Doherty Power Amplifier Load Modulation for Low-Power Efficiency
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Solution Overview
Problem
Radio frequency (RF) power amplifiers face efficiency degradation when operating at low instantaneous power levels due to optimized load impedance designed for maximum efficiency at high power levels, which adversely impacts low power applications.
Innovation Solution
A Doherty power amplifier architecture with separate main and auxiliary power amplifiers and dynamic load modulation techniques, along with digital pre-distortion and signal processing at the baseband level, to maintain high efficiency across both high and low power levels by adjusting load impedance and disabling the auxiliary PA at low power levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If load impedance is optimized for maximum power and efficiency at high power levels, then power efficiency is improved at high power levels, but power efficiency is significantly degraded at low instantaneous power levels
Solution Approach 1:
The power amplifier is divided into two separate signal paths: a main signal path with a first power amplifier optimized for high power operation, and an auxiliary signal path with a second power amplifier optimized for low power operation. Each path processes a portion of the input signal independently, allowing each amplifier to operate in its optimal efficiency range across different power levels.
Solution Approach 2:
The system dynamically adjusts the load impedance seen by each power amplifier through load modulation techniques. The main and auxiliary amplifiers share a common load that is dynamically modulated based on the instantaneous power level, enabling each amplifier to maintain optimal load conditions for its operating range.
2Reliability
If PA average output is backed-off to preserve PAPR headroom, then linearity is improved, but power efficiency is significantly degraded
Solution Approach 1:
By segmenting the power amplifier into main and auxiliary paths, the system can maintain linearity through the main amplifier while the auxiliary amplifier compensates for efficiency losses at low power levels, avoiding the need for excessive back-off.
Solution Approach 2:
The system changes the operating parameters of the two amplifiers differently: the main amplifier operates with higher back-off to ensure linearity, while the auxiliary amplifier operates closer to saturation to maintain efficiency, with the combined output achieving both linearity and efficiency.
3Device complexity
If a single power amplifier is used, then device complexity is reduced, but efficiency cannot be maintained across both high and low power levels
Solution Approach 1:
Both the main and auxiliary power amplifiers share a common load network and output combining structure. The load modulation network serves dual purposes: providing load matching for both amplifiers and enabling dynamic load modulation. This shared infrastructure reduces overall system complexity despite having two amplifiers.
Data Source
AI summary
In one embodiment, an apparatus includes: a digital baseband circuit to receive a digital baseband signal and output a first digital baseband signal and a second digital baseband signal, the second digital baseband signal comprising a scaled version of the first digital baseband signal; a first transmitter signal path coupled to the digital baseband circuit to process the first digital baseband signal and output a first radio frequency (RF) signal; a second transmitter signal path coupled to the digital baseband circuit to process the second digital baseband signal and output a second RF signal; a first power amplifier coupled to the first transmitter signal path to amplify the first RF signal and output an amplified first RF signal; and a second power amplifier coupled to the second transmitter signal path to amplify the second RF signal and output an amplified second RF signal.