Fully Differential Amplifier for Low-Voltage Common-Mode Suppression

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Solution Overview

Problem

Conventional fully-differential amplifiers using differential pairs struggle to operate effectively at low power supply voltages, leading to insufficient gain and instability in common-mode signal suppression, especially in micro CMOS processes where power supply voltages are decreasing.

Innovation Solution

A fully-differential amplifier configuration utilizing single-stage CMOS inverting amplifiers with feedforward or feedback operations to cancel common-mode signals, allowing for sufficient gain and stable common-mode voltage control even at low power supply voltages, and a cascade connection of amplifiers with common-mode feedforward and feedback functions to achieve high gain with minimal current consumption.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional differential pair configuration is used to suppress common-mode signals, then common-mode suppression is achieved, but the amplifier cannot operate effectively at low power supply voltages (0.9V or less)

Engineering Contradiction:

Improvecommon-mode signal suppressionVSAvoidoperation at low power supply voltage

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The amplifier is divided into two independent single-stage inverting amplifiers instead of using a coupled differential pair structure. Each amplifier processes one input signal independently, eliminating the need for matched transistor pairs and allowing operation at low supply voltages while maintaining common-mode rejection through the inverting configuration itself

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent uses inverting amplifiers instead of non-inverting differential pair amplifiers. The inversion property inherently provides common-mode rejection because equal common-mode signals applied to both inputs produce equal inverted outputs, which can be differentially combined to cancel the common-mode component while preserving differential signals

Inventive Principle:

Principle #13The other way round (Inversion)

2Loss of energy

If the power supply voltage is reduced to enable low-power operation, then power consumption decreases, but the output amplification becomes insufficient and common-mode voltage control becomes unstable

Engineering Contradiction:

Improvepower consumptionVSAvoidoutput amplification and common-mode voltage control

Core Design Contradiction:

Loss of energyVSReliability

Solution Approach 1:

A common-mode feedback circuit is implemented that monitors the common-mode voltage at the outputs and adjusts the bias conditions to maintain stable common-mode voltage control. This feedback mechanism ensures that even at low power supply voltages, the amplifier can maintain proper operating points and stable common-mode rejection without requiring high supply voltages

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent changes the operating parameters by using single-stage inverting amplifiers with specific biasing schemes that are optimized for low-voltage operation. The transconductance and output impedance are adjusted through parameter selection to achieve sufficient gain and stability at low supply voltages, unlike conventional differential pairs that require higher voltages to maintain proper operation

Inventive Principle:

Principle #35Parameter changes

3Ease of operation

If single-stage inverting amplifiers are used instead of differential pairs, then operation at low power supply voltages is enabled, but additional circuits are needed to suppress common-mode signals and control output common-mode voltage

Engineering Contradiction:

Improveoperation at low power supply voltageVSAvoidcircuit configuration

Core Design Contradiction:

Ease of operationVSDevice complexity

Solution Approach 1:

The common-mode suppression function and the amplification function are merged into the same inverting amplifier stages. The inverting configuration inherently provides common-mode rejection, eliminating the need for separate differential pair structures. The common-mode feedback circuit is integrated to control output common-mode voltage, combining multiple functions into a unified low-voltage compatible architecture

Inventive Principle:

Principle #5Merging (Combining)

Data Source

PatentUS7619473B2Fully-differential amplifier

Publication Date: 2009.11.17 ROHM CO LTD

AI summary

A fully-differential amplifier able to operate at a low power supply voltage and provided with a common-mode signal suppression function is disclosed. This fully-differential amplifier is provided with a first fully-differential amplifier configured by a single-stage configuration inverting amplifier and canceling out the common-mode signal of the input side by a feedforward means and a second fully-differential amplifier configured by a single-stage configuration inverting amplifier and canceling out the common-mode signal of the output side by a feedback means, the output of the first fully-differential amplifier being connected to the input of the second fully-differential amplifier.