Transformer-Based Doherty PA Linearity via Input Capacitance Compensation
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Solution Overview
Problem
Transformer-based Doherty power amplifiers (PAs) face poor linearity in the saturated output power region and power back-off region, necessitating optimization to improve efficiency and linearity.
Innovation Solution
Incorporating a first and second linear network circuit at the inputs of the main and auxiliary PA paths, respectively, to compensate for input capacitance variations, thereby stabilizing the input capacitance and reducing AM-PM distortion, which enhances the linearity of the Doherty PA.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If power back-off is used to improve linearity, then the three-order cross modulation coefficient is improved, but the efficiency of the PAs in the power back-off region becomes very low
Solution Approach 1:
The PA is divided into two independent paths: a main PA path and an auxiliary PA path. The main PA operates in class AB for linear amplification, while the auxiliary PA operates in class C for efficient power amplification. This segmentation allows each path to be optimized for its specific function, resolving the contradiction between linearity and efficiency.
Solution Approach 2:
The system dynamically switches between the main PA and auxiliary PA based on the input signal level. When the main PA is not saturated, only the main PA works. When the main PA is saturated, the auxiliary PA is turned on to provide additional power while maintaining efficiency. This dynamic operation optimizes both linearity and efficiency across different power levels.
2Use of energy by moving object
If Doherty PAs are used to improve efficiency in the power back-off region, then the average efficiency is improved, but the linearity of the PAs becomes poor
Solution Approach 1:
Different quality requirements are applied to different parts of the system. The main PA path is optimized for linearity with class AB operation, while the auxiliary PA path is optimized for efficiency with class C operation. By assigning different operational characteristics to different paths based on their local requirements, the system achieves both linearity and efficiency simultaneously.
3Reliability
If a 1/4 wavelength line is used for impedance transformation, then the output impedance is transformed, but the input capacitance varies with signal intensity
Solution Approach 1:
The system uses feedback mechanisms through the parallel configuration of main and auxiliary PAs with 1/4 wavelength lines. The output signals from both paths are combined, and the impedance transformation through the 1/4 wavelength lines provides feedback that stabilizes the overall input capacitance despite variations in individual path capacitances with signal intensity.
Data Source
AI summary
A transformer-based Doherty power amplifier includes a main power amplifier path and an auxiliary power amplifier path which are connected in parallel. The main power amplifier path includes a main power amplifier, and the auxiliary power amplifier path includes an auxiliary power amplifier. The transformer-based Doherty power amplifier further includes a first linear network circuit or a second linear network circuit. The first linear network circuit is arranged at an input of the main power amplifier and is used to compensate for variations of an input capacitance of the main power amplifier, so as to improve the linearity of the main power amplifier. The second linear network circuit is arranged at an input of the auxiliary power amplifier and is used to compensate for variations of an input capacitance of the auxiliary power amplifier, so as to improve the linearity of the auxiliary power amplifier.