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

Abstract

This paper presents a study of six physical transmission line transformers (TLTs) designed to provide wideband output matching for laterally diffused metal oxide semiconductor (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 described in the literature, the impact of physical construction methods on impedance matching and real amplifier performance 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 matching using both special-purpose low impedance coaxial cable and a combination of general-purpose coaxial cables connected 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.