Transconductor Predistortion Circuit for Linearity Correction
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Solution Overview
Problem
Existing transconductors in radio frequency transmitters suffer from non-linear voltage-to-current characteristics, leading to reduced signal-to-noise ratio and increased noise performance, especially in modern integrated circuits with limited supply voltage, making them unsuitable for advanced modulation schemes like GSM, EDGE, and UMTS.
Innovation Solution
A transconductor circuit with a predistortion circuit that includes a model transconductor and a feedback network to correct the non-linear voltage-to-current transfer characteristic, allowing the output transconductor to operate linearly by using a scaled replica transconductor and amplifier to adjust the control voltage, thereby maximizing signal range and noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If linearizing resistors and operational amplifiers are added to transistors to form linear transconductors, then the linearity of voltage-to-current characteristics is improved, but the noise performance deteriorates and device complexity increases
Solution Approach 1:
The patent employs a replica transconductor that copies the electrical characteristics of the main transconductor. The replica device replicates the non-linear V-I characteristics without requiring linearizing circuitry, thereby achieving linearity correction through comparison while avoiding the noise and complexity of traditional linearizing components.
Solution Approach 2:
The patent implements a feedback mechanism where the output current of the replica transconductor is compared with the output current of the main transconductor. The difference signal is fed back to adjust the control voltage of the main transconductor, creating a closed-loop system that automatically corrects non-linearities while maintaining low noise performance.
2Manufacturing precision
If linearizing circuitry with bias current sources is added to transistors, then the linearity is improved, but the available voltage swing is reduced due to headroom requirements
Solution Approach 1:
The replica transconductor copies the electrical characteristics of the main transconductor without requiring additional linearizing components that would consume voltage headroom. This allows the main transconductor to operate with maximum voltage swing while the replica provides the linearity reference.
Solution Approach 2:
The patent introduces an operational amplifier as an intermediary that compares the currents from the main and replica transconductors. This intermediary device enables linearity correction without requiring the main transconductor to have additional linearizing components that would reduce voltage swing.
3Manufacturing precision
If traditional linearizing circuits with resistors and amplifiers are used, then the voltage-to-current linearity is improved, but the number of components and circuit complexity increase
Solution Approach 1:
Instead of adding multiple linearizing components to the main transconductor, the patent creates a simplified circuit by adding only a replica transconductor and a comparison amplifier. This copying approach reduces component count while achieving the same linearity correction function.
Solution Approach 2:
The patent extracts the linearity correction function from the main transconductor circuit and implements it separately in the replica transconductor. This separation allows the main transconductor to remain simple while the replica handles the linearity reference, reducing overall circuit complexity.
Data Source
AI summary
A transconductor for providing an output current that is linear in the input voltage (Vin) comprises a main output transconductor (Ms, Mc) and a model transconductor (Msr1, Msr2, Mcr1, Mcr2) is comprised in a predistortion circuit (A), which measures the output of the model transconductor and the overall voltage input (Vin) to provide a control signal (Vc, Vc′) for the transconductors that compensates for their non-linearity.