology, it may not be realised that it dates back to a paper published in 1947. This described work that was undertaken using these amplifiers in analogue computers of the day. However it was not until the 1960s that the concept of these amplifiers could be fully realised with the widespread introduction of integrated circuit technology. With the improvement in integrated circuit technologies in the 1970s, the use of op-amps increased further and the rest as they say is history! Now operational amplifiers have become a fundamental building block used throughout the electronics industry. Even though they have been around for some time, there seems to be little likelihood of their use falling.

What is an Op-Amp?
An operational amplifier is a very close approximation to a perfect amplifier with an infinite gain. In reality op-amps do not quite attain perfection, but with gains often in the region of 100 000 or more, they are sufficiently close!

The operational amplifier has two inputs. One is called the inverting input and is marked with a "-" sign on circuit schematic diagrams. The other is the non-inverting input and this is marked with a "+" sign.

The two inputs gain their names from the way in which they amplify the signals:

• Non-inverting input:   It is found that a positive voltage applied to the non-inverting input will produce a positive swing at the output.



• Inverting input:   A positive voltage applied to the inverting input will produce a negative swing at the output.

If the same voltage is applied to both inputs together then there should be no change at the output. In fact the output is proportional to the difference between the inverting and non-inverting inputs. It is for this reason that these amplifiers are often called differential amplifiers.

Like any electronics circuit, those using operational amplifiers need to have a power supply. Normally op-amps are supplied using dual, i.e. positive and negative supplies. Additionally the supply lines are often not shown as they add confusion to the circuit diagram.

In most cases the operational amplifier will only need five connections for its operation - inverting, non-inverting, output and the two power rails. Very occasionally a further three may be used. These are usually for the "offset null" capability. This is used to reduce any DC offsets that may be present, and for most applications these can be ignored and left disconnected.

Operational amplifier characteristics
Apart from the very high gains there are a number of other attributes expected of operational Their input resistance should be infinite, and the op-amps in use today come very close to this with impedances anywhere from about a quarter of a Megohm upwards. Some using MOSFET input stages have an impedance of hundreds of Megohms.

The output resistance is also important. As may be expected this should be low. In the ideal amplifier this should be zero, but in reality many amplifiers have an output impedance of less than a hundred ohms, and many very much less than this.

Another important feature of the op-amp is its common mode rejection. This refers to the situation where the same signal is applied to both inputs. In an ideal amplifier no output should be seen at the output under these circumstances, however the amplifier will never be perfect. The actual common mode rejection ratio is the ratio between the output level when the signal is applied to both inputs compared to the output when it is applied to just one.

The bandwidth of an op-amp can vary quite widely. An ideal amplifier would have an infinite bandwidth but as one may imagine this would be impossible create, and also very difficult to use and tame in practise. In reality op-amps have a limited bandwidth. Many of the chips used for audio applications may only exhibit their full gain over a relatively small bandwidth, after this the gain falls. Despite this most circuits act to reduce the gain, and enable this smaller level of gain to be maintained over a larger bandwidth.

Op-amp summary
The operational amplifier is a very useful building block for analogue electronics. In view of the widespread use, chips are very cheap and can be used for a wide variety of functions.