Bootstrap Sampling Switch Circuit for Stable On-Resistance
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Solution Overview
Problem
Existing sampling switch circuits face challenges with high input loading and harmonic distortion due to input-dependent on-resistance variations, which limit their performance, especially at higher speeds and in direct RF applications.
Innovation Solution
The proposed sampling switch circuit incorporates a current source and switching circuitry that alternates between a precharge and an output configuration, using a controllable current source to maintain a constant gate-source voltage of the sampling transistor, independent of the input signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a sampling transistor is used as a simple switch in voltage mode sampling, then the circuit structure is simple, but the on-resistance varies with input voltage causing harmonic distortion
Solution Approach 1:
The patent changes the operating parameter of the sampling transistor by applying a constant gate-source voltage (Vgs) through the bootstrap circuit. This parameter change ensures that the on-resistance remains stable and does not vary with input voltage, thereby reducing harmonic distortion while maintaining circuit simplicity
Solution Approach 2:
The bootstrap circuit acts as an intermediary component that mediates between the power supply and the sampling transistor gate. It provides the constant Vgs needed to stabilize the transistor's on-resistance, solving the distortion problem without requiring complex circuit restructuring
2Manufacturing precision
If a bootstrap circuit is used to maintain constant gate-source voltage, then on-resistance stability is improved, but input loading increases
Solution Approach 1:
The bootstrap circuit operates periodically, charging the bootstrap capacitor during the hold phase and using it during the sampling phase. This periodic operation allows the circuit to maintain constant Vgs only when needed during sampling, reducing continuous power dissipation and input loading while preserving on-resistance stability during critical sampling operations
3Loss of energy
If continuous power dissipation is reduced in bootstrap circuits, then energy efficiency is improved, but voltage headroom is reduced
Solution Approach 1:
The circuit uses periodic switching operation where the sampling transistor is ON only during brief sampling intervals and OFF during hold intervals. This periodic action dramatically reduces average power dissipation while maintaining adequate voltage headroom during the active sampling phase when full voltage swing is required
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 configuration reduces input loading and minimizes input-dependent on-resistance variations, enhancing the sampling switch circuit's performance at higher speeds and in applications like direct RF, while maintaining high impedance input.
Implementation Method 1
a capacitor, a current source configured to cause a defined current to flow therethrough and switching circuitry configured to switch between a precharge configuration and an output configuration
Data Source
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AI summary
A sampling switch circuit, comprising an input node, connected to receive an input voltage signal to be sampled, a sampling transistor comprising a gate terminal, a source terminal and a drain terminal, the source terminal connected to the input node, a capacitor, a current source configured to cause a defined current to flow therethrough and switching circuitry configured to alternate between a precharge configuration and an output configuration in dependence upon a clock signal. In the precharge configuration, the switching circuitry connects the capacitor into a current path between said current source and a first voltage reference node to form a potential difference across the capacitor which is dependent on the defined current. In the output configuration, the switching circuitry connects the capacitor between a second voltage reference node and the gate terminal of the sampling transistor so that a voltage level applied at the gate terminal of the sampling transistor is dependent on the defined current.