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Differential amplifier

Differential amplifiers are well-known electronic devices for amplifying a voltage difference between two input signals. Differential amplifiers are utilized to amplify, and produce an output signal which is a function of the difference between two differential, or complementary, input signals and to thereby enable the detection of relatively weak signal levels while inherently rejecting noise common on the differential input lines. Differential amplifier circuits are widely used in the electronics industry and are generally preferred over their single-ended counterparts because of their better common-mode noise rejection, reduced harmonic distortion, and increased output voltage swing. Differential amplifiers are used to amplify analog, as well as digital signals, and can be used in various implementations to provide an output from the amplifier in response to differential inputs. They can be readily adapted to function as an operational amplifier, a comparator, a sense amplifier and as a front-end buffer stage for another circuit. The differential amplifier is often a building block or sub-circuit used within high-quality integrated circuit amplifiers, linear and nonlinear signal processing circuits, and even certain logic gates and digital interfacing circuits. A differential amplifier usually comprises two electrical paths hat are independently coupled to a voltage source at one end, and are together coupled to a voltage or current source at an opposite end. Each electrical path usually comprises a transistor element and a resistance element. In general, a bipolar transistor or BiCMOS transistor is used for a differential amplifier. In recent years, there has been an increasing demand for a system-on-chip configuration and reduction of power consumption, in response to which the CMOS has been widely used. CMOS differential amplifiers are constructed of a number of interconnected transistors. Some transistors are of one carrier type often called n-type Field Effect Transistors (i.e., nFETs), while other transistors are of the opposite carrier type often called p-type Field Effect Transistors (i.e., pFETs). CMOS differential amplifiers are used for various applications because a number of advantages can be derived from these types of amplifiers, as compared to single-ended amplifiers. Differential amplifiers are used where linear amplification having a minimum of distortion is desired. A fully differential amplifier circuit is a special type of amplifier that has two inputs and two outputs. This device amplifies input signals on the two input lines that are out of phase and rejects input signals that have a common phase such as induced noise. The common mode feedback is accomplished by the use of a common mode feedback circuit that monitors the two differential amplifier output lines and provides a feedback signal that adjusts the amplifier's bias current, thereby rejecting the unwanted common mode signals on the amplifier's output.


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