Pre-emphasis Filter for Envelope Tracking Power Amplifier Bandwidth
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Solution Overview
Problem
The power efficiency of power amplifiers in wireless devices is degraded due to low-pass transfer functions in envelope tracking paths, which fail to maintain sufficient bandwidth for high-frequency transmit signals, leading to RF output distortion and increased out-of-band emissions.
Innovation Solution
Applying pre-emphasis to the envelope tracking signal to compensate for the frequency response of the envelope tracking path, using a pre-emphasis filter that estimates and corrects for the amplitude and phase response, ensuring the power amplifier supply voltage maintains sufficient bandwidth and headroom to accurately track the transmit signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low-pass transfer function circuit path is used to produce the PA supply voltage, then the circuit is simple and stable, but the bandwidth of the PA supply voltage is insufficient for high-bandwidth transmit signals
Solution Approach 1:
The pre-emphasis filter applies preliminary action by boosting high-frequency components of the envelope signal before they enter the low-pass transfer function circuit path. This anticipates the bandwidth limitation and compensates for it in advance, allowing the circuit to maintain stability while achieving sufficient bandwidth for high-bandwidth transmit signals like LTE
2Device complexity
If the PA supply voltage bandwidth is limited by the circuit path, then the circuit design is simpler, but the power amplifier efficiency is degraded
Solution Approach 1:
The pre-emphasis filter changes the frequency domain parameters of the envelope signal by applying a frequency-dependent gain that boosts high-frequency components. This parameter transformation allows the signal to maintain sufficient bandwidth characteristics after passing through the low-pass circuit path, thereby preserving power amplifier efficiency without increasing circuit complexity
3Stability of the object's composition
If high-frequency components of the envelope signal are attenuated, then the circuit path is more stable, but the envelope tracking performance is degraded
Solution Approach 1:
The pre-emphasis filter applies preliminary anti-action by intentionally boosting high-frequency components that would otherwise be attenuated by the low-pass circuit path. This counteracts the expected attenuation effect before it occurs, ensuring that the envelope tracking accuracy is maintained despite the inherent stability limitations of the low-pass circuit path
Data Source
AI summary
Embodiments provide systems and methods for producing an envelope tracking (ET) PA supply voltage with minimum headroom relative to the desired RF output signal of the PA. In an embodiment, the PA supply voltage is generated by applying an ET signal to an ET path of the wireless device, and by further applying pre-emphasis to the ET signal to compensate for a frequency response of the ET path. Embodiments operate by estimating the frequency response of the ET path and determining the pre-emphasis based on the estimate of the frequency response of the ET path. In embodiments, the pre-emphasis is applied using a pre-emphasis filter, which may be within or outside of the ET path. Further, the pre-emphasis may be applied to the ET signal in digital or analog form.