Bandwidth-Compressed Envelope Tracking for Linear Power Amplifiers
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Solution Overview
Problem
Power amplifiers face efficiency trade-offs between linearity and heat dissipation, with full-bandwidth envelope tracking leading to inefficiencies and non-linearity, especially in digital modulation schemes, resulting in significant power loss as heat and distortion.
Innovation Solution
Implementing a bandwidth-compressed envelope tracking signal that is narrower than the full-bandwidth signal, using a tapped delay line and filtering to identify peak magnitudes and modulate the power amplifier supply node, thereby enhancing efficiency and maintaining linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed power supply is used to operate the power amplifier, then linearity is improved, but efficiency deteriorates with significant power dissipated as heat
Solution Approach 1:
The power supply voltage is made dynamic by modulating it according to the envelope of the input signal. The envelope detector extracts the signal envelope, and this envelope signal controls the power supply voltage to the amplifier, allowing the amplifier to operate efficiently across varying signal conditions while maintaining linearity through appropriate biasing.
Solution Approach 2:
The operating parameters of the power amplifier are changed by varying the power supply voltage in accordance with the signal envelope. This parameter modulation allows the amplifier to transition between different operating regions, achieving high efficiency when the signal is present while maintaining linearity through proper envelope tracking.
2Loss of energy
If the power amplifier is operated closer to amplitude compression to improve efficiency, then efficiency is improved, but linearity deteriorates causing distortion and noise
Solution Approach 1:
The amplifier operates dynamically near compression for efficiency while the envelope tracking supply modulation maintains linearity. The power supply voltage follows the signal envelope, allowing the amplifier to be driven harder without sacrificing linearity, as the supply voltage adjusts to prevent actual compression.
Solution Approach 2:
The envelope detector provides feedback about the signal amplitude, which is used to modulate the power supply voltage. This feedback mechanism ensures the amplifier receives appropriate supply voltage to maintain linearity even when operated near compression, preventing distortion while maximizing efficiency.
3Measurement precision
If a full-bandwidth envelope signal is used for envelope tracking, then the envelope tracking is accurate, but the signal chain becomes inefficient and requires excessive bandwidth
Solution Approach 1:
Only the essential envelope information is extracted from the full-bandwidth signal. The envelope detector extracts the amplitude modulation component, which contains the necessary information for power supply modulation. This extracted envelope signal has much lower bandwidth requirements, improving signal chain efficiency while maintaining accurate envelope tracking.
Solution Approach 2:
Instead of processing the full-bandwidth signal, a simplified copy of the essential envelope information is created. The envelope detector produces a low-bandwidth replica of the amplitude variations, which is sufficient for controlling the power supply voltage and achieves the same envelope tracking function with reduced bandwidth requirements.
Data Source
AI summary
An envelope tracking scheme can be used, such as to modulate a supply node of a power amplifier circuit to improve efficiency. For example, a magnitude or amplitude envelope of a signal to be modulated can be scaled and used to drive a node, such as a drain, of the power amplifier circuit. An envelope tracking signal can be generated such as having a bandwidth that is compressed as compared to a full-bandwidth envelope signal. A peak-value “look ahead” technique can be used, for example, so that amplitude compression or clipping of the transmit signal is suppressed when the bandwidth-compressed envelope tracking signal is used to modulate a supply node of the power amplifier used to amplify the transmit signal.