2 Why are Baluns Commonly Used?

The term balun is a portmanteau of balanced and unbalanced, indicating that a balun will transition between a balanced (also called differential) transmission line (where opposite signals both travel in transmission lines) and an unbalanced (also called single ended) transmission line (where the return current travels in the ground). However, this description obscures the simplicity of the balun. A balun has equal power outputs just like a Wilkinson power divider, resistive power divider, or quadrature hybrid coupler. However, it has a 180° phase difference between outputs. This is in contrast to the power dividers, which have 0° phase difference, and quad hybrids, which have 90° phase difference. Also, baluns are passive elements that do not require a supply voltage and more than likely do not add much noise. Baluns are considered by some designers to be the standard component of choice when converting between single-ended and differential signals for narrowband applications.

An ideal balun is a device that has a single-ended port and a differential port. A differential port may be considered as two separate ports or a combined port. The following S-parameter equations describe an ideal balun which has port 1 as input and port 2 and 3 are output ports.

Note what is implied by this:

• A balun is a three-port power splitter
• The two outputs will be equal and opposite
• The unbalanced input is matched to the input transmission line impedance (usually 50 Ω)
• Unlike an isolator or circulator, a balun is a reciprocal device that can be used bidirectionally

Also note what is not implied by this:

• The two outputs are not necessarily matched
• The outputs of the balun may or may not be the same impedance as the input
• There is no constraint on S23, so the outputs may or may not have isolation
• Therefore there may be a different return loss on the outputs for differential and common-mode signals