Class AB Op-Amp Current Clamp for High Quiescent Control
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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 operating outside the linear range, leading to inefficiency and excess heat, especially in applications using multiple Op-Amps.
Innovation Solution
The control circuit includes a voltage clamp and current limiting transistors to manage the current in the sense branch, limiting high quiescent current when the Op-Amp operates outside its linear range while maintaining normal current flow within the 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 operating outside linear range
Solution Approach 1:
The patent implements dynamic control of the sense branch current by introducing a current limiting transistor that adjusts its conduction state based on the operating conditions. When the Op-Amp operates outside the linear range, the transistor limits the current to prevent excessive quiescent current, while allowing normal current flow within the linear range to maintain proper class AB control functionality.
Solution Approach 2:
The patent changes the current parameter in the sense branch dynamically by using a current limiting transistor. The transistor modifies the current flow characteristics based on the output voltage level, reducing the current when operating outside the linear range and maintaining normal current levels within the linear range, thus resolving the contradiction between adaptability and energy consumption.
2Adaptability or versatility
If current feedback loop is used in low voltage class AB Op-Amp, then class AB control is facilitated, but excess heat is generated due to high quiescent current
Solution Approach 1:
The current limiting transistor dynamically adjusts the sense branch current based on operating conditions, reducing current when the Op-Amp operates outside the linear range. This dynamic current control directly reduces power dissipation and heat generation in the sense branch, while preserving the class AB control functionality when needed.
Solution Approach 2:
The patent converts the potentially harmful high quiescent current into a controlled parameter by using the current limiting transistor. The transistor transforms the harmful effect (excessive current and heat) into a beneficial controlled current flow that maintains functionality while reducing waste, effectively converting the problem into a solution.
3Reliability
If output transistor gate voltage approaches supply rail, then amplifier operation is maintained, but current in sense branch increases much beyond typical value
Solution Approach 1:
The current limiting transistor acts as an intermediary between the sense branch and the rest of the circuit. It mediates the current flow by limiting it when the gate voltage approaches the supply rail, preventing excessive current while allowing the amplifier to continue operating. This intermediary component resolves the contradiction by controlling the current without disrupting the overall amplifier operation.
Data Source
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AI summary
A circuit (300) is provided. In some examples, the circuit includes a first transistor (336) having a gate and a drain coupled together and a current source (334) coupled to the drain of the first transistor (336). A second transistor (308) has a drain coupled to a source of the first transistor (336). A third transistor (338) has a gate coupled to the gate of the first transistor (336). A fourth transistor (310) has a drain coupled to a source of the third transistor (338) and a gate of the fourth transistor (310) is coupled to a gate of the second transistor (308). In some examples, the third transistor (338) is configured to limit a first current (326) between the third transistor (338) and the fourth transistor (310) based on an output voltage (318).