Doherty High-Frequency Amplifier Impedance Compensation for Wide Bandwidth
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Solution Overview
Problem
Existing Doherty amplifiers fail to achieve a wide band operation, especially at high output power levels, due to limitations in frequency characteristics compensation across the active regions where both carrier and peak amplifiers operate.
Innovation Solution
Incorporating an impedance compensation circuit with specific electrical lengths and characteristic impedances in the transmission lines, ensuring the imaginary part of impedance at the center frequency is opposite in polarity, allowing for broader frequency characteristics compensation across all operational regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a third transmission line having an electrical length equal to one-half wavelength of a center frequency is connected between the node between the first transmission line and the second transmission line and the output terminal of the peak amplifier, then the frequency characteristics of the third transmission line compensate for the frequency characteristics of the load impedance of the carrier amplifier in the active region where the peak amplifier does not operate, but the output signal of the carrier amplifier does not pass through the third transmission line in the active region where both amplifiers operate, preventing compensation for frequency characteristics at high output power
Solution Approach 1:
The transmission line system is segmented into multiple sections: the first transmission line (one-quarter wavelength) connects the carrier amplifier output to the node, the second transmission line (one-quarter wavelength) connects the node to the output terminal, and the third transmission line (one-half wavelength) connects the node to the peak amplifier output. This segmentation allows different transmission lines to serve different functions in different operational regions, enabling frequency characteristics compensation across the entire bandwidth while maintaining proper signal paths in both low and high output power regions.
2Adaptability or versatility
If the imaginary part of the load impedance of the carrier amplifier is positive at a frequency lower than the center frequency and negative at a frequency higher than the center frequency, then the phase characteristics of the first and second transmission lines cause frequency characteristic deterioration, but adding compensation circuits increases device complexity
Solution Approach 1:
The third transmission line is designed to serve multiple functions: it compensates for the frequency characteristics of the carrier amplifier load impedance in the active region where the peak amplifier does not operate, and it also compensates for the frequency characteristics of the peak amplifier load impedance in the active region where both amplifiers operate. This multi-functionality allows a single transmission line to address frequency characteristic deterioration across different operational regions without significantly increasing device complexity.
Data Source
AI summary
A high-frequency amplifier includes: a carrier amplifier amplifying a first signal; a peak amplifier amplifying a second signal; a first transmission line connected between output terminals of the carrier amplifier and the peak amplifier, and having an electrical length equal to one-quarter wavelength of a center frequency in the predetermined frequency band; a second transmission line connected between one end of the first transmission line and the output terminal of the high-frequency amplifier, and having an electrical length equal to one-quarter wavelength of the center frequency; and an impedance compensation circuit with one end connected to a node between the first transmission line and the second transmission line. At the center frequency, an imaginary part of an impedance during viewing of the impedance compensation circuit from the node is opposite in polarity from an imaginary part of an impedance during viewing of the second transmission line from the node.