Differential Amplifier

# Differential Amplifier - DIFFERENTIAL AMPLIFIER The advent...

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ECE 414 1 M H Miller DIFFERENTIAL AMPLIFIER The advent of the integrated circuit led to the development of circuit configurations complementing the strengths of the technology and bypassing weaknesses. The high degree to which the characteristics of like components on the same substrate can be matched led to extensive use of symmetry as a filtering mechanism. The differential amplifier circuit configuration in particular has assumed considerable prominence in this respect. Physical and Electrical Symmetry The 'Differential Amplifier' is associated closely with integrated circuit technology; this technology provided both incentives for use of the circuit configuration as well as the means to construct effective circuits of this type. The differential amplifier is used extensively in modern monolithic electronic circuitry. The figure below illustrates the general character of a differential device. The circuit, a specific circuit configuration need not be specified for the present purpose, is assumed to be a linear circuit and to have physical mirror symmetry. The half-circuits on either side of the mirror symmetry line correspond both in topology and also in the properties of respective corresponding circuit elements. The currents I A and I B represent corresponding currents flowing within their respective half-circuits. V A and V B are corresponding voltages (relative to a common ground) in the respective half-circuits. On the other hand I a and I b are currents flowing out of one respective half-circuits, and into the other. The application of symmetry to circuit behavior is based on a philosophical assumption that nature is not maliciously perverse, and so similar circuit elements operated under similar conditions will behave similarly. Two special cases illustrate certain properties of the sort of symmetrical arrangement illustrated that are of special interest. Suppose input signals S 1 and S 2 are provided to the circuit. (The actual signal might be S 1 -S 2 , i.e., applied between the input terminals, but it is convenient here to reference each separately to a common ground point). Two cases of particular interest correspond to the inputs having certain general symmetry properties. In the one case, the 'common-mode' case, both signals are the same, i.e., S 1 =S 2 . In the other case, the 'differential-mode' case, the signals are electrically antisymmetrical, i.e., S 1 = -S 2 . Consider the common-mode case first. Because of the physical symmetry of the two half-circuits and the electrical symmetry of the input signals corresponding voltages and currents in each half circuit will be equal. Thus, for example, I a = I b , and KCL requires I a + I b = 0, i.e., both currents must be zero. A general conclusion is that there is no common-mode current flowing between the two halves even if there are physical connections between them.

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