Example of 4 different circuit configurations, using a low-pass filter, to demonstrate. Fig. 1. Unbalanced, asymmetrical circuit. Fig. 2. Unbalanced, symmetrical circuit. Fig. 3. Balanced, asymmetrical circuit. Fig. 4. Balanced, symmetrical circuit.
Balanced circuit From Wikipedia, the free encyclopedia In electrical engineering, a balanced circuit is electronic circuitry for use with a balanced line, or the balanced line itself. Balanced lines are a common method of transmitting many types of electrical 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 induces equal noise voltages into both wires. By measuring the voltage difference between the two wires at the receiving end, the original signal is recovered while the noise is rejected. Any inequality in the noise induced in each wire is an imbalance and will result in the noise not being fully rejected. 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 midway 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. |
Example of 4 different circuit configurations, using a low-pass filter, to demonstrate. Fig. 1. Unbalanced, asymmetrical circuit. Fig. 2. Unbalanced, symmetrical circuit. Fig. 3. Balanced, asymmetrical circuit. Fig. 4. Balanced, symmetrical circuit. |
| There are a number of ways that a balanced line can be driven and the signal detected. In all methods, for the continued benefit of good noise immunity, it is essential that the driving and receiving circuit maintain the impedance balance of the line. It is also essential that the receiving circuit detects only differential signals and rejects common-mode signals. It is not essential (although it is often the case) that the transmitted signal is balanced, that is, symmetrical about ground. |
Fig. 5. Balanced line connected passively using transformers.
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| Transformer balance The conceptually simplest way to connect to a balanced line is through transformers at each end shown in figure 5. Transformers were the original method of making such connections in telephony, and before the advent of active circuitry were the only way. In the telephony application they are known as repeating coils. Transformers have the additional advantage of completely isolating (or "floating") the line from earth and earth loop currents, which are an undesirable possibility with other methods. |
Fig. 6. Balanced line connected to active balanced circuitry.
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| The side of the transformer facing the line, in a good quality design, will have the winding laid in two parts (often with a centre tap provided) which are carefully balanced to maintain the line balance. Line side and equipment side windings are more useful concepts than the more usual primary and secondary windings when discussing these kinds of transformers. At the sending end the line side winding is the secondary, but at the receiving end the line side winding is the primary. When discussing a two-wire circuit primary and secondary cease to have any meaning at all, since signals are flowing in both directions at once. |
Fig. 7. Balanced line actively driven with an asymmetrical signal, but connected
to balanced impedances.
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Where Zin is the input impedance of the line. These are clearly not
symmetrical since V− is much smaller than V+. They are not even opposite
polarities. In audio applications V− is usually so small it can be taken
as zero.[8]