Current-Mode Filter Circuit for Low-Voltage Wideband Linearity

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Solution Overview

Problem

Conventional Gm-C filters face challenges in achieving accurate frequency characteristics, wide dynamic range, and linearity, especially in high-frequency regions, and lack applicability for current-mode band-pass and high-pass filters, requiring large circuit areas and high voltage operations.

Innovation Solution

A current-mode filter circuit with a minimum configuration, incorporating a transconductance adjustment circuit suitable for low voltage and small area, and capable of operating as high-pass and band-pass filters, is developed, using a current mirror circuit with field effect transistors and capacitive elements, allowing for independent control of filter parameters without relying on transconductance parameters.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a conventional Gm-C filter is used, then the filter can operate in voltage mode, but it cannot achieve accurate frequency characteristics in high-frequency regions due to parasitic pole proximity

Engineering Contradiction:

Improvefrequency characteristics accuracyVSAvoidparasitic pole effect

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent replaces the voltage-mode Gm-C filter with a current-mode filter architecture. This substitution changes the fundamental operating mode from voltage to current, thereby eliminating the parasitic pole issues that plague voltage-mode filters at high frequencies. The current-mode operation allows for accurate frequency characteristics even in high-frequency regions.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Use of energy by moving object

If a Gm-C filter operates in voltage mode with low voltage operation, then power consumption is reduced, but dynamic range and linearity cannot be ensured

Engineering Contradiction:

Improvepower consumptionVSAvoiddynamic range and linearity

Core Design Contradiction:

Use of energy by moving objectVSReliability

Solution Approach 1:

The patent substitutes voltage-mode operation with current-mode operation. In current-mode filters, the signal is processed through current mirrors and transconductance stages that inherently provide better dynamic range and linearity even at low supply voltages. This is achieved through the use of MOS transistors operating in saturation region with proper biasing, allowing low power consumption while maintaining signal integrity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Productivity

If conventional current-mode filters are designed, then they can achieve wider band and linearity, but they require large circuit areas and high voltage operations

Engineering Contradiction:

Improvebandwidth and linearityVSAvoidcircuit area

Core Design Contradiction:

ProductivityVSArea of stationary object

Solution Approach 1:

The patent designs a universal current-mode filter structure that can function as low-pass, band-pass, or high-pass filters by simply changing the connection configuration of capacitive elements and transistors, without requiring separate circuits for each filter type. This multi-functionality reduces the overall circuit area while maintaining wide bandwidth and linearity characteristics.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent achieves different filter responses (low-pass, band-pass, high-pass) by changing the connection parameters and configuration of existing components rather than changing the fundamental circuit topology. By adjusting which capacitive elements are connected to which transistor nodes, the same current-mode structure can provide multiple filter functions with optimized bandwidth and linearity.

Inventive Principle:

Principle #35Parameter changes

4Productivity

If conventional current-mode filters are designed, then they can achieve wider band and linearity, but they require high voltage operations

Engineering Contradiction:

Improvebandwidth and linearityVSAvoidoperating voltage

Core Design Contradiction:

ProductivityVSTemperature

Solution Approach 1:

The patent employs current-mode operation with MOS transistors that can maintain wide bandwidth and linearity at low supply voltages. The current mirrors and transconductance stages are designed to operate efficiently in the saturation region with minimal voltage headroom, eliminating the need for high voltage operations while preserving the performance benefits of current-mode filtering.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS8350619B2Filter circuit and optical disk device including the same

Publication Date: 2013.01.08 PANASONIC HOLDINGS CORP

AI summary

A current-mode filter includes a first, a second, and a third transistor having the same channel polarity. The drain of the first transistor is connected to the source of the second transistor functioning as a gate grounded circuit. The drain of the second transistor is connected to the gates of the first and third transistors. A first and a second capacitive element are connected to the gate and drain of the first transistor. The current source supplies a bias current to each of the first and second transistors. The drain of the first transistor is used as an input terminal. An output signal is extracted from a drain current of the third transistor. Therefore, only one transconductance adjustment circuit is enough.