Load-Modulated RF Amplifier Gain Shaping for Linear Predistortion

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Solution Overview

Problem

Designing a satisfactory radio-frequency power amplifier for electronic devices is challenging due to its non-linear gain characteristics, particularly in load-line modulated amplifiers, which exhibit gain expansion and abrupt transitions, making it difficult to implement digital predistortion circuits effectively.

Innovation Solution

Implementing a load modulated radio-frequency amplifier with an adjustable load impedance and a gain shaping circuit that dynamically tunes the load impedance based on instantaneous signal amplitude, combined with a digital predistortion circuit to linearize the amplifier gain by predistorting baseband signals and adjusting the load impedance accordingly.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a load-line modulated amplifier is used to achieve wide dynamic range, then the amplifier can operate across a wide range of signal power levels, but the gain becomes non-linear with abrupt transitions and gain expansion

Engineering Contradiction:

Improvedynamic rangeVSAvoidgain linearity

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The patent applies dynamics by making the load impedance adjustable and controllable. The load impedance is dynamically tuned based on the instantaneous signal amplitude to maintain optimal amplifier performance across different operating conditions, enabling smooth gain transitions while preserving wide dynamic range capability

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent changes the load impedance parameter as a function of signal amplitude. By adjusting the load impedance dynamically according to the instantaneous signal level, the amplifier maintains linear gain characteristics across its entire operating range, resolving the contradiction between wide dynamic range and gain linearity

Inventive Principle:

Principle #35Parameter changes

2Stability of the object's composition

If digital predistortion is applied to linearize amplifier gain, then gain linearity improves, but the complexity of the system increases due to additional circuits and signal processing

Engineering Contradiction:

Improvegain linearityVSAvoidcircuit complexity

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

The patent introduces a gain shaping circuit as an intermediary element that sits between the amplifier core and the adjustable load. This circuit dynamically adjusts the load impedance based on signal characteristics, providing gain linearization through a relatively simple analog mechanism rather than complex digital predistortion processing

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent replaces complex digital signal processing and predistortion circuits with an analog gain shaping mechanism that directly controls the load impedance. This substitution simplifies the overall system by using continuous analog control instead of discrete digital processing steps

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Ease of manufacture

If the load impedance is kept constant, then the amplifier circuit is simpler to design and implement, but the gain transitions abruptly between different signal power levels

Engineering Contradiction:

Improvedesign simplicityVSAvoidgain transition smoothness

Core Design Contradiction:

Ease of manufactureVSStability of the object's composition

Solution Approach 1:

The patent transitions from a static, fixed load impedance design to a dynamic, adjustable load impedance system. The load impedance is continuously adapted based on the instantaneous signal amplitude, enabling smooth gain transitions while maintaining reasonable design complexity through systematic control approaches

Inventive Principle:

Principle #15Dynamics

4Power

If the amplifier operates at high signal power levels, then the output power is sufficient for transmission, but the non-linear gain characteristics become more pronounced

Engineering Contradiction:

Improveoutput powerVSAvoidgain characteristics

Core Design Contradiction:

PowerVSStability of the object's composition

Solution Approach 1:

The patent dynamically changes the load impedance parameter in response to signal power level variations. At high signal power levels, the load impedance is adjusted to compensate for non-linear effects, maintaining stable and linear gain characteristics while allowing the amplifier to operate at high output power levels

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS12537548B2Load modulated radio-frequency amplifier with digital predistortion

Publication Date: 2026.01.27 APPLE INC

AI summary

An electronic device may include wireless circuitry. The wireless circuitry may include at least a digital predistortion circuit, an upconversion circuit, and a load-line modulated amplifier circuit. The digital predistortion circuit can be configured to receive a reference baseband signal from one or more processors and to selectively output a predistorted version of the reference baseband signal. The upconversion circuit can be configured to receive a signal from the digital predistortion circuit and to output a radio-frequency signal. The load-line modulated amplifier circuit can be configured to amplify the radio-frequency signal. The load-line modulated amplifier circuit can include an adjustable load component. The adjustable load component can have a constant impedance when an instantaneous signal amplitude of the reference baseband signal is within a first range and can be tuned to have a varying impedance when the instantaneous signal amplitude of the reference baseband signal is within a second range.