RF and Microwave Engineering: Fundamentals of Wireless Communications

6.11 Balanced-to-Unbalanced Circuits

In Section 4.9 we discussed balanced and unbalanced transmission lines. Microstrip lines, striplines and coaxial lines are unbalanced transmission lines. Symmetrical two-wire lines and coupled microstrip lines represent balanced transmission lines. A balanced line is commonly driven with symmetrical signals where the voltages at the terminals have equal magnitude but opposite signs with respect to ground U₁ = − U₂ (see Equation 4.75).

If we connect a balanced transmission line or component to an unbalanced transmission line or component baluns (balanced-to-unbalanced transformers) are used. For low and medium frequencies a transformer with a secondary centre tap provides a broadband conversion between unbalanced and balanced systems (Figure 6.43a). Such a transformer can additionally be used for impedance matching. Figure 6.43b shows an unbalanced input signal u₀(t) and balanced output signals u₁(t) and u₂(t) where u₁(t) = − u₂(t).

Figure 6.43 Balanced-to-unbalanced circuit (balun) with transformer.

At higher frequencies transformers become unattractive due to increased losses. Therefore, transmission line structures are commonly used to provide narrowband conversion. Figure 6.44a shows a balun with two λ/4-lines to provide a 180° phase shift. The unbalanced signal on a microstrip line is matched to a power divider and split into two signals ...