RF Pre-Distortion Bias Circuit for Gain Expansion and EVM
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Solution Overview
Problem
Current WLAN FEM products face challenges in achieving the required linearity, particularly in reducing error-vector-magnitude (EVM) levels to −47 dB or lower for standards like 802.11ax and 802.11be, due to limitations in amplifier stage linearity.
Innovation Solution
An analog amplitude pre-distortion circuit with a bias circuit comprising a transistor, resistor, variable reactance component, and capacitor, which detects RF signal amplitude and applies bias voltages at specific frequency ranges to the amplifier stage, reducing gain expansion and improving linearity by presenting different impedances at various frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If amplifier stage is used to amplify RF signal, then signal power is increased, but gain expansion and AM-to-AM distortion occur reducing linearity
Solution Approach 1:
The bias circuit applies a preliminary bias voltage to the amplifier stage input based on the detected RF signal amplitude. This pre-distortion action occurs before the main amplification process, allowing the amplifier to operate in a more linear region and reducing gain expansion and AM-to-AM distortion while maintaining the required signal power output.
2Manufacturing precision
If bias voltage is applied to amplifier stage input, then linearity is improved, but frequency response may be affected
Solution Approach 1:
The bias circuit dynamically adjusts the bias voltage applied to the amplifier stage input based on the detected RF signal amplitude. The circuit presents different impedances at different frequency ranges, with lower impedances at lower frequencies and higher impedances at higher frequencies. This dynamic adaptation maintains linearity improvement across the entire frequency spectrum without compromising the frequency response.
3Adaptability or versatility
If variable reactance component is used in bias circuit, then frequency-selective biasing is achieved, but device complexity increases
Solution Approach 1:
The variable reactance component in the bias circuit changes its electrical parameters (reactance value) based on the frequency of the RF signal. This allows the circuit to present different impedances at different frequency ranges, achieving frequency-selective biasing that maintains linearity across the operating bandwidth. The parameter change is automatic and controlled by the signal frequency itself, avoiding the need for complex external control mechanisms.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enhances the linearity of the RF amplification process, reducing gain expansion and AM-to-AM distortion, thereby achieving lower EVM levels and improving the overall performance of WLAN FEM products.
Implementation Method 1
a variable reactance component coupled to the control terminal... present second impedances at a second range of frequencies to the amplifier stage input; wherein the first impedances are lower than the second impedances and wherein the first range of frequencies are lower than the second range of frequencies
Data Source
AI summary
An analog amplitude pre-distortion circuit and method. The circuit includes an RF input for receiving an RF signal. The circuit also includes an amplifier stage comprising an amplifier stage input coupled to the RF input, wherein 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 transistor having a first current terminal, a second current terminal and a control terminal, wherein the first current terminal is coupled to the amplifier stage input and wherein the second current terminal is coupled to a reference potential. The bias circuit also includes a resistor coupled between the amplifier stage input and the control terminal. The bias circuit also includes a variable reactance component coupled to the control terminal. The bias circuit further includes a capacitor coupled between the control terminal and the reference potential.