Differential Mixer Transconductor With Feedback Linearization
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Solution Overview
Problem
Conventional mixers suffer from non-linear voltage-current transfer functions due to the characteristics of n-channel transistors, limiting their application in high linearity requirements such as wireless local area networks (WLAN) or code division multiple access (CDMA) transmitters.
Innovation Solution
The mixer circuit incorporates a transconductor design with a first and second operational amplifier, a first and second current mirror, and resistors coupled between the input terminals of the operational amplifiers, achieving a linear voltage-current transfer function through negative feedback and current mirroring.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional n-channel transistors are used in the transconductor, then the device complexity is reduced, but the voltage-current transfer function becomes non-linear
Solution Approach 1:
The patent employs negative feedback by coupling the output of operational amplifiers back to their inverting input terminals. This feedback mechanism linearizes the voltage-current transfer function of the transconductor, transforming the inherently non-linear transistor characteristics into a linear relationship between differential input voltage and output current.
Solution Approach 2:
The patent introduces operational amplifiers and current mirrors as intermediary elements between the input voltage and output current. These intermediaries process and condition the signals, with the operational amplifiers providing high-gain feedback control and the current mirrors ensuring precise current replication, thereby achieving linear transfer characteristics.
2Manufacturing precision
If operational amplifiers and current mirrors are added to achieve linear transfer function, then the linearity is improved, but the device complexity increases
Solution Approach 1:
The patent combines multiple functional blocks (operational amplifiers, current mirrors, and transistors) into an integrated transconductor circuit. The operational amplifiers and current mirrors work together in a unified feedback architecture, where the current mirrors replicate currents between differential pairs and the operational amplifiers provide coordinated feedback control, achieving linearization through their combined action.
Solution Approach 2:
The operational amplifiers serve multiple functions simultaneously: they provide high-gain feedback for linearization, act as voltage buffers, and enable differential signal processing. The current mirrors also perform multiple roles including current replication, impedance transformation, and bias current distribution, reducing the need for separate dedicated circuits for each function.
Data Source
AI summary
A mixer in a smaller signal differential model includes a load circuit, a switch quad, and a transconductor. The switch quad further including a first current path and a second current path is coupled to the load circuit. The connecting node of the switch quad and the load circuit is a differential-output terminal. The transconductor further includes a first resistor, a first operational amplifier, a second operational amplifier, a first current mirror, and a second current mirror. The resistor is coupled between two first input terminals of the first operational amplifier and the second operational amplifier. A current control terminal of the first current mirror is coupled to the first input terminal of the first operational amplifier, and a current mirroring terminal of the first current mirror is coupled to the first current path. A current control terminal of the second current mirror is coupled to the first input terminal of the second operational amplifier, and a current mirroring terminal of the second current mirror is coupled to the second current path. A differential voltage is applied between two second input terminals of the first operational amplifier and the second operational amplifier.