Optimizing Shortwave Wideband RF Amplifier: Study of Transmission Line Transformer Construction Methods

This paper presents a study of six physical trans- mission line transformers (TLTs) designed to provide wide- band output matching for laterally diffused metal oxide semi- conductor (LDMOS) transistors within a push-pull amplifier operating in the 1.8-30 MHz spectrum with an output power of 600 W. While the mathematical model of TLTs is well de- scribed in the literature, the impact of physical construction methods on impedance matching and real amplifier perfor- mance is more challenging to ascertain. This paper compares six different TLTs built on various ferrite cores and employing different implementations of transmission lines. Return loss below -14 dB was achieved from 1.65 to 37.4 MHz, with most of the tested transformers exhibiting return loss better than -10 dB up to 50 MHz. The study also presents the impact of transmission line implementation on impedance match- ing using both special-purpose low impedance coaxial cable and a combination of general-purpose coaxial cables con- nected in parallel. Comparison of three chosen transformers in a real RF amplifier shows that using parallel transmission lines can lead to a return loss comparable to that of a special- purpose coaxial cable, although at the cost of lower efficiency and output power. Second harmonic cancellation effect was also investigated for three transformers.