Folded Cascode Current Source with Dual Mirrors
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Solution Overview
Problem
Existing current sources/sinks in analog integrated circuits face limitations in achieving high output resistance and wide voltage swing, which restricts their performance in bias and amplifier operations.
Innovation Solution
A folded cascode configuration using dual pnp-npn current mirrors for current feedback between the base and emitter, enhancing output resistance while maintaining constant collector current, and implemented with BJTs or MOSFETs to achieve low input voltage and wide bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If cascade or Wilson current sources are used to improve output resistance, then output resistance increases (Rout=βro/2), but output voltage swing is limited (VMIN=VBE+VCE,sat=0.9V)
Solution Approach 1:
The current source is divided into two separate mirrors: a reference mirror and an output mirror. This segmentation allows independent optimization of each mirror's function, enabling the output mirror to achieve both high output resistance and wide voltage swing by decoupling the reference setting function from the output delivery function.
Solution Approach 2:
The patent transitions from a single-plane current source design to a folded cascode configuration that utilizes multiple dimensional aspects of transistor operation. By folding the cascode structure and implementing dual mirrors, the design achieves high output resistance through the cascode stacking while simultaneously enabling wide voltage swing through the folded configuration that reduces the minimum output voltage requirement.
2Reliability
If gain-boost or regulated cascode current sources are used, then output resistance improves via negative feedback, but device complexity increases
Solution Approach 1:
The patent implements feedback through the dual mirror configuration where the reference mirror current is fed back to control the output mirror. This feedback mechanism automatically adjusts the output current to maintain high output resistance without requiring complex gain-boost circuits or regulated cascode structures, thereby achieving high reliability with reduced complexity.
Solution Approach 2:
The patent uses a reference mirror that copies the reference current to the output mirror. This copying mechanism simplifies the circuit by using a straightforward current replication approach rather than complex feedback amplifiers, achieving high output resistance through the precision current copying while maintaining low device complexity.
3Device complexity
If basic BJT current source is used, then circuit simplicity is maintained, but output resistance is limited (Rout=ro)
Solution Approach 1:
The current source is segmented into reference and output mirrors with distinct functions. This segmentation enables the output mirror to achieve high output resistance through the folded cascode configuration while the reference mirror maintains circuit simplicity by using a basic current copying structure.
Solution Approach 2:
The patent applies the folded cascode dimensionality change to the output mirror specifically, allowing it to achieve high output resistance without complicating the overall circuit. The reference mirror remains simple while the output mirror utilizes the folded cascode structure to elevate output resistance from ro to βro/2 or higher.
Data Source
AI summary
Systems and methods for providing a high performance current source are described. In an example implementation, the current source includes transistors in dual current mirror configuration. The dual mirror configuration employs current feedback to increase the output resistance of the current source while achieving a wide voltage swing.