Bias control for linearizing class AB amplifiers using envelope detection

Abstract

This paper propose a linearization method that is to control the operating point of a class AB amplifier according to its output power. The proposed linearization method is presented in this paper and the performance test results using two-tone signal are presented also. Power amplifiers with a good linearity are required for communication systems in order to use the limited frequency resources efficiently. The IMD(Inter-Modulation Distortion) generated by nonlinear characteristics of an amplifier causes adjacent channel interferences which decrease the efficiency of frequency usage especially when the output power of the amplifier is nearly saturated. Both MCPA(Multi-Carrier Power Amplifier) and HPA(High Power Amplifier) are used in mobile communication systems. In general, MCPAs are used in base station transmitters where a large capacity and high linearity is needed but HPAs are used in a small capacity system such as repeaters because the output power of the HPAs should be backed off by a large amount for guaranteeing the required linearity. Repeaters are commonly used for increasing cell coverage by delivering signals from the base station to the regions which can not receive the signal with an adequate power. Most of repeaters are installed outside and the performance of the repeater must be maintained even though the environment changes. MCPAs have used linearization techniques, such as a feedforward and a digital predistortion(1). Those linearization techniques, however, are not suited for an amplifier with a low price and small size. The characteristics of analog predistorters using diodes are easily changed by temperature and environmental changes and feedback systems can process narrow band signals due to the stability problems(2). Therefore diode predistorters and feedback systems are not adequate to low cost and small size amplifiers also. Therefore, another linearization method is needed for small and medium power amplifiers used in such an application like repeaters. One of those methods is to use the envelope of a transmit signal.