The operational amplifier is an electronic integrated circuit with multistage amplifying circuit. Its input stage is a differential amplifying circuit, which has high input resistance and the ability to suppress zero drift. The intermediate stage mainly carries on the voltage amplification, has the high voltage amplification factor, is generally composed of the common emitter amplifier circuit; The output pole is connected with the load, which has the characteristics of strong load capacity and low output resistance. Operational amplifiers are widely used.

An op-amp has two inputs a (inverting input), b (in-phase input), and an output o. Also known as inverting input and non-inverting input and output, respectively. When the voltage U- is applied to the a terminal and the common terminal (the common terminal is the point at which the voltage is zero, it corresponds to the reference node in the circuit.) When the actual direction of the output voltage is higher from the a end than the common end, the actual direction of the output voltage U is from the common end to the o end, that is, the directions of the two are opposite. When the input voltage U+ is added between the b terminal and the common terminal, the actual direction of U and U+ is exactly the same as that of the common terminal. For the sake of distinction, terminals a and b are marked with "-" and "+" respectively, but do not mistake them for positive and negative polarity of the voltage reference direction.