Ring Oscillator Bias Circuit for Temperature-Stable Low-Power Frequency

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Solution Overview

Problem

Current low-power voltage controlled oscillators, such as current-starved oscillators, exhibit a negative temperature gradient, causing the oscillation frequency to decrease with rising temperature, which cannot be effectively trimmed off-chip, posing a challenge for temperature stability in digital and smart phone circuits.

Innovation Solution

A ring oscillator device with a bias circuit using a current source with a main n-type metal-oxide semiconductor transistor, providing a control current proportional to a temperature-independent reference voltage and a gate-source voltage with a negative temperature coefficient, which compensates for temperature-dependent frequency drifts, ensuring temperature stability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Use of energy by moving object

If a current-starved oscillator is used to achieve low power consumption, then power consumption is reduced, but temperature stability deteriorates due to negative temperature gradient

Engineering Contradiction:

Improvepower consumptionVSAvoidtemperature stability

Core Design Contradiction:

Use of energy by moving objectVSStability of the object's composition

Solution Approach 1:

The patent converts the harmful negative temperature coefficient of the oscillation frequency into a beneficial feature by using a current source with positive temperature coefficient to compensate for it. The main NMOS transistor's gate-source voltage has negative temperature coefficient, which when used in the current source creates a positive temperature coefficient current that compensates the oscillation frequency drift, thereby improving temperature stability while maintaining low power consumption

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent changes the temperature coefficient parameter of the control current from negative to positive by utilizing the negative temperature coefficient of the NMOS transistor's gate-source voltage. This parameter change enables the current source to provide compensation current that counteracts the temperature-dependent frequency drift, resolving the temperature stability issue

Inventive Principle:

Principle #35Parameter changes

2Use of energy by moving object

If current-starved delay elements are used to reduce operation current, then operation current is reduced, but frequency stability deteriorates due to temperature effects

Engineering Contradiction:

Improveoperation currentVSAvoidfrequency stability

Core Design Contradiction:

Use of energy by moving objectVSReliability

Solution Approach 1:

The patent implements a feedback mechanism where the bias circuit continuously monitors and adjusts the control current based on temperature variations. The main NMOS transistor's temperature-dependent gate-source voltage provides automatic feedback that modulates the current source output, creating a compensating effect that maintains frequency stability despite temperature changes and low operation current

Inventive Principle:

Principle #23Feedback

3Reliability

If MOS transistor mobility decreases with increasing temperature, then transistor resistance increases, but oscillation frequency drops due to negative temperature gradient

Engineering Contradiction:

Improvetransistor resistance stabilityVSAvoidoscillation frequency

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The patent applies a counterweight approach by introducing a control current with positive temperature coefficient that opposes and compensates for the negative temperature gradient effect. The main NMOS transistor's temperature-dependent characteristics are harnessed to generate this compensating current, which counteracts the frequency drop caused by increased transistor resistance at higher temperatures

Inventive Principle:

Principle #8Anti-weight (Counterweight)

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 a temperature-stable oscillation frequency with low operation current, eliminating the need for a level shifter and allowing for a wide output swing, thus addressing the temperature instability issue in current-starved oscillators.

Implementation Method 1

the gate-source voltage of the main NMOS-transistor comprises a negative temperature coefficient

Methodology Applied

Scientific EffectTemperature coefficient effect:

Data Source

PatentUS10992288B2Oscillator device

Publication Date: 2021.04.27 INVENSENSE INC

AI summary

In an embodiment an oscillator device includes a ring oscillator circuit with at least one delay stage with an output of a last delay stage fed back to an input of a first delay stage, wherein each of the delay stages is configured to receive a charging current and to provide a delay that is dependent on the charging current and at least one of the delay stages includes a metal-oxide-semiconductor field-effect transistor and a bias circuit including an output terminal coupled to an input terminal of the ring oscillator circuit, wherein the bias circuit is configured to receive a temperature-independent reference voltage and includes a current source with a main NMOS-transistor, the current source configured to provide a control current to the ring oscillator circuit which is proportional to a difference of the temperature-independent reference voltage and a gate-source voltage of the main NMOS-transistor, and wherein the gate-source voltage of the main NMOS-transistor includes a negative temperature coefficient.