Common-Gate RF Transconductor for IM2 Cancellation in Mixers
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Solution Overview
Problem
Direct-conversion receivers face challenges with envelope distortion due to even-order nonlinearity, particularly in down-conversion mixers, which result in undesirable spectral components at the baseband, degrading receiver sensitivity and requiring high IIP2 performance.
Innovation Solution
The implementation of a transconductor circuit with a common-gate or common-base configuration, where biasing transistors sense and compensate low-frequency IM2 products, enhancing IIP2 and reducing the need for mixer IIP2 calibration or trimming, while maintaining low-voltage operation suitable for CMOS technology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If down-conversion mixers are used in direct-conversion receivers, then integration level increases and complexity reduces, but even-order nonlinearity generates envelope distortion and degrades receiver sensitivity
Solution Approach 1:
The patent converts the harmful IM2 distortion products into a useful compensation signal. The biasing transistors sense the low-frequency IM2 products generated by the RF input transconductor and feed back compensating signals to cancel these distortions, thereby transforming the harmful nonlinearity into a benefit for improving IIP2 performance
Solution Approach 2:
The patent implements a feedback mechanism where biasing transistors continuously sense the IM2 distortion products at the output of the RF input transconductor and feed back compensating signals to the input stage. This feedback loop dynamically cancels the even-order nonlinearity, improving receiver sensitivity without increasing overall system complexity
2Reliability
If mixer IIP2 calibration or trimming is implemented, then IIP2 performance improves, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent enables the mixer to self-correct its IIP2 performance through automatic distortion compensation. The biasing transistors automatically sense and compensate IM2 products without requiring external calibration equipment or trimming circuits, making the system self-sufficient and eliminating complex calibration procedures
Solution Approach 2:
The patent extracts the calibration function from the overall system by implementing distortion compensation directly within the mixer circuitry itself. The biasing transistors perform the compensation function locally at the distortion source, eliminating the need for separate external calibration or trimming circuits
3Reliability
If common-gate or common-base transconductor configuration is used, then IIP2 performance improves through IM2 compensation, but circuit design complexity increases
Solution Approach 1:
The patent merges the biasing function with the distortion sensing and compensation function into a single integrated circuit block. The biasing transistors simultaneously provide DC biasing for the RF input transconductor and sense/cancel IM2 distortion products, combining multiple functions into one compact circuit structure that minimizes overall complexity
Data Source
AI summary
A transconductor input circuit for a down converting quadrature mixer stage of a direct-conversion receiver comprises a pair of common-gate input transistors whose source electrodes are coupled to a differential radio frequency (RF) input signal outputted from an interstage RF filter. The transconductor circuit further comprises a pair of equally-sized biasing transistors for biasing the pair of common-gate input transistors. Source electrodes of the biasing transistors are coupled to the source electrodes of the transistors to sense the differential radio frequency input signal for canceling intermodulation distortion.