Self-Biased Cascode OTA for Mirror Voltage 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 mobile battery-powered 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 by using cascode transistors with lower threshold voltages and eliminating the need for additional biasing circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current mirrors are used in OTAs, then the circuit can be implemented with standard components, but severe mismatches occur in drain-to-source voltages of mirror transistors
Solution Approach 1:
The current mirror is segmented into two distinct parts: a first current mirror and a second current mirror. Each mirror operates with controlled voltage conditions, preventing the severe mismatches that occur in conventional single-stage current mirrors. This segmentation allows each mirror to function optimally without interfering with the other's voltage requirements.
Solution Approach 2:
A control voltage is introduced as an intermediary element to regulate the drain-to-source voltages of the mirror transistors. This control voltage acts as a mediator that balances the voltage conditions across different current mirrors, ensuring matched operation without requiring complex circuit modifications.
2Reliability
If extra bias voltages are used to control drain-to-source voltages, then transistor matching is improved, but power consumption and circuit size increase
Solution Approach 1:
The biasing function is merged with the existing current mirror structure. The control voltage for drain-to-source matching is generated within the current mirror circuit itself rather than requiring separate external biasing circuits. This integration eliminates additional power consumption and circuit area while maintaining voltage matching.
Solution Approach 2:
The current mirror circuit generates its own control voltage internally to maintain proper drain-to-source voltage matching. The circuit serves itself by using a portion of its own output or internal node voltages to control the operating conditions, eliminating the need for external biasing resources and reducing overall power consumption.
3Adaptability or versatility
If conventional OTAs are designed for specific power supply voltages, then optimization for that voltage is achieved, but versatility across wide voltage ranges is limited
Solution Approach 1:
The OTA circuit is designed with dynamic voltage control capabilities through the control voltage mechanism. This allows the drain-to-source voltages to adapt dynamically to different power supply conditions while maintaining proper transistor matching. The circuit can operate effectively across a wide range of power supply voltages without requiring redesign, as the control voltage automatically adjusts to maintain optimal operating conditions.
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.