Balun

Types of balun

Classical transformer type

In classical transformers, there are two electrically separate windings of wire coils around the transformer's core. The advantage of transformer-type over other types of balun is that the electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or are otherwise electrically incompatible; for that reason they are often called isolation transformers.

This type is sometimes called a 'voltage balun'. The primary winding receives the input signal, and the secondary winding puts out the converted signal. The core that they are wound on may either be empty (air core) or, equivalently, a magnetically neutral material like a porcelain support, or it may be a material which is good magnetic conductor like ferrite in modern high-frequency (HF) baluns, or soft iron as in the early days of telegraphy.

The electrical signal in the primary coil is converted into a magnetic field in the transformer's core. When the electrical current through the primary reverses, it causes the established magnetic field to collapse. The collapsing magnetic field then induces an electric field in the secondary winding.

The ratio of loops in each winding and the efficiency of the coils' magnetic coupling determines the ratio of electrical potential (voltage) to electrical current and the total power of the output. For idealized transformers, although the ratio of voltage to current will change in exact proportion to the square of the winding ratio, the power (measured in watts) remains identical. In real transformers, some energy is lost inside to heating of the metallic core of the transformer, to the AC and DC resistances of the winding conductors, and lost outside to the surrounding environment because of imperfect magnetic coupling between the two coils.

Autotransformer type

An ideal balun consists of two wires (primary and secondary) and a core: the current in the primary wire generates a magnetic field in the core, which in turn induces an electric field in the secondary wire. An autotransformer balun has only one coil, or is made of two or more coils that have an electrical connection, wound around a core. One can also make an autotransformer from an ordinary transformer by cross-wiring the primary and secondary windings. Baluns made with autotransformer windings are also called voltage baluns, since they produce balanced output voltage, but not necessarily balanced current.

In all autotransformers, the single winding must have at least one extra electrical connection – called a tap or tap point – between the two ends of the winding. The current sent into the balun through one pair of connections acts as if it were a primary coil, and magnetizes the entire core. When the electric current in the input segment of the coil changes, the induced magnetic field collapses and the collapse of the magnetic field in the core induces an electric current in the entire coil. Electrical connections to parts of the coil different from the input connections have higher or lower voltages depending on the length of the coil that the output is tapped from.

Unlike transformer-type baluns, an autotransformer balun provides a path for DC current to ground from every terminal. Since outdoor antennas are prone to build-up of static electric charge, the path for the static to drain to ground through an autotransformer balun can be a distinct advantage.

Transmission-line transformer type

Transmission line or choke baluns can be considered as simple forms of transmission line transformers. This type is sometimes called a current balun, since it ensures equal current on both sides of its output, but not necessarily equal voltage. These are normally called ununs, because they go from unbalanced to unbalanced or un-un. Baluns are balanced to unbalanced or bal-un.

A more subtle type results when the transformer type (magnetic coupling) is combined with the transmission line type (electro-magnetic coupling). Most typically the same kind of transmission line wires are used for the windings as carry the signal from the radio to the antenna, although these baluns can be made using any type of wire. The resulting devices have very wideband operation. Transmission line transformers commonly use small ferrite cores in toroidal rings or two-hole, binocular, shapes.

The Guanella transmission line transformer (Guanella 1944) is often combined with a balun to act as an impedance matching transformer. Putting balancing aside a transformer of this type consists of a 75 Ω transmission line divided in parallel into two 150 Ω cables, which are then combined in series for 300 Ω. It is implemented as a specific wiring around the ferrite core of the balun.

Classic magnetic transformers are limited in maximum frequency due to the permeability of the magnetic material, which declines rapidly above MHz frequencies. This limits the operating frequency to around 1 GHz. Microwave systems require baluns in the 1-100 GHz range for applications including mixers, push-pull amplifiers, and interface to differential analog to digital and digital to analog converters.

Transmission line transformer baluns with a high frequency transmission line such as a wirewound pair are able to operate at significantly higher frequencies (to 8 GHz and above) by combining capacitive coupling with magnetic coupling typical of classic magnetic transformers.

Planar and Marchand baluns

To operate at even higher frequencies, magnetic loading becomes ineffective. Nathan Marchand proposed a 'conversion transformer' that was capable of operation over a 1:3 bandwidth ratio in 1944. This basic circuit has been widely adapted and modified for use in planar structures (such as MMICs and RFICs) at frequencies up to at least 100 GHz.

These types of planar baluns work exciting a mode in a ground plane or shield, which is then 'floated' away from the main ground. This allows extraction of the negative signal in addition to the main transmission line, creating positive and negative signal pairs. By connecting the positive and negative signal paths in various configurations dozens of balun configurations have been proposed and published in the electronics literature.