Operational Amplifier

The standard operational amplifier (op-amp) symbol is shown in figure(a).It has two input terminals,the inverting input(-) and the noninverting input(+) and an output terminal.The typical op-amp operates with two dc supply voltages,one positive and the other negative as shown in figure(b).Usually these dcvoltage terminals are left off the schematic symbol for simplicity but are always understood to be there.Some typical op-amp IC packages are shown in figure(c).

The Ideal Op-Amp:

To illustrate what an op-amp is.Let's consider its ideal characteristics.A practical op-amp of course,falls short of these ideal standards,but it is much easier to understand and analyze the device from an ideal point of view.

First,the ideal op-amp has infinite voltage gain and infinite bandwidth.Also it has an infinite input impedence(open),so that it does not load the driving source.Finally,it has a zero output impedence.These characteristics are illustrated in figure.The input voltage Vin appears between the two input terminals and the output voltage is AvVin as indicated by the internal voltage source symbol.The concept of infinite input impedence is a particularly valuable analysis tool for the various op-amp configuration,which will be discussed.

The practical Op-Amp:

Although modern integrated circuit (IC) op-amps approach parameter values that can be treated as ideal in many cases,the ideal device can never be made.

Any device has limitations,peak-to-peak output voltage,for example,is usually limited to slightly lass than the two supply voltages.Output current is also limited by internal restrictions such as power dissipation and component ratings.

Characteristics of a practical op-amp are very high voltage gain,very high input impedence,very low output impedence and wide bandwidth.Three of these are lebelled in figure:

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