Doherty Power Amplifier Biasing for Linear RF Efficiency
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Solution Overview
Problem
Doherty power amplifiers (PAs) are traditionally considered unsuitable for linear PA applications in handsets due to size, complexity, and non-linear behavior, requiring predistortion linearizers in base station applications.
Innovation Solution
A power amplifier system that splits an RF signal into two portions with different phases and powers, using a Doherty amplifier circuit with a carrier amplifier and a peaking amplifier, where the carrier amplifier operates in Class AB mode and the peaking amplifier operates in Class B mode, to reduce AM/AM and AM/PM distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a Doherty PA is used for linear PA applications, then power efficiency is improved, but device complexity and non-linear behavior increase
Solution Approach 1:
The RF signal is divided into two separate portions with different phases and powers, which are then amplified by separate carrier and peaking amplifier paths before being recombined. This segmentation allows each amplifier to operate in optimized bias modes (Class AB for carrier, Class B for peaking) to achieve high power efficiency while managing complexity through functional division
Solution Approach 2:
Different biasing modes are applied to different amplifier paths based on their specific functions. The carrier amplifier uses Class AB biasing for linear operation, while the peaking amplifier uses Class B biasing for efficiency at peak powers. This localized optimization of operating conditions resolves the contradiction between efficiency and linearity
2Use of energy by moving object
If a Doherty PA is used for linear PA applications, then power efficiency is improved, but non-linear behavior increases
Solution Approach 1:
The signal is intentionally split into asymmetric portions with different phases (10-20 degrees apart) and different power levels. The carrier path handles the majority of the signal at lower power, while the peaking path handles the peak portions at higher power. This asymmetric division allows each amplifier to operate in its optimal efficiency region while maintaining overall linearity
Solution Approach 2:
The patent changes the operating parameters of the amplifiers by applying different biasing modes (Class AB vs Class B) and adjusting signal phases and power levels. These parameter changes enable the system to achieve high power efficiency across varying output power conditions while maintaining linear operation through the specific phase and power relationships between the two amplifier paths
3Device complexity
If equal phases and equal powers are used in amplifier paths, then device complexity is reduced, but AM/AM and AM/PM distortion increase
Solution Approach 1:
The patent introduces specific phase shifts (10-20 degrees) and power imbalances between the two amplifier paths to optimize linearity. By carefully controlling these parameters, the system achieves reduced AM/AM and AM/PM distortion without requiring additional complex linearization circuits, thus resolving the contradiction between simplicity and linearity
Data Source
AI summary
Systems and methods related to linear load modulated power amplifiers. A power amplifier (PA) system can include a divider that splits a signal into two portions, a first portion directed to an attenuator that attenuates the first portion so that the first portion and the second portion have different powers and a second portion directed to a phase shift component that shifts a phase of the second portion so that the first portion and the second portion have different phases. The PA system can also include a Doherty amplifier circuit where a carrier amplifier amplifies the attenuated first portion and a peaking amplifier amplifies the phase-shifted second portion. The carrier amplifier includes a Class AB driver stage and a Class B output. The peaking amplifier includes a Class B driver stage a Class B output stage.