A balanced circuit is an electrical or electronic circuitry where the signal or power is transferred symmetrical in two wires. Unlike in the unbalanced circuit where the voltage is seen in one wire and the common, the output of the balanced circuit is usually two wires with one wire output at 180 degrees phase difference from the other wire. The balanced circuit is used for high-power or high-electromagnetic interference immunity.
For better noise performance, the simplest balanced circuit is a transformer output that has a secondary winding, which is either floating or has a center tap for common connection. Most audio and telecommunication baseband applications make use of balanced circuits to convey signals between equipment cabinets. Common balanced lines are twisted pairs that may be bundled as in the 25-pair cable that is also shielded to further take advantage of the electromagnetic interference immunity.
More sophisticated balanced circuits may use a bridge-type transistor amplifier stage or dual operational amplifier output. The resulting output at idle is 0 volts alternating current (VAC), which is the same as in the transformer output. An advantage of electronic drive over transformer coupled is the decreased size and weight of the resulting circuit. The frequency response of pure electronic and transformer-less circuits is usually superior to the transformer output counterparts.
For a transformer coupled signal, although the signal pair is balanced, there could be a common-mode noise that may be induced as a common and in-phase noise voltage on both wires of the balanced circuit. This noise is rejected by a feature known as common-mode rejection, which is the ability of operational amplifiers to ignore any signal that is common in both inverting and non-inverting inputs. Ideally, if the operational amplifier inputs are shorted, any unbalanced signal input swing, within rated levels, results in 0 volts (V) operational amplifier output, and this is common-mode rejection.
Balanced circuits use miscellaneous components for better handling of electrical noise. In communication circuits, repeating coils are used for long spans of balanced lines to reduce noise such as the hum produced by electrical circuits and the transient cracking noise with lightning discharge. These devices also reduce noise from earth loop currents that are caused when alternating current (AC), which is supposed to run through the neutral wires, is mistakenly routed into ground cabling. This happens when the AC power termination has a neutral-to-ground connection in the equipment side instead of the AC mains power distribution cabinet.
