Impedance mismatches in the circuit cause energy to reflect back to the source, reducing the amount of power available to the load and potentially damaging the source. Matching the output impedance of the power supply to the input impedance of the power supply to the input impedance of the electrical load maximizes the reflected energy of the power supply and optimizes circuit efficiency.
A symmetrical balanced line has two conductors with equal current flow in opposite (differential) directions. Twisted pair cables are examples of balanced wire cables. Asymmetric unbalanced (differential) orientation. Twisted pair cables and ribbon cables are examples of balanced wire cables. One conductor of an unbalanced (single-ended) line, such as coaxial cable, is grounded.
When a balanced power source is to be isolated from an unbalanced load, an impedance matching transformer is called a balun. If the impedance of the load matches the impedance of the source, then impedance matching is not required and the balun has a 1:1 impedance ratio. When the load impedance does not match the source in a 1:N impedance ratio, a 1:N impedance ratio transformer is required.
A classic example of an impedance mismatched circuit is a 300Ω ribbon cable (balanced) for an antenna connected to a 75Ω coaxial (unbalanced) receiver input. Without a balun to match the load’s impedance to the source’s impedance, energy is reflected back from the antenna’s lower power input signal, providing a reduced signal to the receiver.
300Ω to 75Ω Balun
The balun provides a 180° phase shift and ideally balanced impedance. Wirewound broadband RF transformers provide excellent broadband baluns, providing two 90° phased wires for a 180° offset. Baluns are used with antennas, mixers, and push-pull amplifiers to create the correct phase relationship and match impedances. Wire-wound RF transformers are commonly used in push-pull amplifier applications in the frequency range of a few kHz to around 2 GHz. Alternative configurations may include a center tap for bias or ground.
Center hits Ruthroff Balun
Ruthroff-style baluns provide isolation as well as impedance matching. If isolation is not required, the bandwidth can be extended to frequencies up to 3.5 GHz when connected as a Guanella (transmission line) transformer.
1：1 Guanella Balun
Simple Balun connection example
When connecting a 200Ω balanced antenna to a 50Ω unbalanced (coaxial) cable, connect the high impedance (200Ω) balanced antenna to the high impedance side of the transformer and the low impedance (50Ω) unbalanced side to the low impedance transformer side. This connection reduces the high impedance of the antenna to match the 50Ω cable.
200Ω to 50Ω Balun
High frequency narrowband balun
For high frequency narrowband applications where isolation is not required, a lumped element design can be implemented. The availability of tight tolerance surface mount chip inductors and capacitors minimizes variations in production. Designers can use circuit simulation to optimize component values to achieve the desired impedance matching network using LC components. An online calculator can be used to estimate initial design values. At GHz frequencies, an accurate inductor model including parasitic elements is essential to approach a reality-based design. Coilcraft’s air core and chip inductor models are measurement based to provide an accurate simulation of your LC balun or matching network.