Using active IF load-pull to investigate electrical base-band induced memory effects in high-power LDMOS transistors

Memory effects are generally attributable to thermal, electrical, packaging and/ or surface effects. This behaviour in turn impacts overall linearity and importantly the suitability of a power amplifier (PA) to linearisation through pre-distortion. It is assumed that electrical memory introduced by the low-frequency baseband impedance environments associated with the power amplifier bias insertion networks being frequency dependent represents a significant contributor to overall observed memory effects in high-power LDMOS PA design. In this work, baseband or IF active load-pull is used to provide an effective way to engineer all the significant IF components generated as a result of multi-tone excitation, independent of modulation frequency. Specific IF impedance environments are presented to a device with this approach in order to probe the sensitivity to IF impedance variations. These investigations are performed on a 12 W LDMOS device characterised at 2.1 GHz within a purpose built, high-power measurement system, that allows the collection of both RF and IF voltage and current waveforms along with all associated impedances.