A balance-to-unbalance (balun) is a transformer that alters the impedance and wiring configuration of an available signal. In its simplest forms, electrical and electronic circuits will be using the unbalanced configuration, wherein the signal is available in a single signal wire and a common wire. For better performance, the signal may be sent over a wire via a balanced wiring configuration where the signal is available in two wires each with a signal, and, therefore, as many as three wires may be utilized, with the common wire included. In the balanced configuration, there are two signal wires and an optional common signal path that is usually grounded through a defined resistance.
The balun is very similar to a classical transformer. The balanced input is the primary winding, while the secondary winding will have one signal wire and a common connection. In the antenna transformer, which is a transmission-line transformer, the outdoor antenna may be a dipole with two outputs that are both signal wires, so this setup may use a balanced-type antenna cable that is seen as two wires running parallel to each other. This cable is terminated in the balun with two wires, while the output is usually a 75-ohm unbalanced output that is a coaxial cable connector into the rear of the receiver. The setup uses a small balun, which is two few-turn windings on a ferrite bead or a small ferrite core.
For radio, the balun is used for passing radio frequency signals from balanced sources like dipoles and loop antennas into unbalanced equipment such as coaxial cables and coaxial cable ports found in receivers and transmitters. In radio and video usage, it may be used to alter the impedance of interconnected circuits and may set the frequency-dependent phase characteristics as a delay line. The effect is experienced as good signal quality through higher output power and/or lessened distortion.
In audio, there are balanced microphones with three-terminal connectors, as well as unbalanced microphones that are terminated in a coaxial cable connector that has a single wire in the inner core surrounded by a shield that is wired to the common connection. If the shield connection is disconnected temporarily and amplifier volume is up, while plugging the microphone, the noise that reaches the preamplifier input is heard as a humming sound. This is a lumped unbalanced-to-balanced transformation that is implemented when an unbalanced microphone drives a balanced speaker system.
An autotransformer has only one winding for the input and output, so this type of transformer may not be simple to use as a balun. The isolation transformer is commonly used as a balun because of its ability to isolate the input from the output circuit, which makes it possible to maintain the balanced circuit on the input side and the unbalanced circuit on the output side. In a balun, the common line of the unbalanced secondary winding may also connect to the center tap of the primary winding if that center tap exists.