Design and Digital Linearization of a High Efficiency Variable Conduction Angle Doherty Amplifier

Introduction The need for highly efficient, linear microwave power amplifiers has grown rapidly, especially as mobile and satellite communication systems have been developed and expanded [1]. These systems employ multilevel digital modulation techniques to achieve higher-data-rate and multimedia services. This results in a band-limited multi-carrier signal, which is spectrally efficient but exhibits time-varying amplitude and phase characteristics. For acceptable linearity in handling such multicarrier signals with high peak-to-average envelope ratios, the amplifier output power must be backed off from its peak value. This has the disadvantages of reducing amplifier efficiency, increasing energy consumption, and reducing the operating time of portable power sources. Such a tradeoff between linearity and efficiency is difficult to achieve with conventional amplifiers. One of the promising techniques for improving efficiency under output power backoff is the load-line modulation technique used in the Doherty amplifier [2]– [4]. A simplified basic block diagram of the Doherty amplifier is shown in Figure 1. Amplifier 1 is referred to as the carrier amplifier. Amplifier 2 is referred to as the peak amplifier. The input power is divided equally, with a quarter-wave delay at the input of the peak amplifier. After being amplified, the output signals are combined in phase at the output of the quarter-wave transformer (with characteristic impedance Zc). Detailed analysis of the characteristics of the Doherty amplifier configurations based on carrier and peak amplifiers both biased in Class B is reported in [3]. In these configurations, an additional control circuit is needed to turn on the peak amplifier when the carrier amplifier saturates. Several designs of the Doherty amplifier [3]-[8] have reported nonconstant gain and phase behavior, caused partially by the inherent nonlinearity in the carrier and peak amplifiers when biased in Class B (or AB) and Class C, respectively. Such nonlinearities of the AM-AM and AM-PM characteristics affect negatively the integrity of the transmitted signal and cause an increase The need for highly efficient, linear microwave power amplifiers has grown rapidly.