Investigation on power amplifier linearization technique : an approach of baseband injection

Abstract

Due to the overcrowding of communication spectrum, high spectral efficiency is particularly important for the transmission of data at the highest possible rate for a given channel bandwidth. Different modulation methods with non-constant envelope signal are implemented to improve channel capacity. However, these modulation formats need a stringent requisition on the amplifier’s linearity as the amplification of such signals will generate unwanted frequencies in adjacent channels caused by the nonlinear effect of the amplifier. Different linearization methods have been reported to enhance the linearity of amplifier, such as feedforward, feedback, pre-distortion and second-order signal injection techniques. In this research study, various linearization techniques that have been applied to power amplifiers in communication system are reviewed. The secondorder intermodulation products injection technique is investigated particularly, because of its outstanding performance on the third-order intermodulation suppression and its simplicity of the involved circuit design. The objective of this work is to identify the limitation of the conventional injection technique and to improve its ability. It addresses the use of novel techniques by only utilizing the low-frequency component to completely reduce the intermodulation distortion of the power amplifier which tends to operate near the saturation region. In the proposed method, dual second-order signals are injected to the input and output ports of the amplifier to achieve better amplifier linearity. An experimental twotone test reveals that over 40 dB suppression on third-order intermodulation distortion at 1.95GHz has been demonstrated on the bipolar and field-effect transistors, compared with the single-signal injection, around 20dB more improvement has been acquired at the 1dB compression point of the amplifier. Therefore, this method provides a simple and efficient way to improve amplifier linearity. This report presents an analysis and experimental result to verify the idea. Keywords: envelope injection, power amplifiers, second-order injection, linearization, Volterra analysis