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In electrical engineering, an unbalanced circuit is one in which the transmission properties between the
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of the circuit are different for the two
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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 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 function (mathematics), mathematical function that mathematical model, models the system's output for each possible ...
. 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 Passivity (engineering), passive two-terminal electronic component, electrical component that stores energy in a magnetic field when an electric current flows through it. An inductor typic ...
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 electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
.


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 outp ...
.


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 transmis ...
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 Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
. 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 communications cable 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 balanced ...
line, which is an intrinsically balanced format, makes the line susceptible to common-mode interference. For this reason,
balanced line In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other c ...
s are normally driven from
balanced circuit 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 Signal transmission, transmitting many types of electrical Signal (electronics ...
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 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