In electrical engineering, an unbalanced circuit is one in which the transmission properties between the

ports A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ha ...

of the circuit are different for the two poles of each port. It is usually taken to mean that one pole of each port is bonded to a common potential (

single-ended signalling Single-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires. One wire carries a varying voltage that represents the signal, while the other wire is connected to a reference voltage, usually ...

) but more complex topologies are possible. This common point is commonly called ''ground'' or ''earth'' but it may well not actually be connected to

electrical ground In electrical engineering, ground or earth is a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth. Electrical circuits may be conne ...

at all. Unbalanced circuits are to be contrasted to balanced circuits where the transmission paths are impedance balanced (the impedances are identical).

Examples

Passive filter

The figure shows two versions of a simple

low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...

, unbalanced version (A) and balanced version (B). Both circuits have exactly the same effect as filters, they have the same

transfer function In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a mathematical function that theoretically models the system's output for each possible input. They are widely used ...

. However, on the unbalanced circuit, the bottom pole of the input port is connected directly to the bottom pole of the output port. Thus, the impedance between the top poles is greater than the impedance between the bottom poles from input to output. For a circuit to be balanced the impedance of the top leg must be the same as the impedance of the bottom leg so that the transmission paths are identical. To achieve this, the

inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a ...

in the balanced version is split into two equal inductors, each with half the original

inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of t ...

Tuned amplifier

The figure shows the circuit of a typical tuned amplifier. The lower pole of the input port is connected directly to the lower pole of the output port. This connection also forms the negative rail of the supply voltage. This scheme is typical of many electronic circuits that are not required to have differential inputs or outputs. An example of a circuit that does not follow this pattern is the

differential amplifier A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...

Advantages and disadvantages

The basic advantage of using an unbalanced circuit topology, as compared to an equivalent balanced circuit, is that far fewer components are required. The difficulties come when a port of the circuit is to be connected to a

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

or to an external device that's designed for balanced operation. Many transmission lines are intrinsically an unbalanced format such as the widely used

coaxial cable Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a ...

. In such cases the circuit can be directly connected to the line. However, connecting an unbalanced circuit to, for instance, a

twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...

line, which is an intrinsically balanced format, makes the line susceptible to

common-mode interference Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage. In most electrical circuits the signal is transferr ...

. For this reason,

balanced line In telecommunications and professional audio, a balanced line or balanced signal pair is a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other ci ...

s are normally driven from

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 t ...

s. One option is to redesign the circuit so that it is properly impedance balanced. If that is not possible or desirable, a

balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...

, a device for interfacing balanced and unbalanced circuits, may be used.

References

*Don Davis, Eugene Patronis, ''Sound System Engineering'', p. 433, CRC Press, 2014 . *Douglas Self, ''Audio Power Amplifier Design'', pp. 649-654, Taylor & Francis, 2013 . *R.S. Sedha, ''A Textbook of Electronic Circuits'', p. 627, S. Chand, 2008 {{ISBN, 8121928036. category:Electronic circuits