Cascode Current Mirror Feedback Biasing for Low Headroom
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Solution Overview
Problem
Existing cascode current mirrors require high headroom voltage, which can be a significant portion of the available supply voltage, especially at lower voltage ranges, making them impractical for operation at lower voltages and limiting their scalability and power efficiency.
Innovation Solution
The proposed cascode current mirror circuit incorporates a feedback block that generates the gate voltage for the cascode transistor of the input leg based on an intermediate current from the output leg, reducing the required headroom voltage by utilizing a mirrored current instead of the direct input current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cascode current mirror configuration is used to increase output resistance, then the output resistance is improved, but the headroom voltage requirement increases
Solution Approach 1:
The patent introduces an intermediate current mirror stage that acts as a mediator between the input current and the cascode transistor gate. This intermediate stage generates a controlled current that, when passed through a resistor, produces the required gate voltage without requiring the full headroom voltage of traditional cascode configurations. The intermediate stage thus enables the cascode structure to achieve high output resistance while operating at lower supply voltages.
Solution Approach 2:
The patent changes the parameter relationship by using an intermediate current mirror with a different current ratio than the main current mirror. By adjusting the transistor size ratios in the intermediate stage and using a specifically sized resistor, the gate voltage is optimized to enable cascode operation at reduced headroom while maintaining the desired output resistance characteristics.
2Use of energy by moving object
If additional current sources are added to reduce headroom voltage, then the headroom voltage is reduced, but the device complexity increases
Solution Approach 1:
The intermediate current mirror serves multiple functions: it provides the gate bias voltage for the cascode transistor, it generates the controlled current for the resistor voltage drop, and it maintains proper current relationships throughout the circuit. This multi-functionality reduces the need for separate dedicated biasing circuits, thereby limiting the increase in overall device complexity while achieving reduced headroom voltage.
3Use of energy by moving object
If transistor sizes are increased to maintain performance at lower voltages, then the voltage headroom is reduced, but the transistor area increases
Solution Approach 1:
The patent carefully adjusts the transistor size parameters in the intermediate current mirror stage to achieve the required current ratios without excessive area. By optimizing the width-to-length ratios of the intermediate stage transistors and using a resistor with appropriate resistance value, the circuit achieves low headroom operation with minimal transistor area overhead compared to scaling up all transistors in the main current mirror.
Data Source
AI summary
Methods and devices for a cascode current mirror with low headroom voltage are presented. According to one aspect, a gate voltage to a cascode transistor of an input leg of the current mirror is provided by a feedback block that operates from a mirrored current output by an output leg of the current mirror. The feedback block includes a feedback current mirror that outputs a mirrored current for conduction through a self-biasing diode-connected transistor that generates the gate voltage to the cascode transistor of the input leg. According to yet another aspect, the cascode current mirror includes a start-up circuit coupled between an input to the input leg and a gate of the cascode transistor of the input leg, the start-up circuit generating a start-up voltage during a transition mode of operation of the cascode current mirror. According to one aspect, a transistor is used as the start-up circuit.