Voltage Mode Differential Driver With Parasitic Capacitance Cancellation

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Solution Overview

Problem

High-speed digital communication requires driver circuits that balance high operating speeds with low power consumption, as conventional current mode drivers consume excessive power and voltage mode drivers suffer from poor harmonic response and line impedance matching.

Innovation Solution

A high-speed, low-power voltage mode differential signal driver apparatus utilizing a cross-coupled transistor pair with an RC impedance circuit to cancel parasitic capacitance, improving edge rate performance while maintaining low power consumption.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If current mode drivers are used, then good harmonic response and line impedance matching are achieved, but power consumption increases

Engineering Contradiction:

Improveharmonic response and line impedance matchingVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The driver is divided into two functional segments: a voltage mode output stage for low power consumption and a current mode buffer stage for driving the RC impedance circuit. This segmentation allows each segment to optimize for its specific function while working together to achieve both low power and good signal integrity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The RC impedance circuit acts as an intermediary between the voltage mode output stage and the transmission line. It provides negative impedance to cancel parasitic capacitance, enabling the voltage mode driver to achieve harmonic response and impedance matching characteristics traditionally associated with current mode drivers.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Use of energy by moving object

If voltage mode drivers are used, then power consumption is reduced, but harmonic response and line impedance matching deteriorate

Engineering Contradiction:

Improvepower consumptionVSAvoidharmonic response and line impedance matching

Core Design Contradiction:

Use of energy by moving objectVSReliability

Solution Approach 1:

The RC impedance circuit serves as an intermediary that compensates for the deficiencies of voltage mode drivers. By providing negative impedance, it cancels parasitic capacitance effects, thereby improving harmonic response and impedance matching without requiring the driver itself to be in current mode.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The impedance characteristics of the driver are dynamically adjusted through the RC circuit, which presents a negative impedance that varies with frequency. This parameter change enables the system to achieve better harmonic response and impedance matching across the operating bandwidth.

Inventive Principle:

Principle #35Parameter changes

3Speed

If data rates are increased, then communication speed improves, but parasitic capacitance effects worsen edge rate performance

Engineering Contradiction:

Improvedata rateVSAvoidedge rate performance

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The parasitic capacitance, which normally degrades edge rate performance at high speeds, is converted into a benefit through the RC impedance circuit. The circuit is designed to present a negative impedance that specifically targets and cancels the parasitic capacitance, turning the harmful effect into an advantage for high-speed operation.

Inventive Principle:

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

Solution Approach 2:

The RC impedance circuit provides preliminary anti-action by pre-compensating for parasitic capacitance effects before they can degrade the signal. The negative impedance is configured to counteract the capacitive loading, preparing the circuit for high-speed operation and preventing edge rate deterioration.

Inventive Principle:

Principle #9Preliminary anti-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

The solution supports high operating speeds and improved edge rate performance while maintaining low power consumption, effectively addressing the limitations of conventional driver circuits.

Implementation Method 1

the RC impedance circuit appears as a negative impedance which can be used to cancel out some or all of the stray or parasitic capacitance of a driven circuit

Methodology Applied

Scientific EffectNegative impedance:

Implementation Method 2

The driver output circuit includes first and second circuit branches individually including two transistors, an intervening resistor and a current source

Methodology Applied

Scientific EffectTransistor amplification:

Data Source

PatentUS8884655B2Low-power voltage mode high speed driver

Publication Date: 2014.11.11 TEXAS INSTRUMENTS INC

AI summary

Differential voltage mode signal driver circuitry is presented in which a differential current mode amplifier input stage provides a differential signal, and an output stage includes a pair of bipolar transistors receiving the differential signal and being connected in series with a pair of cross-coupled field effect transistors that are coupled to corresponding current sources, where a negative impedance circuit is connected between the field effect transistors to substantially cancel a parasitic capacitance of a driven output circuit.