Parallel Bipolar RF Amplifier Bias Offsets for IM3 Cancellation
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Solution Overview
Problem
RF amplifiers with bipolar transistors suffer from third-order intermodulation distortion (IM3) due to their non-linear behavior, causing interference with adjacent transmission channels, and existing techniques for reducing this distortion are not sufficient to achieve optimal linearity.
Innovation Solution
The RF amplifier design incorporates a parallel arrangement of branches with bipolar transistors in a degenerative emitter configuration, where each branch has a base coupled to the input and a collector coupled to a common collector node, with emitter degeneration impedance between the emitter and a common rail, and the bases are biased at different voltages to ensure IM3 distortion currents are in antiphase, effectively canceling out the third-order Taylor coefficient.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If bipolar transistors are used in RF amplifiers, then high gain and efficiency are achieved, but third-order intermodulation distortion (IM3) increases causing interference with adjacent channels
Solution Approach 1:
The amplifier is divided into multiple parallel branches, each containing a bipolar transistor with different bias voltage. This segmentation allows each transistor to contribute differently to the overall transfer characteristic, enabling cancellation of third-order distortion terms while maintaining high gain operation.
Solution Approach 2:
Each transistor branch is given a different local bias condition (different bias voltages) to create specific transfer characteristics. This local quality differentiation enables the combined output to have improved linearity while individual transistors operate in their optimal high-gain region.
2Reliability
If out-of-band matching is used to reduce IM3 distortion, then linearity is improved, but the solution is still insufficient to achieve optimal linearity
Solution Approach 1:
The bias voltage parameter is varied across different transistor branches to fundamentally change the transfer characteristic and enable distortion cancellation. This parameter change approach provides more precise control over linearity compared to traditional out-of-band matching techniques.
Solution Approach 2:
The amplifier uses asymmetric biasing where each transistor branch operates at a different bias point, creating asymmetric transfer characteristics that when combined, cancel out the third-order distortion terms. This asymmetric approach provides superior linearity control compared to symmetric conventional designs.
Data Source
AI summary
An RF amplifier is described including an input, an output, a parallel arrangement of a first branch and at least one further branch, each branch comprising a bipolar transistor in a degenerative emitter configuration having a base coupled to the input, a collector coupled to a common collector node, and an emitter degeneration impedance arranged between the emitter and a common rail. The common collector node is coupled to the output, the base of the first branch bipolar transistor is biased at a first bias voltage and the base of the at least one further branch bipolar transistor is biased at a bias voltage offset from the first bias voltage. In operation of the RF amplifier a IM3 distortion current output by the first branch bipolar transistor is in antiphase to a IM3 distortion current output by the at least one further branch bipolar transistor.