The operational amplifier is perhaps the most versatile of amplifier circuits, used many different applications as a gain component due to high stability, gain and input impedance, as well as the fact that very little external components are needed for operation.
Internally, the OpAmp is based around a transistorized differential amplifier; two transistors connected to the same emitter resistor, where one of the inputs is inverted and added to the other to essentially subtract one from the other, the difference amplified by a certain factor and fed as the output.
The basic opamp is a simple differential amplifier. Most commercially available opamps have extra internal circuitry to compensate for temperature change, different voltage source values and compensation to get an exact 0v when both inputs are disconnected.
There are two characteristics that make opamps so versatile: The voltage difference across its inputs will be very close to 0v, and its inputs draw virtually no current. This characteristics are only valid only under Negative Feedback.
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