INTERNAL CIRCUIT :
The operational amplifier is a direct-coupled high gain amplifier usable from 0 to over 1MHz to which feedback is added to control its overall response characteristic i.e. gain and bandwidth. The op -amp exhibits the gain down to zero frequency. The internal block diagram of an op-amp is shown in the fig The input stage is the dual input balanced output differential amplifier. This stage generally provides most of the voltage gain of the amplifier and also establishes the input resistance of the op-amp. The intermediate stage is usually another differential amplifier, which is driven by the output of the first stage. On most amplifiers, the intermediate stage is dual input, unbalanced output. Because of direct coupling, the dc voltage at the output of the intermediate stage is well above ground potential. Therefore, the level translator (shifting) circuit is used after the intermediate stage downwards to zero volts with respect to ground. The final stage is usually a push pull complementary symmetry amplifier output stage. The output stage increases the voltage swing and raises the ground supplying capabilities of the op-amp. A well designed output stage also provides low output resistance.
