Transformers find wide and important applications in RF circuits. Historically, transformers were used in power systems for voltage step-up and step-down. Since power transmission is more efficient at high voltages, transformers are used to boost signals to tens of thousands of volts for long-range transmission. For safety, though, we prefer to work with much lower voltage levels, and thus a transformer is used to step down the voltage to hundreds of volts before delivery into homes and factories. Electronic components, though, even operate at lower voltages. Before the widespread use of switching power supplies, transformers were ubiquitous in performing this task. For ultra low-noise applications, such as sensitive measurements, transformer-based designs still reign supreme as the method of choice.

The name “transformer,” in fact, derives from this function. As such, it's an ideal element for transforming voltages. Consequently, due to conservation of energy, transformers are equally good at stepping down/up currents. Thus, transformers are doubly good at impedance transformation. A desirable quality of transformers is their broadband operation. Whereas passive LC circuits can easily double as impedance transformation circuits, they do so only over a narrow bandwidth.

An ideal transformer is broadband, faithfully duplicating, inverting, and scaling voltages and currents independent of frequency with one important exception. Since its behavior derives from magnetic induction, it cannot perform it's function for static or DC signals.