Analog Amplitude Pre-Distortion Biasing for RF Amplifier Linearity
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Solution Overview
Problem
Existing WLAN FEM products face challenges in achieving high linearity, particularly in meeting error-vector-magnitude (EVM) levels below −47 dB as required by advanced WiFi standards such as 802.11ax and 802.11be.
Innovation Solution
An analog amplitude pre-distortion circuit is introduced, comprising an RF input, an amplifier stage, and a bias circuit with a detector stage and a bias application stage. The bias circuit detects the amplitude of the RF signal, produces a correction signal, and applies a bias voltage at specific frequency ranges and impedances to the amplifier stage, thereby improving linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If an amplifier stage is used to amplify RF signal, then signal power is increased, but gain expansion and gain compression occur leading to poor linearity
Solution Approach 1:
The bias application stage applies a pre-distortion bias voltage to the amplifier stage input before amplification occurs. This preliminary action compensates for the expected non-linearities (gain expansion and compression) that will occur during amplification, thereby improving linearity while maintaining the power amplification function
Solution Approach 2:
The detector stage continuously monitors the amplitude of the RF signal and generates a correction signal based on detected amplitude variations. This feedback mechanism dynamically adjusts the bias voltage applied to the amplifier stage, compensating for non-linearities in real-time and improving overall linearity
2Manufacturing precision
If a bias voltage is applied to correct gain expansion, then linearity is improved, but additional circuit complexity is introduced
Solution Approach 1:
The detector stage and bias application stage are integrated into a unified bias circuit that works in conjunction with the amplifier stage. The detector and bias application functions are merged into a coordinated system that shares common components and signal paths, reducing overall circuit complexity while maintaining linearity correction capability
Data Source
AI summary
An analog amplitude pre-distortion circuit includes a Radio Frequency, RF, input for receiving an RF signal. The circuit also includes an amplifier stage comprising an amplifier stage input for receiving the RF signal from the RF input, where the amplifier stage is operable to amplify the RF signal to produce an amplified RF signal. The circuit further includes a bias circuit. The bias circuit includes a detector stage for detecting an amplitude of the RF signal, and for producing a correction signal based on the amplitude of the RF signal. The bias circuit also includes a bias application stage coupled to the amplifier stage input.