Analog Amplitude Pre-Distortion Circuit for Low-EVM RF Amplifiers
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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 -47dB or lower for standards like 802.11ax and 802.11be, due to limitations in RF signal amplification 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 different frequency ranges to the amplifier stage, reducing gain expansion and improving linearity by presenting varying impedances to the amplifier stage input.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional RF amplification is used, then the amplifier can provide sufficient gain, but gain expansion and AM-to-AM distortion occur reducing linearity
Solution Approach 1:
The bias circuit applies preliminary amplitude-dependent biasing to the amplifier stage before the RF signal is amplified. By detecting the RF signal amplitude and applying appropriate bias voltages through the transistor and variable reactance component, the amplifier's gain is pre-adjusted to compensate for expected gain expansion and distortion, thereby improving linearity while maintaining sufficient gain.
2Device complexity
If a fixed bias circuit is used, then the circuit is simple, but it cannot compensate for amplitude-dependent gain variations
Solution Approach 1:
The bias circuit transitions from a fixed configuration to a dynamic one by incorporating a variable reactance component (such as a variable capacitor or inductor) controlled by a controller. This allows the bias impedance to be adjusted in real-time based on the RF signal amplitude, enabling the circuit to compensate for amplitude-dependent gain variations and improve linearity while managing complexity through programmable control.
3Manufacturing precision
If amplitude-dependent biasing is applied across all frequencies, then linearity improves, but low-frequency biasing is degraded
Solution Approach 1:
The bias circuit implements frequency-dependent biasing by using different impedance paths for different frequency ranges. A capacitor coupled to the control terminal provides a low-impedance path for low-frequency bias signals while presenting high impedance to RF frequencies. This allows amplitude-dependent biasing to be applied effectively at low frequencies for linearity improvement without degrading the bias response or introducing distortion at RF frequencies.
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 RF amplification, 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 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... The bias circuit is operable to: detect an amplitude of the RF signal
Implementation Method 2
a capacitor coupled between the control terminal and the reference potential
Implementation Method 3
a variable reactance component coupled to the control terminal... present second impedances at a second range of frequencies to the amplifier stage input
Data Source
Figure 1
Figure 2~3B
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.