Operational amplifier

Operational amplifiers (op-amps) are high-gain DC coupled amplifiers with two inputs and a single output, and have been used as comparators, audio amplifiers, filters, etc. An operational amplifier is characterized by having two inputs, that is, an inverting or negative input and a non-inverting or positive input. The operational amplifier includes an output, that is, a single-ended amplifier, or two outputs, that is, a double-ended amplifier which is also known as a fully differential amplifier. Operational amplifiers are used in many electronic circuits to condition, manipulate and amplify signals. An typical operational amplifier amplifies a voltage difference on the inputs to generate a desired output voltage. The operating characteristics of a particular operational amplifier are dependent upon its circuit topology. Generally, the operational amplifier consists of a number of stages, each containing internal sub-stages. The operating class of the operational amplifiers is defined according to the polarization of the active elements that supply power to the load and can be divided, among the various ones existing, into class A, in which the active elements always operate in a conduction zone and are polarized at about the center of it, class B, in which the active elements are polarized at the locking limit of the conduction zone, class AB, in which the active elements are weakly polarized within the conduction zone, and in class C, in which the active elements operate far from the conduction zone. Single stage differential amplifier circuits are used in many electronic applications, such as programmable logic arrays. For programmable logic arrays differential amplifier circuits are designed to vary the common-mode gain and common-mode rejection ratio utilizing more than one amplifier stage and/or with additional complex electronic circuitry. Two-stage operational amplifiers typically include a first gain stage connected to inputs of the amplifier and a second gain stage driven by the first gain stage. The second gain stage provides the output of the amplifier. Both the first gain stage and the second gain stage are operated at respective bias currents. In metal oxide semiconductor (MOS) amplifiers, the first gain stage is typically operated at bias currents which are comparable in magnitude to the bias currents of the second gain stage so that maximum gain and bandwidth may be achieved. Multiple-stage operational amplifiers typically include a cascade of one or more gain stages and an output driver stage for driving an output load. The output stage is, for example, a Class AB amplifier that provides high low-frequency gain. To achieve an overall high open loop gain (e.g. greater than 150 dB), a multiple-stage opamp normally requires three or more gain stages.