A new "active" predistorterwith high gain and programmable gain and phase characteristics using cascode-FET structures

A monolithic-microwave integrated-circuit (MMIC)-compatible miniaturized "active" predistorter using cascode FET structures is presented. The predistorter has added functionality of gain, as well as programmable gain,and phase variation characteristics, which are required to compensate for the nonlinear distortion of a wide range of power amplifiers (PAs). Thanks to the inherent gain of the predistorter, a need for an additional buffer amplifier is eliminated. Furthermore, it can eventually replace the first-stage amplifier in the multistage PAs, making this approach well suited to MMIC implementation. A simple analysis is performed to understand the phase variation mechanisms in the proposed predistorter and to identify the dominant sources of phase variation. From the analysis, the origins of nonlinear distortion of predistorter were found to be G(m) of the upper FET and R-ds of the lower FET. It was also found from the analysis that,the gain and phase variation can be programmed by controlling the bias and size of the transistors. To demonstrate the general usefulness of this predistorter, the cascode predistorter was applied to linearize waft-level MMIC amplifiers for code-division multiple-access handset applications, as well as 30-W high power amplifiers (HPAs) for base-station applications. Adjacent channel power ratio (ACPR) improvement of 3-5 dB was achieved with off-chip predistorter when applied to 0.9-W monolithic amplifiers. The predistorter was also integrated with a 1.6-W MMIC PA on a single chip, replacing the first-stage transistor of the amplifier. An integrated predistorter showed ACPR improvement of 2.5 dB at 28-dBm output power. When applied to a base-station HPA, the spectral regrowth is suppressed by approximately 10 dB at the average output power of 44.7 dBm. The "active" predistorter of this study can be a viable general-purpose linearization technique that can be applied to a wide range of amplifiers.