Self-Biased Cascode OTA for Drain-Voltage Mirror Matching
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Solution Overview
Problem
Analog amplifiers, particularly operational transconductance amplifiers (OTAs), face challenges in achieving good performance, cost-effectiveness, and versatility across a wide range of power supply voltages, especially in low-voltage mobile applications, due to severe mismatches in drain-to-source voltages of mirror transistors.
Innovation Solution
The implementation of self-biased cascode current mirrors in operational transconductance amplifiers reduces the need for extra bias voltages, lowering power consumption, size, and cost, while maintaining performance through the use of cascode transistors with lower threshold voltages, which eliminates the need for additional biasing circuits and mitigates offset issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional current mirrors are used in OTAs, then the circuit can be implemented with standard transistors, but severe mismatches occur in drain-to-source voltages of mirror transistors
Solution Approach 1:
Cascode transistors are introduced as intermediary elements between the mirror transistors and the output nodes. These cascode transistors act as mediators that equalize the drain-to-source voltages across mirror transistors by providing a controlled voltage drop, thereby eliminating the severe voltage mismatches that would otherwise occur in conventional current mirror configurations.
Solution Approach 2:
The invention changes the voltage parameters in the circuit by introducing self-biasing mechanisms that dynamically adjust the drain-to-source voltages. The cascode current mirrors modify the voltage distribution across the transistor network, ensuring that mirror transistors operate at matched voltage levels despite process variations, thereby improving manufacturing precision.
2Device complexity
If self-biased cascode current mirrors are implemented, then power consumption and size are reduced, but additional circuit complexity is introduced
Solution Approach 1:
The cascode current mirrors are designed with self-biasing capabilities where the circuits generate their own bias voltages through internal transistor action. The cascode transistors automatically establish the required voltage drops and current levels without external biasing networks, making the circuit self-sufficient and eliminating the need for additional biasing components.
Solution Approach 2:
The biasing function is merged into the current mirror structure itself. Rather than having separate biasing circuits, the cascode current mirror combines the current mirroring function with the biasing function in a single integrated structure. The cascode transistors simultaneously perform current mirroring and voltage regulation, reducing overall device complexity.
3Reliability
If extra bias voltages are used to maintain performance, then amplifier performance is maintained, but power consumption and device size increase
Solution Approach 1:
The cascode current mirror structure serves multiple functions simultaneously: it provides current mirroring, voltage equalization, and self-biasing all within the same transistor network. This multi-functionality eliminates the need for separate biasing circuits and reduces the overall circuit area while maintaining amplifier performance through integrated voltage and current control.
Data Source
AI summary
Apparatus and methods provide an operational transconductance amplifier (OTA) with one or more self-biased cascode current mirrors. Applicable topologies include a current-mirror OTA and a folded-cascode OTA. In one embodiment, the self-biasing cascode current mirror is an optional aspect of the folded-cascode OTA. The self-biasing can advantageous reduce the number of biasing circuits used, which can save chip area and cost. One embodiment includes an input differential pair of a current-mirror OTA.