Feedback Impedance Modulator for Power Amplifier AMPM Linearity
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Solution Overview
Problem
Power amplifiers face challenges in supporting wideband signal modulation due to high Peak-to-Average Power Ratio (PAPR), leading to distortion in Amplitude Modulation to Amplitude Modulation (AMAM) and Amplitude Modulation to Phase Modulation (AMPM) of output signals, particularly in advanced communication systems like 5G, where conventional Average Power Tracking (APT) techniques result in inefficiency and poor linearity.
Innovation Solution
A power amplifier with a pre-distortion circuit that corrects AMPM distortion by distorting phase at the input terminal in an opposite direction to the original distortion, using an impedance modulator with a variable capacitance that increases as power increases, effectively canceling out total AMPM distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If Average Power Tracking (APT) technique is used to support wideband signal with high PAPR, then the power amplifier can handle wide frequency bandwidth, but AMAM distortion and AMPM distortion occur due to nonlinearity of HBT and poor linearity
Solution Approach 1:
The patent applies pre-distortion technique by modifying the input signal characteristics before amplification. The pre-distortion circuit intentionally introduces opposite distortion to the input signal, so that when the nonlinear power amplifier processes this pre-distorted signal, the distortions cancel out, resulting in linear output signal. This preliminary action prevents AMPM and AMAM distortion before they occur in the main amplification stage.
Solution Approach 2:
The patent implements a feedback mechanism where the output signal is monitored and fed back to the pre-distortion circuit. This feedback loop allows the system to dynamically adjust the pre-distortion parameters based on actual amplifier performance, ensuring optimal linearity compensation across varying operating conditions and maintaining signal fidelity in wideband operation.
2Manufacturing precision
If bias voltage is increased to compensate AMPM distortion, then AMPM distortion is reduced, but power efficiency deteriorates significantly
Solution Approach 1:
Instead of increasing bias voltage after distortion occurs, the patent applies pre-distortion to the input signal that anticipates and counteracts the expected AMPM distortion. This preliminary modification of the input signal allows the amplifier to operate at optimal bias conditions while still achieving linear output, avoiding the efficiency penalty of increased bias voltage.
Solution Approach 2:
The patent changes the signal parameters (amplitude and phase) of the input signal dynamically based on operating conditions. The pre-distortion circuit modifies signal characteristics in real-time to compensate for amplifier nonlinearity, allowing the system to maintain linearity without changing the bias voltage or operating point, thus preserving power efficiency.
3Adaptability or versatility
If conventional APT technique is used for high PAPR signals, then wideband operation is enabled, but linearity deteriorates in high power region close to saturated power
Solution Approach 1:
The pre-distortion circuit applies signal modification before the high power amplification stage, preparing the input signal to compensate for expected nonlinear behavior in the high power region. This preliminary action ensures that even when operating close to saturated power, the output signal maintains linearity because the distortion was pre-corrected.
Solution Approach 2:
The patent implements dynamic pre-distortion that adapts to varying power levels and operating conditions. The pre-distortion parameters are adjusted in real-time based on the instantaneous signal conditions, allowing the system to maintain optimal linearity across the entire power range from low to high power regions, unlike static conventional APT techniques.
Data Source
AI summary
A power amplifier and power amplification circuit are described herein. An illustrative power amplifier is disclosed to include an input terminal, a drive amplifier connected to the input terminal, and an impedance modulator having a capacitance that is adjusted inversely and proportionately relative to a signal output by the drive amplifier, wherein the impedance modulator provides a feedback loop between an output of the drive amplifier and the input terminal.