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Revision:Operational Amplifier

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Physics > Operational Amplifier

Image:U741.jpg
The classic 741 operational amplifier in an 8-pin DIP package.

The Operational Amplifier (colloquially 'Op Amp') is one of the most useful and widely used components in electronic devices today. Originally used to perform mathematical operations on analogue electrical signals, it was the prime building-block in the world's first real analogue electronic computers.

Contents


The Ideal Operational Amplifier

Image:OpAmpSymbol.png
BS3939 Symbol for an Operational Amplifier.

The Operational Amplifier is essentially a form of voltage operated differential amplifier. It looks at the difference between the voltages applied to its two separate inputs, multiplies this difference by a factor of gain, and then puts this resultant voltage onto it's output. We can express this algebraically as shown below:


V_{out} = (V_{+} - V_{-}) \times \alpha</dd>


where \alpha is the gain factor, and V_{+} and V_{-} are the voltages applied to the non-inverting input (labelled +) and inverting input (labelled -) respectively.


The ideal (theoretical) operational amplifier has an infinitely large gain factor \alpha. This means that the tiniest different in voltage between the two inputs results in an infinitely large output voltage. In reality, an infinite gain factor is impossible to achieve, and so real Op Amps have very very large gain factors, typically of around 100,000. This may seem to make the real-world Op Amp sound useless, since even a 0.0001 Volt difference between the two inputs will result in the output being pushed as high or as low as it can possibly go (limited by the supply rails). This problem can be solved, however, by introducing negative feedback, as detailed in the later section.

The input impedances of the ideal operational amplifier are also infinite. This means that the ideal Op Amp draws zero current from the circuits on its inputs, hence having no effect on the behaviour of the circuit. Again, in reality, this is not quite the case. A modern real-world operational amplifier, will have typical input impedances of around 10,000,000 Ohms. This means that some, albeit a very small, amount of current is drawn by the Op Amps inputs, and in most circuits this presents no problem, but in certain situations, this will affect operation and will need to be accounted for.


Negative Feedback

Applications

The Comparator

The Non-inverting Amplifier

The Inverting Amplifier

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