RF Amplifier Bias Circuit With Filtered Stable Bias Point
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Solution Overview
Problem
Heterojunction bipolar transistors in power amplifiers for mobile communication terminals experience thermal resistance issues, leading to decreased transconductance and gain, causing linearity deterioration and RF signal distortion, especially in linear modulation schemes.
Innovation Solution
A bias circuit configuration using four bipolar transistors and a filter circuit to stabilize bias signals, attenuate high-frequency components, and maintain a stable bias point, preventing nonlinear operation and gain fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the RF signal output is increased, then the power output of the amplifier is improved, but the thermal resistance causes heat generation leading to decreased transconductance and gain, resulting in linearity deterioration
Solution Approach 1:
The patent changes the bias point parameter dynamically based on output level. At low and middle output levels, the bias point is set to a first value that prevents linearity deterioration, while at high output levels, it switches to a second value that allows for different performance characteristics. This parameter switching resolves the contradiction by adapting the bias condition to the operating region.
Solution Approach 2:
The invention introduces dynamic control of the bias point through a control circuit that monitors the output level and adjusts the bias accordingly. The bias point transitions from a static fixed value to a dynamic variable that changes with operating conditions, enabling the system to maintain optimal performance across different power output levels while preventing linearity deterioration in the critical low and middle output regions.
2Device complexity
If a conventional bias circuit is used, then the circuit complexity is low, but the bias point becomes unstable at high output levels causing gain fluctuations and nonlinear operation
Solution Approach 1:
The patent implements a feedback mechanism where the control circuit monitors the output level of the amplifier and uses this information to adjust the bias point of the transistor. The feedback signal is derived from the output and fed back to the bias control, creating a closed-loop system that automatically stabilizes the bias point according to the operating conditions, preventing the instability that occurs in conventional open-loop bias circuits.
Solution Approach 2:
The invention introduces a control circuit as an intermediary element between the power amplifier and the bias supply. This intermediary component processes the output level information and generates appropriate bias control signals, acting as a mediator that translates output conditions into suitable bias adjustments, thereby stabilizing the bias point without requiring direct modification of the power amplifier itself.
Data Source
AI summary
A bias circuit includes first and second bipolar transistors, first and second field-effect transistors, and a filter circuit. The first field-effect transistor supplies a bias signal to an amplifier. The filter circuit is connected between a collector terminal of the first bipolar transistor and the ground through a base terminal of the first bipolar transistor. The filter circuit has frequency characteristics for attenuating a high frequency component of an RF signal to be input to the amplifier.