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differential amplifier[‚dif·ə′ren·chəl ′am·plə‚fī·ər]
An electronic circuit that is designed to amplify the difference between two voltages measured with respect to a common reference, usually designated as ground. By convention, the net difference of two voltages measured with respect to a common reference is called the differential-mode voltage, while the sum of the voltages, usually divided by two to give an average value, is called the common-mode voltage.
An ideal differential amplifier thus has exactly the same gain from each input to its output, and the amplifier produces an output that is directly proportional to its differential-mode voltage. The amplifier delivers zero output in response to common-mode voltages. If these gains are not exactly equal, then equal (common-mode) voltages applied at each input terminal will not be equal at the amplifier output and their difference will not cancel completely. The common-mode gain, the ratio of the output response of a real differential amplifier to the input signal applied equally to each input terminal, is a measure of this gain mismatch.
Differential amplification is very useful when the signal to be amplified exists in an electrically noisy environment, since the noise voltage is usually a common component of both input voltages and, hence, will cancel when the difference of the amplifier inputs is taken. See Amplifier
For a physical differential amplifier to work properly, the electrical paths of each input signal through the amplifier must be nearly identical. Thus, the most important requirement for a differential amplifier is that it be constructed with transistors with closely matched electrical characteristics. Integrated circuits with amplifier transistors physically close to each other meet the required close matching requirement and are ideally suited for the production of differential amplifiers. See Integrated circuits
Differential-amplifier circuits that are suitable for integrated-circuit fabrication can use either metal oxide semiconductor field-effect transistors (MOSFETs) or bipolar junction transistors (BJTs). The input transistor pair must be matched closely. For best performance, the two load transistors also should be matched. See Transistor