Rail-to-Rail Precharge Buffer With Level Shifters for Accurate Sampling
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Solution Overview
Problem
Purely transistor-based open loop buffers fail to accurately represent input signals at or near the rail voltages, limiting the output of class AB amplifiers to transition between positive and negative rail voltages, necessitating a solution for an amplifier precharger that provides rail-to-rail voltages with high input impedance, low noise, and low power consumption.
Innovation Solution
A precharge buffer system incorporating a pair of voltage level shifters and amplifier transistors, with each level shifter providing a respective level shift to counteract the threshold voltage of the transistors, and a bypass signal path for mutually exclusive activation states, enabling precise sampling of input signals across the rail extremes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a class AB push-pull stage with level shifting transistors is used, then the amplifier can provide good representation of input signals within the operating range, but the bias stage cannot accept inputs at the supply extremes due to constraints of the electrical components
Solution Approach 1:
A dedicated precharge buffer stage is introduced as an intermediary between the input signal source and the class AB push-pull amplifier. This precharge buffer specifically handles the rail-to-rail voltage range, preparing the signal for the main amplifier stage which then operates within its optimal range. The precharge buffer acts as a mediator that extends the overall input voltage range without compromising the accuracy of the main amplifier.
Solution Approach 2:
The amplifier system is divided into two functional segments: a precharge buffer stage that handles the extreme voltage ranges near the supply rails, and a class AB push-pull stage that handles the mid-range signals with high precision. This segmentation allows each stage to be optimized for its specific operating range, with the precharge buffer managing rail-to-rail transitions and the main amplifier providing accurate representation within its operating range.
2Speed
If an open loop buffer design is used for fast response, then the buffer responds quickly to input changes, but purely transistor based designs do not provide good representation of input signals at the rail voltages
Solution Approach 1:
The precharge buffer performs preliminary action by precharging the sampling load to the correct voltage level before the main amplifier processes the signal. This preliminary voltage establishment occurs in the open-loop domain for speed, and then the main amplifier refines the signal representation in a closed-loop configuration, combining the benefits of fast response with accurate representation.
3Productivity
If level shifting transistors are used to bias the push-pull stage, then the amplifier can operate in class AB mode, but the bias stage cannot accept inputs at the positive and negative supply extremes
Solution Approach 1:
The precharge buffer serves as an intermediary stage that handles the extreme voltage inputs that would otherwise be rejected by the level shifting bias network. By processing these extreme voltages first, the precharge buffer allows the class AB amplifier to receive signals within its acceptable input range, maintaining both the efficiency benefits of class AB operation and the ability to accept rail-to-rail inputs.
Data Source
AI summary
A method and system that may include a pair of amplifier transistors and an output coupled to a load device. The precharge buffer may be controlled by an activation signal. The precharge buffer may also include a pair of level shifters. Each level shifter may be provided in association with a respective one of the transistors, and each may provide a respective level shift to an input signal at a common signal source based on a reference voltage. Outputs of the level shifters may be coupled to the respective transistors. The precharge buffer may also include a bypass signal path extending from the common signal source to the load device. A signal path may be controlled by another activation signal, and the precharge buffer and the bypass signal may be enabled during mutually exclusive states of the activation signal.