Envelope Tracking Modulator Offset Current for Lower Amplifier Loss
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Solution Overview
Problem
Existing envelope tracking power supplies for RF power amplifiers suffer from significant power dissipation in the output stage of the linear amplifier due to asymmetry in the modulator output voltage, leading to unequal average source and sink currents and resulting in higher power dissipation in the current source compared to the current sink.
Innovation Solution
Incorporating a DC offset current at the high frequency output stage using a second voltage supply lower than the output stage voltage, provided via an inductor, to balance the average source and sink currents, thereby reducing power dissipation by modifying the current flow through the addition of an offset current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a linear amplifier is used in the high frequency path to track high frequency variations in the reference signal, then the power supply can provide high frequency tracking capability, but the output stage of the linear amplifier dissipates significant power due to asymmetry in the modulator output voltage
Solution Approach 1:
The patent applies asymmetry by introducing a DC offset current that deliberately creates an asymmetric current distribution between the current source and current sink. The current source carries both the AC signal current and the DC offset current, while the current sink carries only the AC signal current. This asymmetric arrangement compensates for the inherent asymmetry in the modulator output voltage waveform, balancing the average currents and reducing power dissipation in the output stage.
2Reliability
If the average source current and sink current are unequal due to asymmetry in modulator output voltage, then the linear amplifier can track the reference signal, but the current source dissipates more power than the current sink
Solution Approach 1:
The patent changes the parameter of current distribution by introducing a DC offset current. This modifies the operating point of the output stage, shifting the average current levels of the current source and current sink. By adjusting this DC offset parameter, the system achieves equal average currents while maintaining the ability to track the reference signal accurately.
3Loss of energy
If a DC offset current is added to balance the average source and sink currents, then power dissipation is reduced, but an additional voltage supply is required
Solution Approach 1:
The patent introduces a DC offset current as an intermediary element that mediates between the asymmetric voltage waveform and the current balancing requirement. This DC offset current acts as a bridge, allowing the system to achieve current balance without requiring complex circuit modifications. The additional voltage supply serves solely to provide this DC offset, making it a minimal complexity addition that achieves the desired power dissipation reduction.
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
This approach reduces power dissipation in the output stage by approximately 20% by adjusting the current flow, minimizing the power dissipated in the current source and increasing it in the current sink, thereby enhancing the efficiency of the power modulator.
Implementation Method 1
The DC offset current may be provided via an inductor connected between the further power supply and the high frequency output.
Data Source
AI summary
There is disclosed an envelope tracking power supply arranged to generate a modulated supply voltage in dependence on a reference signal, comprising a first path for tracking low frequency variations in the reference signal and a second path for tracking high frequency variations in the reference signal, the second path including a linear amplifier, wherein the output of the linear amplifier comprises a current source and a current sink connected to the high frequency output, there further being provided a DC offset current at the high frequency output.