What is the difference between a Balun and a RF Transformer?

Though certain types of baluns and RF transformers may be similarly constructed, and the circuit diagram symbols are easily confused, the designation of balun and RF transformer imply different operational characteristics and purposes, with some overlapping characteristics. Baluns convert between balanced and unbalanced lines, and RF transformers perform impedance matching, voltage or current step-up or step-down, and DC-isolation between two circuits. However, there are some balun configurations that perform both balun and RF transformer functions, known as autotransformer baluns or voltage baluns. 

The term “Balun” comes from the function of converting between a balanced signal pathway and an unbalanced signal pathway. A balanced signal pathway requires two 180 degree out-of-phase signals in the same transmission pathway, where an unbalanced signal is a single signal with a ground plane or pseudo-ground return path. A balun can be fabricated by appropriately taping and configuring a winding-based transformer, transmission line-based transformer, with a delay line, or other means. An RF choke can be used in the place of a balun for some applications, by attenuating one of the signals in the transmission pathway and allowing the other to pass.

RF transformers use magnetic coupling between two or more circuits to transform, or isolate, the voltage, current, or impedance amongst the coupled circuits. Common RF transformers are composed of twisted-pair, or coaxial, transmission lines would around toroidal ferromagnetic cores, though there are also thick-film, thin-film, and planar RF transformers. Hence, there are many overlapping properties, functions, and fabrication methods between Baluns and RF transformers, and some types of Baluns can be considered modified RF transformers, though this is not always the case for every Balun.