Op-amp bias tracking for current mirror linearity
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Solution Overview
Problem
Current mirror circuits face challenges in accurately tracking voltage at node X between transistors M1 and M3, leading to indeterminate voltage, which affects current mirroring and the linearity range of the power amplifier core, especially under process variation and supply modulation.
Innovation Solution
Incorporating an op-amp in the bias branch to receive a divided voltage and drive the gates of transistors to maintain a determinate voltage at node X, coupled with voltage dividers and capacitors to ensure accurate bias tracking and decouple input and output nodes, thereby stabilizing the voltage at node Y and extending the linearity range across PVT parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a simple current mirror configuration is used, then the device complexity is low, but the voltage at node X becomes indeterminate, affecting current mirroring accuracy and linearity range
Solution Approach 1:
The patent introduces an operational amplifier that continuously monitors the voltage at node X and adjusts the gate voltage of transistor M3 to maintain node X at Vdd/2. This feedback mechanism ensures accurate voltage tracking between node X and node Y, resolving the indeterminate voltage issue while maintaining current mirroring accuracy across process variations and supply modulation.
Solution Approach 2:
The operational amplifier acts as an intermediary element between the bias branch and the power amplifier core. It mediates the voltage relationship by actively controlling the gate of M3 to ensure that node X voltage tracks node Y voltage, thereby enabling accurate current mirroring without directly modifying the basic current mirror structure.
2Adaptability or versatility
If process variation and supply modulation are considered, then the linearity range is improved, but the voltage tracking becomes indeterminate without additional control
Solution Approach 1:
The operational amplifier implements a feedback loop that actively compensates for process variations and supply voltage changes. By continuously adjusting the gate voltage of M3 based on the voltage at node X, the circuit maintains determinate voltage levels and accurate tracking under varying conditions, thereby improving both reliability and adaptability.
Solution Approach 2:
The circuit transitions from a static bias configuration to a dynamic control system. The operational amplifier continuously adapts the bias conditions in response to changing voltages at node X, enabling the circuit to maintain accurate voltage tracking and current mirroring across different operating conditions, process variations, and temperature ranges.
Data Source
AI summary
A current mirror includes a bias branch, which includes first and second transistors in series between a voltage source and ground, a voltage divider coupled between the voltage source and ground, an op-amp configured to receive a divided voltage of the voltage divider and a voltage of a node between the first and second transistors, and drive a gate of the second transistor to pull the node to the divided voltage. The current mirror further includes a power amplifier core coupled to the bias branch. The power amplifier core includes first and second drive transistors configured in series between the voltage source and ground. Gates of the first transistor and the first drive transistor are coupled, and gates of the second transistor and the second drive transistor are coupled.