Doherty Amplifier Impedance Matching for Higher Backoff
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Solution Overview
Problem
Current Doherty amplifiers face inefficiencies in amplifying signals with large peak to average power ratio (PAPR), as they struggle to maximize backoff amount and efficiency, particularly due to the limitations in impedance matching between the main and auxiliary amplifiers.
Innovation Solution
A Doherty amplifier design that includes a transistor for main amplifier operating in class AB, a transistor for auxiliary amplifier operating in class C, a series capacitor to reduce impedance capacitance at the composite point, and an output matching circuit to match impedance with the load, enhancing efficiency by increasing the backoff amount and achieving high-efficiency amplification for signals with large PAPR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the parasitic capacitance of transistors is used to form a 90-degree delay circuit, then the circuit complexity is reduced, but the backoff amount is insufficient and efficiency improvement is limited
Solution Approach 1:
An inductor is introduced as an intermediary component between the auxiliary amplifier transistor and the composite point. This inductor works in conjunction with the parasitic capacitance to form a resonant circuit that provides the necessary phase delay, replacing the need for a separate 90-degree delay circuit while increasing the backoff amount for improved efficiency.
Solution Approach 2:
The patent changes the electrical parameters of the circuit by introducing an inductor with specific inductance value that resonates with the parasitic capacitance at the operating frequency. This parameter change transforms the circuit from a simple parasitic capacitance-based delay to a resonant circuit that provides both phase delay and increased backoff capability.
2Productivity
If the backoff amount is increased to improve efficiency for signals with large PAPR, then the efficiency improves, but the impedance matching between main and auxiliary amplifiers becomes more difficult
Solution Approach 1:
The inductor serves as an impedance transformation intermediary that facilitates proper impedance matching between the auxiliary amplifier and the composite point even at increased backoff levels. By carefully selecting the inductor value, the circuit achieves both high backoff for efficiency and proper impedance matching.
Solution Approach 2:
The patent employs dynamic impedance transformation through the resonant circuit formed by the inductor and parasitic capacitance. This dynamic approach allows the circuit to maintain proper impedance matching across different operating conditions, including the increased backoff range required for high-efficiency operation with large PAPR signals.
Data Source
AI summary
A Doherty amplifier including: a first transistor for main amplifier having a source-to-drain parasitic capacitance and operating in class AB; a transmission line whose input end is connected to an output end of the first transistor and whose output end is connected to a composite point; a second transistor for auxiliary amplifier having a source-to-drain parasitic capacitance and operating in class C; a series capacitor whose input end is connected to an output end of the second transistor and whose output end is connected to the composite point, and to reduce the capacitance value of impedance seen from the composite point toward the output end of the second transistor at the time of a backoff operation; and an output matching circuit connected between the composite point and a point of connection to an output load, to match the impedance of the composite point to the impedance of the output load.