The RF power amplifier is a critical component in wireless communication systems, particularly in the vicinity of the transmitting antenna. It plays a vital role in amplifying signals before they are transmitted over the air. Key performance metrics of an RF power amplifier include linearity, efficiency, and noise figure. Among these, linearity and efficiency are especially important, as they directly impact the quality and reliability of the transmitted signal.
With the growing number of wireless users and the expansion of broadband services, the frequency spectrum has become increasingly congested. To accommodate more communication channels within limited bandwidth, advanced modulation techniques with higher spectral efficiency are required. However, the nonlinearity of the power amplifier causes distortion, particularly when the signal envelope fluctuates. This distortion becomes more significant as the demand for high-quality transmission increases, placing stricter requirements on the linearity of the power amplifier.
To address this issue, several linearization techniques have been developed, including feedforward, negative feedback, and predistortion. Feedforward methods require additional hardware, such as auxiliary amplifiers and complex control circuits, making them costly and bulky. Negative feedback, while effective, reduces the amplifier’s gain and limits its operation to narrow frequency bands, which makes it unsuitable for modern wideband applications. Predistortion, on the other hand, is a widely used technique that introduces a controlled nonlinear signal at the input of the amplifier to counteract its inherent nonlinearity.
Predistortion works by adding a nonlinear circuit at the front end of the power amplifier, designed to generate a signal that cancels out the amplifier's nonlinear distortion. The basic principle involves generating a signal that is equal in amplitude but opposite in phase to the distortion introduced by the amplifier. This results in a more linear output from the system. Predistortion can be implemented at different stages—RF, IF, or baseband—and this paper focuses on RF predistortion.
A new predistorter design is proposed in this work, based on a co-parallel diode pair arranged in a bridge configuration. The design aims to eliminate unwanted fundamental frequency components that remain after traditional predistortion methods. A schematic of the new predistorter is shown in Figure 3, illustrating the key components involved in the compensation process.
In this design, the input signal is split into two paths using a 3 dB splitter. One path goes through the predistorter, while the other is delayed and adjusted in both amplitude and phase using a variable attenuator and phase shifter. The delayed and adjusted signal is then combined with the output of the predistorter, allowing for the cancellation of the fundamental frequency component. This approach ensures that only the desired intermodulation distortion remains, improving the overall linearity of the system.
The circuit was simulated using ADS2008U1, with a two-tone signal at 940 MHz and 950 MHz applied to the input. The simulation results showed a significant reduction in the fundamental frequency component, demonstrating the effectiveness of the new design. The improved predistorter was also integrated into a power amplifier circuit, and the results confirmed a substantial improvement in third-order intermodulation distortion, exceeding 40 dBc.
This research presents a novel predistorter that not only enhances the linearity of the RF power amplifier but also allows for adaptive control of the phase and amplitude adjustments. By dynamically adjusting the circuit based on the level of distortion in the output, the system can maintain optimal performance under varying conditions. This adaptive approach offers a more flexible and efficient solution compared to conventional predistortion techniques, making it suitable for next-generation wireless communication systems.
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