The operational amplifier is perhaps the most versatile of amplifier
circuits, used many different applications as a gain component due to
high stability, gain & 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 & added to the other to essentially
subtract one from the other, the difference amplified by a certain
factor & 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, & its inputs
draw virtually no current. This characteristics are only valid only
under Negative Feedback
