Passive Phase Compensation in Doherty Amplifier Input Paths
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Solution Overview
Problem
Doherty amplifiers face efficiency losses due to phase coherency issues between main and peaking amplifier paths, leading to non-ideal load modulation and reduced peak power capability across the operational band, especially at high power levels.
Innovation Solution
Incorporating passive phase and delay compensation circuits in the main and peaking input paths of Doherty amplifiers, utilizing transmission line based series delay elements and shunt 90-degree short-circuit stubs to equalize group delays and optimize load impedance dispersion, thereby ensuring phase coherency and maximizing efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the Doherty amplifier operates with standard impedance inversion at the band center frequency, then the main amplifier sees a high value load line impedance at backoff, but phase coherency is lost at frequencies other than the center frequency due to group delay differences
Solution Approach 1:
The patent applies preliminary action by introducing phase compensation circuits in advance to equalize the group delays of the main and peaking amplifier paths before the phase coherency loss occurs. This is achieved by adding series delay elements and shunt stubs that pre-correct the phase differences, ensuring that both paths arrive at the summing node with matched phases across the entire frequency band, not just at the center frequency.
Solution Approach 2:
The patent uses intermediary elements (phase compensation circuits consisting of series delay elements and shunt stubs) as mediators between the main and peaking amplifier paths. These intermediaries equalize the group delays by introducing controlled phase shifts, allowing the two paths to maintain phase coherency across the frequency band while still achieving the required impedance inversion characteristics.
2Power
If the peaking amplifier conduction increases to supply more current at high power levels, then the load line impedance of the main amplifier output decreases, but this causes non-ideal load modulation and reduces peak power capability
Solution Approach 1:
The patent applies feedback principles by using the phase compensation circuits to continuously monitor and correct the phase relationships between the main and peaking amplifier paths. The compensation circuits provide feedback equalization that maintains ideal load modulation characteristics across the frequency band, ensuring that the dynamic load line changes in the main amplifier path do not degrade peak power capability.
3Manufacturing precision
If the impedance inverter is designed for 90 degrees electrical length at band center frequency, then correct load modulation is achieved at the center frequency, but bandwidth limitations cause phase coherency loss at other frequencies
Solution Approach 1:
The patent applies dynamics by making the phase compensation circuits frequency-adaptive. The series delay elements and shunt stubs are designed to provide frequency-dependent phase shifts that dynamically equalize the group delays across the entire frequency band. This allows the system to maintain both correct load modulation at the center frequency and phase coherency across the full bandwidth, effectively extending the operational versatility.
Data Source
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AI summary
An embodiment of a Doherty amplifier includes first and second amplifier paths with first and second amplifiers, respectively, a power divider, a series delay element, and a short-circuited stub. The power divider is configured to receive a radio frequency (RF) signal and to divide the RF signal into first and second input signals that are produced at first and second power divider outputs. The series delay element is coupled between the first power divider output and the first amplifier. The short-circuited stub is coupled between the first power divider output and the first amplifier or between the second power divider output and the second amplifier. The first amplifier path is characterized by a first frequency-dependent insertion phase, the second amplifier path is characterized by a second frequency-dependent insertion phase, and a slope of the first or second frequency-dependent insertion phase is altered by the short-circuited stub.