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.
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. 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. The equipment side winding of the transformer does not need to be so carefully balanced. In fact, one leg of the equipment side can be earthed without effecting the balance on the line as shown in figure 5. With transformers the sending and receiving circuitry can be entirely unbalanced with the transformer providing the balancing.
Active balance Active balance is achieved using differential amplifiers at each end of the line. An
op-amp implementation of this is shown in figure 6, other circuitry is possible. Unlike transformer balance, there is no isolation of the circuitry from the line. Each of the two wires is driven by an op amp circuit which are identical except that one is inverting and one is non-inverting. Each one produces an asymmetrical signal individually but together they drive the line with a symmetrical signal. The
output impedance of each amp is equal so the impedance balance of the line is maintained. While it is not possible to create an isolated drive with op-amp circuitry alone, it is possible to create a floating output. This is important if one leg of the line might become grounded or connected to some other voltage reference. Grounding one leg of the line in the circuit of figure 6 will result in the line voltage being halved since only one op-amp is now providing signal. To achieve a floating output additional feedback paths are required between the two op-amps resulting in a more complex circuit than figure 6, but still avoiding the expense of a transformer. A floating op-amp output can only float within the limits of the op-amp's supply rails. An isolated output can be achieved without transformers with the addition of
opto-isolators.
Impedance balance As noted above, it is possible to drive a balanced line with a single-ended signal and still maintain the line balance. This is represented in outline in figure 7. The amplifier driving one leg of the line through a
resistor is assumed to be an ideal (that is, zero output impedance) single-ended output amp. The other leg is connected from ground through another resistor of the same value. The impedance to ground of both legs is the same and the line remains balanced. The receiving amplifier still rejects any common-mode noise as it has a differential input. On the other hand, the line signal is not symmetrical. The voltages at the input to the two legs,
V+ and
V− are given by; :V_+ = V_\mathrm {in} \frac{Z_\mathrm {in}+R_1}{Z_\mathrm {in}+2R_1} :V_- = V_\mathrm {in} \frac{R_1}{Z_\mathrm {in}+2R_1} 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. ==Balanced to unbalanced interfacing==