MOSFET Predistorter Gate Capacitance 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 signals that cause nonlinear operation due to mobility degradation and velocity saturation in MOSFET transconductance amplifiers.
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, thereby promoting linear operation without excessive current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If power amplifier current is increased to promote linear operation, then linearity is improved, but battery life is reduced and current consumption increases
Solution Approach 1:
A predistorter circuit is introduced as an intermediary component between the input signal and the power amplifier. This predistorter pre-distorts the input signal in the opposite direction of the amplifier's expected nonlinearity, so that the combined effect produces a linear output. This allows the amplifier to operate at lower current levels while maintaining linearity through the predistortion compensation mechanism.
Solution Approach 2:
The predistorter applies preliminary distortion to the input signal before it reaches the power amplifier. By pre-compensating for the anticipated nonlinear effects in advance, the amplifier receives a pre-conditioned signal that requires less current adjustment during amplification, thereby reducing overall current consumption while maintaining output linearity.
2Measurement precision
If transconductance amplifier current is increased to meet noise performance requirements, then noise performance is improved, but linearity is degraded due to mobility degradation and velocity saturation
Solution Approach 1:
The predistorter acts as a mediator that compensates for the nonlinear effects (mobility degradation and velocity saturation) that occur when the transconductance amplifier operates at high current levels. By pre-distorting the input signal, the system can maintain both the high current operation needed for noise performance and the linearity required for faithful signal reproduction.
Solution Approach 2:
The patent converts the harmful nonlinear effects (mobility degradation and velocity saturation) into a beneficial predistortion pattern. By intentionally applying the opposite distortion in the predistorter, the harmful nonlinearities that occur in the amplifier are transformed into a net linear response, allowing high current operation for noise performance without sacrificing linearity.
3Adaptability or versatility
If bias voltage is used to control amplifier gain, then gain controllability is improved, but linearity is degraded when large signals drive the amplifier into nonlinear operation
Solution Approach 1:
The predistorter serves as an intermediary that compensates for the linearity loss caused by large signal operation. It pre-distorts the input signal based on the expected amplifier characteristics, so that even when the amplifier operates in its nonlinear region with bias voltage control, the overall system maintains linearity through the compensating predistortion.
Solution Approach 2:
The predistorter applies preliminary compensation for nonlinear effects before the signal enters the bias-controlled amplifier stage. This pre-action ensures that when large signals are applied and the amplifier operates with reduced linearity, the predistortion has already prepared the signal to achieve linear overall performance, preserving both gain controllability and linearity.
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 enhances the linearity of transconductance amplifiers, reducing nonlinear operation and maintaining noise performance and bias voltage gain controllability, as demonstrated by improved effective transconductance characteristics.
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.