RF Transmitter Predistortion MOSFET for PA Linearity
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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
Engineering 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
Solution Approach 1:
The predistorter circuit applies preliminary nonlinear transformation to the input signal before it reaches the power amplifier. This predistortion compensates for the amplifier's nonlinear characteristics in advance, allowing the amplifier to operate in a more linear region and reducing the need for excessive current to achieve linearity.
Solution Approach 2:
The invention changes the capacitance parameters dynamically by using voltage-controlled variable capacitors in the predistorter circuit. By adjusting capacitance values based on operating conditions, the system optimizes linearity across different power levels without requiring constant high current operation.
2Reliability
If degeneration is combined with increased current to promote linearity, then linearity is improved, but bias voltage gain controllability is hampered
Solution Approach 1:
The invention separates the linearity enhancement function from the gain control function. The predistorter handles linearity compensation independently while the bias voltage circuit maintains full controllability of gain, avoiding the trade-off present in degenerated amplifier configurations.
3Measurement precision
If transconductance amplifier operates at increased current levels to meet noise performance, then noise performance is improved, but current consumption increases
Solution Approach 1:
The predistorter circuit performs preliminary signal conditioning that reduces the need for high current operation in the transconductance amplifier. By pre-compensating for nonlinearities, the amplifier can achieve acceptable noise performance at lower current levels.
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 minimizing current consumption and maintaining bias voltage gain controllability, leading to improved RF power amplifier performance.
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
Data Source
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.