Although the Doherty power amplifier offers higher efficiency compared to traditional amplifiers, it typically suffers from poor linearity. To address this issue, digital predistortion (DPD) technology is commonly employed to enhance its performance. As communication speeds continue to rise, the demand for wider bandwidth in Doherty amplifiers has also increased. This makes it essential to develop a broadband digital predistortion platform that can effectively evaluate and improve the linearity of these amplifiers. In this study, a broadband Doherty power amplifier was linearized using a digital predistortion system based on the R&S signal and spectrum analyzer (FSW26). Experimental results demonstrated that the Doherty amplifier, when combined with DPD, significantly improved linearity across various operating conditions, including single-frequency, dual-frequency, and multi-frequency scenarios.
With the rapid development of communication technologies, especially the widespread use of carrier aggregation, the required signal bandwidth for transmitters has grown substantially. This has led to the emergence of concurrent multi-frequency transmitters, which must support both single-carrier and multi-carrier signals simultaneously. As a result, the bandwidth of the transmission channel has expanded dramatically. However, the power amplifier remains one of the main limiting factors in achieving higher bandwidth. The Doherty power amplifier, while more efficient than the conventional class AB amplifier, typically has a relatively narrow bandwidth—often around 10%. To meet the demands of modern communication systems, designing broadband Doherty amplifiers has become a key focus in recent years.
Due to its inherent nonlinearity, the Doherty power amplifier usually requires digital predistortion to be used in real-world applications, particularly in base stations. Therefore, beyond efficiency and bandwidth, the effectiveness of linearization techniques is a critical concern for industry professionals. After the design of a Doherty amplifier is completed, it's crucial to test the linearity performance of the digital predistortion system to ensure the overall performance meets the required standards.
Digital predistortion is a technique designed to counteract the nonlinear behavior of power amplifiers. Normally, power amplifiers operate efficiently when they are in saturation. For example, in a Doherty amplifier, the main amplifier operates in saturation at lower power levels, while both the main and auxiliary amplifiers work in saturation at peak power. This allows the Doherty amplifier to achieve high efficiency at both low and high power levels. However, when an amplifier is in saturation, it introduces nonlinear distortion, causing the output signal to have a broader bandwidth than the input. This distortion can degrade signal quality and lead to interference.
Figure 1: The true principle diagram
To mitigate this distortion, digital predistortion works by pre-processing the input signal in the digital domain. This involves adding components that are equal in magnitude but opposite in phase to the distortion generated by the power amplifier. These components help cancel out the nonlinear effects, resulting in a more linear output. Figure 1 illustrates the principle of digital predistortion in terms of gain. It shows that when the input signal is large, the amplifier’s gain decreases. However, the predistortion module compensates for this by increasing the gain at those points. When combined, the two stages create a linear response, ensuring the input and output signals maintain a consistent gain relationship. This approach is essential for maintaining signal integrity in high-performance communication systems.
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