Cascode Current Mirror Feedback Biasing for Low Headroom Voltage
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Solution Overview
Problem
Cascode current mirrors require high headroom voltage, which is a significant portion of the supply voltage, especially at lower voltage levels, making them impractical for operation in nanometer scale technologies and limiting their use in portable devices.
Innovation Solution
A cascode current mirror circuit with a feedback block that generates the gate voltage for the input leg using a mirrored current from the output leg, combined with a start-up helper circuit to stabilize the gate voltage during transition modes, reducing the required headroom voltage to one gate-to-source voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cascode current mirror configuration is used to increase output resistance, then the current mirror performance is improved, but the headroom voltage requirement increases to two Vgs drops
Solution Approach 1:
The patent introduces a feedback mechanism where the gate voltage of the cascode transistor is dynamically adjusted based on the drain voltage of the main transistor. A feedback transistor senses the voltage difference and adjusts the cascode gate voltage accordingly, allowing the cascode current mirror to maintain high output resistance while reducing the headroom voltage requirement from two Vgs drops to approximately one Vgs drop plus a small overhead voltage.
Solution Approach 2:
The patent changes the operating parameters of the cascode transistor by dynamically adjusting its gate voltage rather than using a fixed bias voltage. This allows the cascode transistor to operate in different regions depending on the instantaneous voltage conditions, enabling the circuit to achieve high output resistance with reduced headroom voltage requirement.
2Use of energy by moving object
If additional current sources are added to reduce headroom voltage to one Vgs drop, then voltage requirement is reduced, but device complexity increases
Solution Approach 1:
The feedback transistor serves multiple functions: it senses the voltage difference, generates the appropriate gate voltage for the cascode transistor, and provides current compensation. This multi-functionality reduces the need for separate dedicated components, thereby reducing overall circuit complexity while achieving the voltage reduction goal.
Solution Approach 2:
The circuit uses its own internal signals to generate the required gate voltage. The feedback mechanism automatically adjusts the cascode gate voltage based on the operating conditions without requiring external control circuits or additional biasing networks, making the circuit self-sufficient and reducing overall complexity.
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.