MOSFET Varactor Bias Circuit for Low-Voltage Temperature Compensation

Solution Overview

Problem

Conventional varactor temperature compensation schemes are ineffective at low power supply voltages, leading to increased varactor capacitance with temperature, which affects the output frequency of voltage-controlled oscillators, and are complex and costly due to the need for precise current source matching.

Innovation Solution

A bias circuit using a diode-connected metal-oxide semiconductor field effect transistor (MOSFET) with a switched-capacitor common-mode feedback circuit generates a differential voltage from the gate-to-source voltage, effectively compensating for temperature-induced capacitance changes without requiring large MOSFET sizes, even at low power supply voltages.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If a BJT-based bias circuit is used to compensate for temperature-induced capacitance changes, then the varactor capacitance stability is improved, but the power supply voltage requirement increases and circuit density decreases

Engineering Contradiction:

Improvevaractor capacitance stabilityVSAvoidpower supply voltage

Core Design Contradiction:

Stability of the object's compositionVSPower

Solution Approach 1:

The patent changes the transistor type from BJT to MOSFET and operates the MOSFET in subthreshold region, fundamentally altering the voltage-capacitance relationship. The gate-to-source voltage of the subthreshold MOSFET has a different temperature coefficient that can compensate for varactor drift while operating at lower supply voltages (e.g., 1.8V or 1.2V instead of requiring higher voltages for BJT threshold compensation).

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the BJT threshold voltage mechanism with a MOSFET gate-to-source voltage mechanism. By taking the derivative of the MOSFET's gate-to-source voltage with respect to temperature in the subthreshold region, the circuit generates a compensating signal that replaces the traditional BJT-based compensation approach, enabling operation at lower power supply voltages.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Stability of the object's composition

If a BJT-based bias circuit is used to compensate for temperature-induced capacitance changes, then the varactor capacitance stability is improved, but the circuit area increases

Engineering Contradiction:

Improvevaractor capacitance stabilityVSAvoidbias circuit area

Core Design Contradiction:

Stability of the object's compositionVSArea of stationary object

Solution Approach 1:

The patent changes the transistor type from BJT to MOSFET and operates the MOSFET in subthreshold region, fundamentally altering the voltage-capacitance relationship. The gate-to-source voltage of the subthreshold MOSFET has a different temperature coefficient that can compensate for varactor drift while operating at lower supply voltages (e.g., 1.8V or 1.2V instead of requiring higher voltages for BJT threshold compensation).

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the BJT threshold voltage mechanism with a MOSFET gate-to-source voltage mechanism. By taking the derivative of the MOSFET's gate-to-source voltage with respect to temperature in the subthreshold region, the circuit generates a compensating signal that replaces the traditional BJT-based compensation approach, enabling operation at lower power supply voltages.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Power

If current-source-based bias circuit is used to reduce power supply voltage, then the power consumption is reduced, but the design complexity increases due to current source matching requirements

Engineering Contradiction:

Improvepower supply voltageVSAvoidcurrent source matching complexity

Core Design Contradiction:

PowerVSDevice complexity

Solution Approach 1:

The patent employs a switched-capacitor common-mode feedback circuit that automatically regulates the operating point of the differential pair. This self-regulating mechanism eliminates the need for manual matching of current sources, as the feedback circuit dynamically adjusts to maintain proper bias conditions, thereby reducing design complexity while enabling operation at lower power supply voltages.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The patent employs a switched-capacitor common-mode feedback circuit that automatically regulates the operating point of the differential pair. This self-regulating mechanism eliminates the need for manual matching of current sources, as the feedback circuit dynamically adjusts to maintain proper bias conditions, thereby reducing design complexity while enabling operation at lower power supply voltages.

Inventive Principle:

Principle #23Feedback

4Power

If the MOSFET size is increased to lower the threshold voltage for reduced power supply voltage, then the power supply voltage is reduced, but the circuit density decreases

Engineering Contradiction:

Improvepower supply voltageVSAvoidMOSFET size

Core Design Contradiction:

PowerVSArea of stationary object

Solution Approach 1:

The patent changes the transistor type from BJT to MOSFET and operates the MOSFET in subthreshold region, fundamentally altering the voltage-capacitance relationship. The gate-to-source voltage of the subthreshold MOSFET has a different temperature coefficient that can compensate for varactor drift while operating at lower supply voltages (e.g., 1.8V or 1.2V instead of requiring higher voltages for BJT threshold compensation).

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the BJT threshold voltage mechanism with a MOSFET gate-to-source voltage mechanism. By taking the derivative of the MOSFET's gate-to-source voltage with respect to temperature in the subthreshold region, the circuit generates a compensating signal that replaces the traditional BJT-based compensation approach, enabling operation at lower power supply voltages.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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 maintains varactor capacitance stability over temperature changes while reducing power consumption and preserving circuit density, minimizing the impact of ground bounce and common-mode noise.

Implementation Method 1

The differential voltage is derived from the gate-to-source voltage of the diode-connected transistor

Methodology Applied

Scientific EffectGate-to-source voltage of MOSFET:

Implementation Method 2

A switched-capacitor common-mode feedback circuit controls a bias of the diode-connected MOSFET to produce a differential voltage having a desired common-mode voltage

Methodology Applied

Scientific EffectCommon-mode feedback: Feedback

Implementation Method 3

the capacitance for the varactors is substantially temperature compensated

Methodology Applied

Scientific EffectTemperature compensation:

Data Source

PatentUS10340922B1Bias circuit for temperature-compensated varactor

Publication Date: 2019.07.02 QUALCOMM INC

AI summary

A bias circuit is provided that is configure to control the bias for a diode-connected transistor operating in the sub-threshold region to produce a gate-to-source voltage. A differential tuning voltage derived from the gate-to-source voltage tunes a plurality of varactors.