Designing Wide-band Transformers for HF and VHF Power Amplifiers

In the design of RF power amplifiers, wide-band transformers play an important role in the quality of the amplifier as they are fundamental in determining the input and output impedances, gain flatness, linearity, power efficiency and other performance characteristics. The three forms of transformers that are encountered, unbalanced-to-unbalanced (unun), balanced-to-balanced (balbal), and balanced-to-unbalanced (balun), are used in various combinations to accomplish the desired goals. Careful consideration needs to be given when making choices of the magnetic materials (if any is to be used), the conductors, and the method of construction, as the choices made weigh significantly in the overall performance of the transformer. The type and length of the conductors and the permeability of the magnetic material are the primary factors that determine the coupling, which in turn determines the transmission loss and the low frequency cutoff. The type and length of conductor used and the loss characteristics of the magnetic material also affects the coupling, and further influences the parasitic reactances that affect the high frequency performance.