Schmitt Trigger Inverter with Supply-Independent Hysteresis
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Solution Overview
Problem
Conventional Schmitt trigger inverters are susceptible to noise-induced state changes and oscillations due to a single threshold voltage, which can lead to instability in high noise environments, especially when the supply voltage varies.
Innovation Solution
A Schmitt trigger inverter circuit with a switch that couples a predetermined reference voltage to a node within the first inverter, independent of the supply voltage, generating hysteresis that remains constant despite supply voltage variations, using PFET and NFET devices with bias voltages to limit voltage across devices within operational tolerances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional inverter with a single threshold voltage is used, then the circuit is simple, but the noise immunity is poor and oscillations occur in high noise environments
Solution Approach 1:
The patent divides the single inverter into two cascaded inverters with different threshold voltages. The first inverter has a higher threshold voltage and the second has a lower threshold voltage, creating two distinct switching points that form a hysteresis loop. This segmentation allows the circuit to maintain stability against noise while preserving the basic inverter structure.
Solution Approach 2:
The patent introduces positive feedback by connecting the output of the second inverter back to the input of the first inverter through a feedback network. This feedback mechanism creates the hysteresis effect where the threshold voltage depends on the previous state of the circuit, preventing noise-induced oscillations and improving noise immunity.
2Reliability
If Schmitt trigger architecture is implemented to improve noise immunity, then hysteresis is achieved, but the threshold voltage becomes dependent on supply voltage variations
Solution Approach 1:
The patent changes the operating parameters of the inverters by introducing bias voltages that are referenced to the supply voltage. By using supply-voltage-dependent biasing schemes, the threshold voltages of both inverters shift together with supply voltage variations, maintaining a constant voltage difference between them and thus preserving stable hysteresis characteristics across different supply conditions.
3Reliability
If the inverter threshold voltage is adjusted to improve noise immunity, then hysteresis is enhanced, but the voltage across devices may exceed operational tolerances
Solution Approach 1:
The patent introduces intermediate bias voltage nodes and voltage division networks that act as mediators between the supply voltage and the inverter devices. These intermediary elements ensure that the voltage across each device remains within safe operational limits while still achieving the desired threshold voltage adjustment for noise immunity, preventing direct exposure to full supply voltage swings.
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
The solution provides stable hysteresis and prevents noise-induced state changes, maintaining operational stability across varying supply voltages by ensuring the hysteresis remains constant, thus enhancing noise immunity and operational reliability.
Implementation Method 1
The Schmitt trigger can include a switch coupled to an output of the second inverter. The switch can be selectively enabled responsive to the output signal. The switch can be configured to couple a predetermined reference voltage to a source terminal of the first pull-down device when in an enabled state. Coupling the predetermined reference voltage can alter a threshold voltage of the Schmitt trigger.
Implementation Method 2
The first inverter can include a second pull-up device including a source terminal coupled to a drain terminal of the first pull-up device and a gate terminal coupled to a first bias voltage. The first inverter also can include a third pull-down device including a source terminal coupled to a drain terminal of the first pull-down device, a drain terminal coupled to a drain terminal of the second pull-up device, and a gate terminal coupled to a second bias voltage.
Data Source
AI summary
A Schmitt trigger inverter circuit can include a first inverter. The first inverter can include a first pull-up device, a first pull-down device and a second pull-down device. The first inverter can receive an input signal. The Schmitt trigger inverter circuit can include a second inverter coupled in series with the first inverter and including an output that generates an output signal. The Schmitt trigger inverter circuit further can include a switch coupled to the output of the second inverter circuit and that is selectively enabled by the output signal. The switch can couple a predetermined reference voltage to a source terminal of the first pull-down device when in an enabled state. Coupling the predetermined reference voltage to the source terminal of the first pull-down device can alter a threshold voltage of the Schmitt trigger inverter circuit.