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balanced circuit
A balanced circuit is circuitry for use with a balanced line or the balanced line itself. Balanced lines are a common method of transmitting many types of electrical communication signals between two points on two wires. In a balanced line the two signal lines are of a matched impedance to help ensure that interference induced in the line is common-mode and can be removed at the receiving end by circuitry with good common-mode rejection. To maintain the balance, circuit blocks which interface to the line, or are connected in the line, must also be balanced.
Balanced lines work because the interfering noise from the surrounding environment is induced into both wires equally. By measuring the difference between the two wires at the receiving end, the original signal is recovered while the noise is cancelled. Any inequality in the noise induced in each wire is an imbalance and will result in the noise not being fully cancelled. One requirement for balance is that both wires are an equal distance from the noise source. This is often achieved by placing the wires as close together as possible and twisting them together. Another requirement is that the impedance to ground (or to whichever reference point is being used by the difference detector) is the same for both conductors at all points along the length of the line. If one wire has a higher impedance to ground it will tend to have a higher noise induced, destroying the balance.
==Balance and symmetry==
A balanced circuit will normally show a symmetry of its components about a horizontal line mid-way between the two conductors (example in figure 3). This is different from what is normally meant by a symmetrical circuit which is a circuit showing symmetry of its components about a vertical line at its midpoint. An example of a symmetrical circuit is shown in figure 2. Circuits designed for use with balanced lines will often be designed to be both balanced and symmetrical as shown in figure 4. The advantages of symmetry are that the same impedance is presented at both ports and that the circuit has the same effect on signals travelling in both directions on the line.
Balance and symmetry are usually associated with reflected horizontal and vertical physical symmetry respectively as shown in figures 1 to 4. However, physical symmetry is not a necessary requirement for these conditions. It is only necessary that the electrical impedances are symmetrical. It is possible to design circuits that are not physically symmetrical but which have equivalent impedances which are symmetrical.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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