Class AB Amplifier Quiescent Current Detection Using OTA Feedback
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Solution Overview
Problem
Current methods for measuring current in semiconductor circuits, particularly in class A/B amplifier circuits, face challenges such as power loss, interference with circuit performance, and unpredictability of quiescent current due to variability in transistor ratios and process variations, making accurate current measurement difficult.
Innovation Solution
A circuit utilizing an operational transconductance amplifier (OTA) and a comparator to measure quiescent current by setting up a current mirror and using a feedback circuit to detect divergence, with a current source to set the comparator threshold, allowing for accurate detection of quiescent current with minimal interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If series resistors are used to measure current, then current measurement is achieved, but power loss increases and circuit performance is limited
Solution Approach 1:
The patent introduces an operational transconductance amplifier (OTA) as an intermediary device that converts the current through transistor MN8 into a voltage signal. This voltage is then compared by a comparator to detect when it exceeds a reference voltage, indicating quiescent current has been reached. This intermediary conversion approach allows current measurement without inserting series resistors that would cause power loss.
Solution Approach 2:
The patent replaces the traditional ohmic measurement method (using series resistors and voltage drops) with a transconductance-based electronic measurement system. The OTA converts current to voltage, and the comparator detects the threshold, substituting the mechanical/electrical series resistor approach with a more sophisticated electronic signal processing approach that avoids power loss.
2Measurement precision
If series resistors are used to measure current, then current measurement is achieved, but interference with circuit performance occurs
Solution Approach 1:
The patent uses transistor MN8 as an intermediary element that mirrors the current from the output driver MN0. By measuring the current through MN8 rather than inserting a resistor in the main signal path, the measurement system avoids interfering with the rail-to-rail output driving capability and signal integrity of the class A/B amplifier.
Solution Approach 2:
The patent creates a current copy through transistor MN8 that is proportional to the current in the output driver MN0. This copying approach allows the measurement system to monitor quiescent current without physically interfering with the original current path, thus avoiding interference with circuit performance while maintaining measurement accuracy.
3Measurement precision
If internal nodes are probed in semiconductor circuits, then current measurement is possible, then measurement is achieved, but impedance of the probe alters circuit performance
Solution Approach 1:
The patent introduces a current mirror circuit using transistors MN8 and MN0 as intermediary elements. This current mirror allows the measurement system to access a proportional copy of the quiescent current without directly probing internal nodes with high-impedance probes. The current mirror acts as a buffer that isolates the measurement system from the sensitive internal circuit nodes, preventing impedance-related performance alterations.
4Ease of manufacture
If W/L ratios are set to establish quiescent current, then current setting is achieved, but process variability and voltage effects cause unpredictable current
Solution Approach 1:
The patent implements a feedback mechanism where the comparator monitors the voltage generated by the OTA (which is proportional to the quiescent current) and compares it against a reference voltage. When the quiescent current reaches the desired level, the comparator output changes state, providing feedback that indicates the current has been properly established. This feedback approach compensates for process variability and voltage effects by dynamically detecting when the correct current level is achieved rather than relying solely on fixed W/L ratios.
Data Source
AI summary
An operational transconductance amplifier is configured with an asymmetric output current capability. When current sunk from the output exceeds a rated load, the normally equal input voltages diverge. A comparator coupled to the OTA inputs senses when the output current drain exceeds the rated load and changes state when the OTA input voltages diverge. When used in conjunction with a current mirror transistor of known proportion, current through an output transistor, such as quiescent current through the output stage of a class A/B amplifier, can be accurately detected. A current source in parallel with the OTA output can be used to offset the current sensing trigger point.