What Is a Balun?
A balun, short for “balanced-to-unbalanced,” is a device that converts balanced signals to unbalanced signals and vice versa. Baluns often incorporate impedance transformation using a transformer and are commonly used at antenna connection points and in high-frequency applications such as differential amplifiers on MMIC (Monolithic Microwave Integrated Circuit) semiconductor chips.
Uses of Baluns
Baluns are widely used in television and wireless communication systems for signal conversion between high-frequency outputs and antennas. They are designed to handle characteristic impedances, typically in ratios of 1:1 or 1:4, such as converting a 75-ohm impedance to a different value or converting 50 ohms to 200 ohms. Baluns are available as small, surface-mounted components on low-temperature co-fired ceramic (LTCC) substrates or as wire-wound types using toroidal cores.
Principle of Baluns
Baluns function by converting between balanced (differential) and unbalanced signals. Balanced signals have phases shifted by 180 degrees between the positive and negative sides, allowing symmetrical signal pairs to cancel out common-mode noise. In contrast, unbalanced signals typically have one side grounded, with distinct round-trip signal paths.
There are two main types of baluns: voltage baluns and current baluns. Voltage baluns connect the ground terminal of the unbalanced signal to the midpoint of an inductor on the balanced side, generating balanced voltages in opposite directions. Current baluns, on the other hand, use an inductor in series between the antenna and coaxial cable. They operate by creating a counteracting current in an opposing coil to balance the current flow from the signal source of the unbalanced circuit. In terms of circuit symbols, pole baluns resemble common mode choke coils.