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A balun / ˈ b æ l ʌ n / (from "balanced to unbalanced", originally, but now derived from "balancing unit") [1] is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. [2] A balun can take many forms and may include devices that also transform impedances ...
A circuit that has the specific purpose of allowing interfacing between balanced and unbalanced circuits is called a balun. A balun could be a transformer with one leg earthed on the unbalanced side as described in the transformer balance section above. Other circuits are possible such as autotransformers or active circuits. [4]
Practical impedance-matching devices will generally provide best results over a specified frequency band. The concept of impedance matching is widespread in electrical engineering, but is relevant in other applications in which a form of energy, not necessarily electrical, is transferred between a source and a load, such as in acoustics or optics.
The quarter wave transformer is an alternative to a stub; but, whereas a stub is terminated in a short (or open) circuit and the length is chosen so as to produce the required impedance transformation, the λ/4 transformer is in series with the load and its length and characteristic impedance are designed to produce the required impedance ...
Another application is the design of impedance matching networks. Impedance matching at a single frequency requires only a trivial network—usually one component. Impedance matching over a wide band, however, requires a more complex network, even in the case that the source and load resistances do not vary with frequency.
The mechanical analogy of a transformer is a simple machine such as a pulley or a lever. The force applied to the load can be greater or less than the input force depending on whether the mechanical advantage of the machine is greater or less than unity respectively. Mechanical advantage is analogous to transformer turns ratio in the impedance ...
Choke transformers are sometimes called transmission-line transformers (although see below for a different transformer type with the same name), or Guanella transformers, or current baluns, or line isolators. Although called a "transmission line" transformer, it is distinct from the transformers made from segments of transmission line.
A single impedance has two terminals to connect to the outside world, hence can be described as a 2-terminal, or a one-port, network.Despite the simple description, there is no limit to the number of meshes, [note 6] and hence complexity and number of elements, that the impedance network may have.