Class AB Op-Amp Quiescent Current Control via Matched VDS
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Solution Overview
Problem
Class AB operational amplifiers experience significant variation in output stage quiescent current due to process variation and system voltage changes, leading to unstable current distribution.
Innovation Solution
Incorporating an output stage quiescent current controller that sets the drain-to-source voltage of one transistor equal to another, maintaining a consistent ratio of output stage quiescent current to bias current by matching the W/L ratios of the transistors, thereby controlling the drain-to-source voltage to reduce variations caused by system voltage and process variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If negative feedback is used to maintain output stage quiescent current at a specific multiple of bias current, then the current ratio can be controlled, but the output stage quiescent current varies significantly due to process variation and system voltage changes
Solution Approach 1:
The patent changes the operating parameters of the transistors by controlling their drain-to-source voltages to be equal, which eliminates the channel length modulation effect. This parameter control approach ensures that the output stage quiescent current ratio depends only on the W/L ratios of the transistors, making it insensitive to process variation and system voltage changes.
Solution Approach 2:
The patent employs a feedback mechanism where the drain-to-source voltage of the first transistor is controlled to be equal to that of the second transistor. This feedback loop compensates for variations in system voltage and process parameters, maintaining a stable current ratio that is determined solely by the transistor W/L ratios.
2Power
If drain-to-source voltage varies with system voltage, then channel length modulation effect occurs, but this causes output stage quiescent current to increase with system voltage
Solution Approach 1:
The patent controls the drain-to-source voltage parameter to remain constant and equal for both transistors in the current mirror circuit. By maintaining this parameter at a fixed value regardless of system voltage changes, the channel length modulation effect is eliminated, and the output stage quiescent current becomes independent of system voltage variations.
3Reliability
If gate-to-source voltage and process variation parameter vary with process variation, then transistor characteristics change, but this leads to wider distribution of output stage quiescent currents
Solution Approach 1:
The patent changes the control approach from relying on gate-to-source voltage equality to controlling drain-to-source voltage equality. This parameter change makes the current ratio dependent only on the geometric W/L ratios of the transistors, which are much less sensitive to process variation, thereby concentrating the current distribution.
Solution Approach 2:
The patent uses a current mirror configuration where the second transistor copies the current characteristics of the first transistor. By ensuring equal drain-to-source voltages, the copying accuracy is improved, and the current ratio is precisely determined by the W/L ratio copy, reducing the impact of process variation.
Data Source
AI summary
A class AB operational amplifier includes: a first transistor, for generating a first current; a second transistor, where a second source voltage of the second transistor is equal to a first source voltage of the first transistor, for generating an output stage quiescent current; and an output stage quiescent current controller, coupled to a gate and a source of the first transistor, for controlling a first drain-to-source voltage of the first transistor to be equal to a second drain-to-source voltage of the second transistor. A ratio of the output stage quiescent current to the first current is equal to a ratio of a second W/L ratio of the second transistor to a first W/L ratio of the first transistor.