RF Power Amplifier Predistorter Using Nonlinear MOSFET Capacitance

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

In mobile wireless telephone handsets, power amplifiers face challenges in achieving linear operation without consuming excessive current, degrading noise performance, or sacrificing bias voltage gain controllability, especially when driven by large RF signals.

Innovation Solution

The implementation of a power amplifier circuit that includes an amplifier MOSFET and a predistorter MOSFET, where the predistorter provides nonlinear capacitance at the gate terminal of the amplifier MOSFET, effectively canceling out distortion by capacitively dividing the gate-source voltage between linear and nonlinear capacitances.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If power amplifier current is increased to promote linear operation, then linearity is improved, but current consumption increases and battery life decreases

Engineering Contradiction:

ImprovelinearityVSAvoidcurrent consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The predistorter circuit pre-distorts the input signal before it reaches the power amplifier by introducing a nonlinear capacitance that counteracts the amplifier's nonlinearities. This preliminary action allows the amplifier to operate more linearly at lower current levels, resolving the contradiction between linearity and current consumption

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The predistorter MOSFET acts as an intermediary element between the input signal and the power amplifier. It introduces a compensating nonlinear capacitance that mediates the signal transformation, enabling linear operation without requiring excessive current from the amplifier

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If power amplifier current is increased to promote linear operation, then linearity is improved, but noise performance degrades

Engineering Contradiction:

ImprovelinearityVSAvoidnoise performance

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The predistorter pre-compensates for nonlinearities before the signal enters the power amplifier, allowing the amplifier to operate in a more linear region at lower current levels. This preserves noise performance while achieving the desired linearity through advance signal conditioning

Inventive Principle:

Principle #10Preliminary action

3Reliability

If power amplifier current is increased to promote linear operation, then linearity is improved, but bias voltage gain controllability is sacrificed

Engineering Contradiction:

ImprovelinearityVSAvoidbias voltage gain controllability

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The predistorter circuit pre-distorts the input signal to compensate for amplifier nonlinearities, enabling the amplifier to maintain linear operation at lower current levels. This preserves the effectiveness of bias voltage control for gain adjustment without the need for excessive current

Inventive Principle:

Principle #10Preliminary action

4Power

If a transconductance amplifier is driven by a relatively large signal, then output power is increased, but nonlinear distortion increases due to transistor effects

Engineering Contradiction:

Improveoutput powerVSAvoidlinearity

Core Design Contradiction:

PowerVSReliability

Solution Approach 1:

The predistorter introduces a nonlinear capacitance that produces a distorting effect opposite to that of the power amplifier transistor. This preliminary anti-action counteracts the transistor's nonlinearities, allowing large signal operation with reduced distortion

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The predistorter deliberately introduces nonlinear distortion through the predistorter MOSFET's nonlinear capacitance, which then cancels the harmful nonlinearities of the power amplifier transistor. The harmful distortion is converted into a beneficial cancellation effect

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 configuration enhances the linearity of the transconductance amplifier while maintaining low current consumption and preserving bias voltage gain controllability, as demonstrated by improved linear performance in RF power amplifier circuits.

Implementation Method 1

The source and drain terminals of the predistorter MOSFET are connected together so that it provides a nonlinear capacitance at the gate terminal of the amplifier MOSFET. The gate-source voltage of the amplifier MOSFET is the input voltage signal capacitively divided between the input linear coupling capacitance and the combined non-linear capacitances of the amplifier MOSFET and predistorter MOSFET.

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS9048802B2Radio frequency power amplifier with linearizing predistorter

Publication Date: 2015.06.02 SKYWORKS SOLUTIONS INC

AI summary

A power amplifier circuit includes an amplifier MOSFET and a predistorter MOSFET. The predistorter MOSFET source and drain are connected together, and the predistorter MOSFET is connected between the gate of the amplifier MOSFET and a second bias voltage signal. This biasing of the predistorter MOSFET causes it to provide a nonlinear capacitance at the gate of the amplifier MOSFET. The combined non-linear capacitances of the amplifier MOSFET and predistorter MOSFET provide predistortion that promotes cancellation of the distortion or nonlinearity contributed by the amplifier MOSFET alone.