Bootstrapped MOS Sampling Switches for Linear Track-and-Hold
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Solution Overview
Problem
MOS-based switches in sampling circuits, such as track-and-hold circuits, suffer from non-linearity in output signals due to charge injection dependency on input signals, leading to reduced reliability and potential oxide breakdown from high input signal voltages.
Innovation Solution
Incorporating a second voltage level shifter that shifts the gate voltage during the 'hold' phase to a level lower than the source and drain voltages, and bootstrapping the gate voltage during the 'hold' phase from the input signal, to maintain a constant voltage swing independent of the input signal, thereby reducing the stress on the MOS device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the gate voltage is grounded during the hold phase, then the MOS transistor is turned off, but the voltage swing at the gate becomes dependent on the input signal causing charge injection non-linearity
Solution Approach 1:
The patent changes the gate voltage parameter from a fixed ground reference to a dynamically adjusted voltage that maintains a constant offset from the source voltage during the hold phase. This parameter change ensures the voltage swing remains constant and independent of the input signal, eliminating charge injection non-linearity while keeping the transistor off.
Solution Approach 2:
The patent applies preliminary bootstrapping action during the track phase to establish a constant voltage relationship between gate and source. By pre-establishing this voltage relationship before the hold phase, the system ensures that when the hold phase begins, the gate voltage is already positioned to maintain a constant swing, preventing input signal dependency.
2Adaptability or versatility
If high input signal voltages are applied to the MOS transistor, then the sampling circuit can handle large signal ranges, but the oxide may break down and device lifetime is shortened
Solution Approach 1:
The patent introduces a voltage level shifter as an intermediary component between the input signal source and the MOS transistor gate. This intermediary adjusts the gate voltage to appropriate levels, ensuring that even when input signals have large voltage ranges, the gate voltage remains within safe operating limits, preventing oxide breakdown while maintaining full signal handling capability.
Solution Approach 2:
The patent dynamically adjusts the gate voltage parameter through bootstrapping and level shifting to maintain it within safe operating ranges regardless of the input signal amplitude. This parameter control ensures the voltage stress on the oxide remains below breakdown thresholds while allowing the circuit to process a wide range of input signals.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach improves the linearity of output signals and enhances the reliability of the MOS device by reducing voltage stress, ensuring the MOS transistor operates within safe voltage limits during both 'track' and 'hold' phases.
Implementation Method 1
a second voltage level shifter, selectively coupled between a reference and the gate during the OFF phase, in which the second voltage level shifter shifts a voltage at the gate to a level lower than a voltage at the source and lower than a voltage at the drain
Implementation Method 2
bootstrapping the gate voltage during the 'hold' phase from the input signal, to maintain a constant voltage swing independent of the input signal
Data Source
AI summary
A switch may include a MOS transistor alternatively operating in an ON phase and an OFF phase, a first voltage level shifter, and a second voltage level shifter. The MOS transistor may include a source for receiving an input signal, a drain for connecting to a load, and a gate. The first voltage level shifter may be selectively coupled between the source and the gate during the ON phase, and the second voltage level shifter may be selectively coupled between the gate and the source during the OFF phase.