Class AB Op-Amp Current Limiting Near Supply Rails
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Solution Overview
Problem
Low voltage class AB operational amplifiers (Op-Amps) experience high quiescent current in the current sense branch when the output is close to or at the supply/ground rails, leading to inefficiency and excess heat.
Innovation Solution
A circuit is provided to limit the quiescent current on the current sense branch by using a voltage clamp to bias a current limiting transistor, which adjusts its bias based on the output voltage to prevent high current flow when the Op-Amp operates outside its linear range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If current feedback loop is used in low voltage class AB Op-Amp, then class AB control is facilitated, but high quiescent current occurs in the sense branch when output approaches supply rails
Solution Approach 1:
The patent applies dynamics by making the feedback mechanism adaptive rather than static. The circuit dynamically switches between two feedback paths: one for N-MOS transistors and another for P-MOS transistors, depending on which transistor type is conducting. This dynamic adaptation allows the system to maintain class AB control functionality while preventing high quiescent current by ensuring the feedback loop remains functional across the entire output voltage range, including near supply rails.
2Power
If output transistor gate voltage approaches supply rail, then output voltage approaches ground, but current in sense branch increases significantly
Solution Approach 1:
The patent introduces an intermediary mechanism in the form of a dual feedback path with intermediate switching logic. When the output transistor gate voltage approaches the supply rail, the circuit automatically switches from one feedback path to the other, using the non-conducting transistor type's feedback path. This intermediary switching mechanism prevents the sense branch current from increasing significantly, thereby reducing energy loss while maintaining the full output voltage swing range.
3Adaptability or versatility
If Op-Amp operates outside linear range, then current feedback loop becomes dysfunctional, but current in sense branch goes much beyond typical value
Solution Approach 1:
The patent applies segmentation by dividing the feedback loop into two separate, dedicated paths: one for N-MOS transistors and another for P-MOS transistors. Each path is optimized for its specific transistor type. When the Op-Amp operates outside the linear range, the appropriate feedback path remains functional while the other is inactive. This segmentation ensures that the feedback loop maintains reliability across the entire operational range, preventing dysfunctional behavior and excessive sense branch current.
Data Source
AI summary
A circuit is provided. In some examples, the circuit includes a first transistor having a gate and a drain coupled together and a current source coupled to the drain of the first transistor. A second transistor has a drain coupled to a source of the first transistor. A third transistor has a gate coupled to the gate of the first transistor. A fourth transistor has a drain coupled to a source of the third transistor and a gate of the fourth transistor is coupled to a gate of the second transistor. In some examples, the third transistor is configured to limit a first current between the third transistor and the fourth transistor based on an output voltage.