Two-Stage Microwave Power Amplifier Biasing for Low IM3
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Solution Overview
Problem
Microwave power amplifiers face challenges in simultaneously achieving high efficiency and low third-order intermodulation (IM3) distortion, as existing technologies often compromise between efficiency and linearity, and conventional cascading arrangements can promote intermodulation harmonics and distortion.
Innovation Solution
A two-stage amplifier configuration is employed, where a low distortion first stage is coupled with a high efficiency second stage, using a dynamic biasing method that tracks input and output power levels to modulate DC bias voltages, and a post-distortion approach is used to compensate for distortion introduced by the first stage, with the second stage biased between class-B and class-C for high output power and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional cascading arrangements are used to achieve high efficiency, then efficiency is improved, but intermodulation harmonics and distortion increase
Solution Approach 1:
The amplifier is divided into two distinct stages: a first stage optimized for low distortion with class A or class AB biasing, and a second stage optimized for high efficiency with class B or class C biasing. This segmentation allows each stage to perform its specialized function, resolving the contradiction between efficiency and distortion.
Solution Approach 2:
Different biasing conditions are applied to different stages of the amplifier. The first stage uses class A or class AB biasing to minimize distortion, while the second stage uses class B or class C biasing to maximize efficiency. This local differentiation of operating characteristics resolves the contradiction.
2Use of energy by moving object
If class B or class C biasing is used to improve efficiency, then efficiency is improved, but linearity and distortion performance deteriorate
Solution Approach 1:
The amplifier functionality is segmented into distortion management (first stage) and efficiency optimization (second stage). The first stage handles linearity requirements with class A/AB biasing, while the second stage handles efficiency requirements with class B/C biasing, resolving the contradiction between linearity and efficiency.
Solution Approach 2:
The first stage acts as an intermediary that pre-processes the signal to minimize distortion before it enters the high-efficiency but high-distortion second stage. This intermediary stage protects the overall system linearity while allowing the second stage to operate at high efficiency.
3Reliability
If class A biasing is used to improve linearity, then linearity is improved, but efficiency deteriorates
Solution Approach 1:
The amplifier is segmented so that only the first stage operates in class A or class AB for linearity, while the second stage operates in class B or class C for efficiency. This segmentation allows the system to achieve both linearity and efficiency, unlike a pure class A design.
Solution Approach 2:
Instead of applying class A biasing to the entire amplifier (excessive action for linearity), class A/AB biasing is applied only to the extent necessary in the first stage, while the second stage uses more efficient class B/C biasing. This partial application resolves the efficiency penalty of full class A operation.
Data Source
AI summary
A post-distortion method for cascading amplifier stages in a two-stage microwave power amplifier and a dynamic biasing method using back-end processing for correcting nonlinearity in the power amplifier output. A first or driver stage biased in a near-A region with low distortion is cascaded with a second or power stage biased in a near-C region with high efficiency. The amplitude and phase responses of the two stages compensate another to yield a more linear overall gain for the overall power amplifier. The dynamic biasing scheme modulates the source to drain voltages of the transistors used in the amplifier stages based on the harmonics in amplifier output in order to minimize the harmonics and correct non-linearity in the output.