Current Mirror Pre-Bias Circuit for Faster Current Onset
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Solution Overview
Problem
Current mirror circuits face a long charging time for the onset of output current, which delays the effective operation of active devices, and reducing this time without increasing power consumption is a challenge.
Innovation Solution
A pre-charging circuit is introduced, comprising a sensing transistor and a current sourcing transistor that rapidly charges a common node to a pre-charge voltage near the steady state voltage, reducing the time required for the node to reach the steady state voltage through the reference current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current mirror circuit is used to generate DC bias current, then the output current can be ratiometrically related to the reference current, but the charging time for the onset of output current is long which delays effective operation
Solution Approach 1:
The patent applies preliminary action by introducing a pre-charge circuit that charges the common node to a pre-charge voltage (close to the steady state voltage) before the current mirror begins its normal operation. This preliminary charging action reduces the time required for the node to reach steady state voltage, thereby reducing the delay in output current onset while maintaining accurate current mirroring once steady state is reached.
2Speed
If the charging time is reduced by increasing the reference current, then the transition speed improves, but the power consumption increases
Solution Approach 1:
The pre-charge circuit performs the charging action in advance during a brief transition period, allowing the use of higher current only temporarily during switching. Once the pre-charge voltage is reached, the normal reference current (at normal power levels) completes the charging to steady state. This separates the high-speed transition requirement from continuous power consumption.
Solution Approach 2:
The patent employs periodic action by using a switch controlled by a signal that activates the pre-charge circuit only during the transition from inactive to active state. The pre-charge current flows periodically only when needed for state transitions, not continuously, thereby achieving fast transition speed without continuous high power consumption.
3Speed
If a pre-charging circuit is introduced to reduce transition time, then the operational speed improves, but the device complexity increases
Solution Approach 1:
The patent introduces an intermediary pre-charge circuit that mediates between the power supply and the common node of the current mirror. This intermediary circuit includes a pre-charge transistor and switch that temporarily provide high current during transitions, while the main current mirror circuit remains unchanged and operates at normal power levels during steady state.
Data Source
AI summary
Methods and devices for speeding up the onset of a target current through an output leg of a current mirror are presented. Upon activation of the current mirror, a pre-charge current is sourced to a node of the current mirror that is common to the output leg and an input leg of the current mirror. Sourcing of the pre-charge current is based on sensing, by a first transistor, of a voltage at the common node. Pre-charging of the common node continues up to a cutoff voltage sensed at the common node. Sourcing of the pre-charge current is provided by a second transistor coupled to the common node. Based on the voltage sensed at the common node, the first transistor controls the sourcing of the pre-charge current by the second transistor. Such control is based on a portion of a current from a current source that flows through the first transistor.