Current Mirror Pre-Bias Circuit for Faster Bias Current Startup
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Solution Overview
Problem
Current mirror circuits experience 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 transition time from inactive to active state is long due to slow charging of the common node
Solution Approach 1:
The patent applies preliminary action by introducing a pre-charging circuit that charges the common node to a pre-charge voltage (near the steady state voltage) before the current mirror begins its normal operation. This preliminary charging action significantly reduces the transition time from inactive to active state, as the node starts much closer to its final voltage rather than charging from zero. The pre-charging circuit includes a pre-charge transistor that is activated temporarily to provide the fast charging path.
2Speed
If the charging time of the common node is reduced to speed up operation, then the transition time decreases, but power consumption increases
Solution Approach 1:
The patent applies periodic action by using a pre-charge control signal that temporarily activates the pre-charge transistor only during the transition from inactive to active state. The pre-charging circuit operates periodically when needed (during state transitions) rather than continuously, allowing fast charging when required while consuming minimal power during normal steady-state operation. The pre-charge transistor is turned off once the pre-charge voltage is reached, preventing continuous power dissipation.
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.