The RF power amplifier is a critical component in wireless communication systems, especially at the transmitting end. It plays a vital role in amplifying signals for transmission. Key performance metrics of an RF power amplifier include linearity, efficiency, and noise figure. Among these, linearity and efficiency are the most important. As the number of wireless users increases and broadband services expand, the frequency spectrum becomes more 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 introduces distortion, particularly when the signal envelope fluctuates. This necessitates improved linearity in RF power amplifiers.
Traditional methods to reduce nonlinear distortion include feedforward, negative feedback, and predistortion. Feedforward linearization requires additional power amplifiers and complex control circuits, making it bulky and expensive. Negative feedback reduces the amplifier's gain and limits its operation to narrow frequency bands, suitable only for narrowband systems. Predistortion, on the other hand, is widely used due to its simplicity, stability, low cost, wide bandwidth, and ease of integration. This paper presents a new predistorter design and simulates it using ADS2008U1. The results show that the proposed design effectively improves the nonlinear distortion of the power amplifier.
Predistortion works by introducing a nonlinear circuit before the power amplifier to counteract its nonlinearity. The basic principle is illustrated in Figure 1. The predistorter acts as a nonlinear generator, adjusting its output to match the inverse of the amplifier’s nonlinearity in both amplitude and phase, thus improving overall linearity. Predistortion can be classified into RF, IF, and baseband types, with this paper focusing on RF predistortion.
The traditional predistortion block diagram is shown in Figure 2. However, one drawback is that the predistorter output contains not only distortion but also residual fundamental frequency components. These can interfere with the main signal, reducing the output power. To address this issue, a new predistorter design was developed.
The new predistorter uses a co-parallel diode pair in a bridge configuration, as shown in Figure 3. A 90-degree bridge network helps in signal processing and impedance matching. Capacitors on the bridge compensate for the reactance of the diodes, while maintaining good input and output impedance matching.
Nonlinear output analysis shows that the predistorter produces not only the fundamental frequency but also harmonics and intermodulation products. The third-order intermodulation distortion is particularly significant, and the fundamental frequency must be eliminated to avoid interference.
To cancel the unwanted fundamental frequency, a new circuit was designed. As shown in Figure 4, the input two-tone signal is split, with one part going through the predistorter and the other delayed and adjusted in gain and phase. The adjusted signal is then combined with the predistorter output to cancel the fundamental frequency, leaving only the desired third-order intermodulation distortion.
Simulation using ADS2008U1 confirmed the effectiveness of the design. The traditional predistorter output showed a strong fundamental frequency component, while the improved version reduced it by 40 dBc, as seen in Figures 5 and 6.
For the power amplifier design, the specifications included a frequency range from 900 MHz to 1 GHz, an output power of at least 13.4 dBm, and a third-order intermodulation improvement of over 40.2 dBc. The simulation used a packaged power amplifier module, including the MRF9742 transistor and matching circuits. The predistortion power amplifier circuit is shown in Figure 7.
The simulation results, depicted in Figures 8 and 9, demonstrated a significant reduction in third-order intermodulation distortion after adding the predistorter. This confirms the effectiveness of the proposed design.
In conclusion, this paper introduces a predistorter with fundamental frequency cancellation, significantly improving the linearity of RF power amplifiers. The simulation results show a more than 40 dBc suppression of third-order intermodulation distortion. The adaptive control method allows real-time adjustment of the phase shifter and attenuator based on the output signal, further enhancing the performance of the power amplifier.
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