RF multicarrier amplifier for third-generation systems

The composite RF signal is amplified and fed to the antenna via a transmitter bandpass filter (Figure 1). The coverage and capacity of the base station define the required amount of output power. Several transistor stages connected in series constitute the gain, and the final stage delivers the output power. High-power amplifiers often have multiple transistors connected in parallel at the output. Consequently, the output stage consumes large amounts of power. Silicon (Si) or gallium-arsenide (GaAs) field-effect devices are adequate choices at an operating frequency of 2 GHz. The low-power stages at the input side are biased in a linear mode called class A. The high-power transistors operate in a more efficient but less linear mode called class AB. As seen from the input-output characteristics in Figure 2, class A mode is more linear for small signals. The complex modulation schemes in WCDMA systems have a high peak-toaverage power relationship, which produces amplitude and phase distortion in nonlinear amplifiers. This distortion is even more pronounced as the output power level of the amplifier approaches saturation. Some amplifiers also experience a memory effect—that is, the output signal at a given moment is affected not only by the instantaneous input signal, but also by the previous signal history. Distortion causes signal quality to deteriorate and adjacent channel power (ACP) to increase; it also gives rise to spurious emissions. Amplifier efficiency is dependent on the characteristics of the power transistors and the bias scheme. Efficiency improves with higher output power, but distortion increases rapidly as the power level approaches saturation. The high peak-to-average power relationship is thus fraught with compromise. Figure 3 shows a simulated example of how a two-carrier WCDMA signal is distorted by the gain transfer characteristic of the class AB amplifier. The figure shows the frequency spectrum before and after amplification. The output signal does not satisfy system requirements for spurious emissions. The best solution for multicarrier amplifiers is to operate the power transistors in efficient but slightly non-linear class AB