CTAT Voltage Reference Circuit With BJT Nonlinearity Compensation

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

High-precision temperature sensors based on Bipolar Junction Transistors (BJTs) suffer from non-linearity in base-emitter voltage, leading to inaccurate temperature readings due to inherent device and bias-dependent dependencies.

Innovation Solution

A circuit is designed that uses a pair of BJTs configured in diode mode, with one biased by a proportional to absolute temperature (PTAT) current source and the other by a current source independent of absolute temperature, to generate a delta base-emitter voltage, which compensates for the non-linearity of the first BJT, resulting in a more linear output.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single BJT is used to generate CTAT voltage reference, then the circuit is simple, but the output has significant non-linearity

Engineering Contradiction:

Improvecircuit complexityVSAvoidoutput linearity

Core Design Contradiction:

Device complexityVSManufacturing precision

Solution Approach 1:

The circuit is divided into multiple functional blocks: a first BJT (Q1) generating the primary CTAT voltage, and a second block with two BJTs (Q2, Q3) generating the compensating non-linear component. This segmentation allows each block to have a specific function while collectively achieving the overall goal of non-linearity compensation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention converts the harmful non-linear effect into a beneficial compensation mechanism. The second block deliberately introduces a non-linear component that is complementary to the non-linearity of the first block, so that when combined, they cancel each other out and produce a more linear output.

Inventive Principle:

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

2Manufacturing precision

If multiple compensation blocks are added to reduce non-linearity, then output linearity improves, but device complexity increases

Engineering Contradiction:

Improveoutput linearityVSAvoidcircuit complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The invention changes the operating parameters of the BJTs, specifically using different current densities and temperature dependencies. By carefully selecting the current sources (one PTAT, one independent of temperature) and emitter area ratios, the circuit achieves non-linearity compensation through parameter optimization rather than complex circuit topologies.

Inventive Principle:

Principle #35Parameter changes

3Temperature

If BJT base-emitter voltage is used for temperature sensing, then temperature dependency is achieved, but non-linearity and base-emitter voltage variation occur

Engineering Contradiction:

Improvetemperature sensing capabilityVSAvoidtemperature measurement accuracy

Core Design Contradiction:

TemperatureVSMeasurement precision

Solution Approach 1:

The circuit implements a form of feedback where the output of the first BJT block is fed into the second block, which generates a compensating signal based on the same temperature-dependent parameters. This feedback mechanism allows the circuit to automatically correct for non-linearities without external intervention.

Inventive Principle:

Principle #23Feedback

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 circuit significantly reduces non-linearity in temperature-dependent voltage references, improving the accuracy of temperature readings by at least partially canceling out the non-linear components, with a substantial reduction in output non-linearity from 4.2 mV to around 2 uV peak-to-peak.

Implementation Method 1

The base-emitter voltage, Vbe (T), temperature dependency is known from the art, and is shown in Equation (1): Vbe(T) = VG0 - (XTI/k)ln(T/T0)

Methodology Applied

Scientific EffectTemperature-dependent base-emitter voltage:

Data Source

PatentUS11320319B2Circuit for generating a temperature dependent output

Publication Date: 2022.05.03 ANALOG DEVICES INT UNLTD CO

AI summary

The present disclosure provides a circuit for generating a complimentary to absolute temperature (CTAT) voltage reference. The primary contributor to the voltage reference is first bipolar junction transistor, which is configured in diode mode, to produce the CTAT voltage. Such references include a non-linear component. A pair of bipolar junction transistors are coupled to the first bipolar junction transistor, and are configured to generate a delta base-emitter voltage. By coupling one of the pair to a proportional to absolute temperature current source, and the other to a current course which is substantially independent of absolute temperature, a further non-linear component is introduced, which is complimentary to the non-linear component introduced by the first bipolar junction transistor. The pair of bipolar transistors share a common emitter area size. As such, the non-linear component of the first bipolar junction transistor is compensated by the delta base-emitter arrangement, resulting in a more linear output.