Investigation of electrical base-band memory effects in high-power 20W LDMOS transistors using IF passive load pull

Memory effects are complex phenomena that present major problems in modern high-power linear microwave PA design. Specifically, these effects have a profound influence on spectral symmetry and modulation frequency sensitivity, consequently impacting overall linearity and most importantly, the suitability of a Power Amplifier (PA) to linearisation through pre-distortion. This paper presents detailed, two-tone modulated measurements that clearly show how electrical memory, introduced by non-ideal low-frequency base-band impedances, represents the most significant contributor to overall observed memory effects in high-power LDMOS PA design. The analysis is achieved through the characterisation of a 20W LDMOS device at 2.1 GHz using two-tone excitation and a purpose- built, high- power measurement system, which enables the collection of both RF and IF voltage and current waveforms, together with all associated impedances.