Bootstrap Analog Multiplexer for Harmonic-Free Signal Switching

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Solution Overview

Problem

Analog multiplexers face issues due to metal-oxide-silicon (MOS) transistors in the main pass switches, which introduce harmonics into the output signal if the resistance of the transistor is not constant.

Innovation Solution

The multiplexer employs two bootstrap circuits coupled to the source and gate terminals of the main pass transistor, operating in alternate phases to maintain a constant gate-to-source voltage (VGS), thereby ensuring a constant resistance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If MOS transistor is used as main pass switch, then high frequency analog signal transmission is enabled, but resistance varies causing harmonics in output

Engineering Contradiction:

Improvehigh frequency signal transmissionVSAvoidharmonics in output signal

Core Design Contradiction:

SpeedVSObject-generated harmful factors

Solution Approach 1:

The patent changes the voltage parameter by applying a boosted gate-to-source voltage (VGS) to the MOS transistor. This voltage boosting maintains the transistor in strong inversion mode throughout the signal cycle, stabilizing the resistance and preventing harmonic generation while preserving high-frequency signal transmission capability

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies preliminary action by pre-charging bootstrap capacitors to establish the boosted VGS before the analog signal passes through the transistor. This preliminary voltage establishment ensures the transistor maintains constant resistance during signal transmission, eliminating harmonics before they can be generated

Inventive Principle:

Principle #10Preliminary action

2Object-generated harmful factors

If constant resistance is maintained through boosted VGS, then harmonics are prevented, but device complexity increases due to bootstrap circuits

Engineering Contradiction:

Improveharmonics eliminationVSAvoidbootstrap circuit structure

Core Design Contradiction:

Object-generated harmful factorsVSDevice complexity

Solution Approach 1:

The patent employs periodic action through bootstrap switches that periodically charge and discharge capacitors in synchronization with the multiplexer switching phases. This periodic charging maintains the boosted VGS condition during active signal transmission while simplifying the overall circuit by using standard CMOS switches and capacitors rather than complex continuous control circuits

Inventive Principle:

Principle #19Periodic action

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 solution effectively prevents the introduction of harmonics into the output signal, ensuring that the signal passed from the input to the output remains undistorted.

Implementation Method 1

The multiplexer includes two bootstrap circuits coupled to the source and gate terminals of the transistor. The two bootstrap circuits operate in alternate phases to maintain the constant VGS through operation.

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS20250047279A1Multiplexer with highly linear analog switch

Publication Date: 2025.02.06 STMICROELECTRONICS INT NV

AI summary

A multiplexer includes an input, an output, and a main switch configured to pass a signal from the input to the output. The multiplexer includes two bootstrap circuits that collectively maintain a constant voltage between terminals of the main switch during alternating phases.