Switchable Low-Pass Filter Circuit for Reduced In-Band Droop

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Solution Overview

Problem

Conventional RC filters face a tight tradeoff between in-band droop and out-of-band rejection, degrading performance in both aspects when attempting to improve one.

Innovation Solution

Incorporating a transistor and switchable components in the filter circuitry, allowing for selective activation or deactivation based on frequency ranges to achieve reduced in-band droop without compromising out-of-band rejection.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the cutoff frequency is increased to reduce in-band droop, then in-band signal attenuation is improved, but out-of-band rejection deteriorates

Engineering Contradiction:

Improvein-band signal transmissionVSAvoidout-of-band signal rejection

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The filter circuit transitions from a static RC configuration to a dynamic active filter using a transistor whose operating state can be controlled. The transistor's transconductance varies with input signal amplitude, automatically adjusting the filter characteristics to maintain optimal in-band transmission while preserving out-of-band rejection through nonlinear operation

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The filter utilizes changes in the transistor's operating parameters (specifically transconductance gm) in response to input signal conditions. By operating the transistor in different regions (linear vs. saturation), the effective filter parameters change dynamically, allowing the circuit to achieve reduced in-band droop while maintaining stopband attenuation

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If a conventional RC filter is used, then the circuit structure is simple, but the tradeoff between in-band droop and out-of-band rejection is tight

Engineering Contradiction:

Improvecircuit structureVSAvoidfilter performance

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The transistor acts as an intermediary element that couples the input signal to the RC filter network in a controlled manner. Rather than directly applying the input to the RC circuit, the transistor's controlled current source provides an intermediate signal path that modifies the effective impedance and transfer function, breaking the tight coupling between in-band and out-of-band characteristics

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The filter combines passive RC elements with an active transistor component to create a composite filter structure. This hybrid approach merges the simplicity and frequency-selective properties of passive components with the amplification and nonlinear characteristics of the active transistor, achieving superior overall performance

Inventive Principle:

Principle #40Composite materials

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 proposed filter circuitry provides improved low-pass filtering with reduced in-band droop and maintained out-of-band rejection across different frequency ranges, enhancing overall performance.

Implementation Method 1

a capacitor having a first terminal coupled to the second terminal of the resistor and having a second terminal coupled to a first power supply line

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

a transistor coupled to the first terminal of the resistor or the second terminal of the resistor

Methodology Applied

Scientific EffectField-effect transistor operation:

Implementation Method 3

an inductor coupled to the second terminal of the resistor

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Implementation Method 4

A low-pass filter is a circuit that passes signals having frequencies lower than a certain cutoff frequency while attenuating or rejecting signals having frequencies greater than the cutoff frequency

Methodology Applied

Scientific EffectLow-pass filtering: Filter (electronic)

Data Source

PatentUS20260012159A1Low-Pass Filter Circuitry

Publication Date: 2026.01.08 APPLE INC

AI summary

Filter circuitry is provided that includes a resistor having a first terminal coupled to an input of the filter circuitry and having a second terminal coupled to an output of the filter circuitry, a capacitor having a first terminal coupled to the second terminal of the resistor and having a second terminal coupled to a power supply line, and a transistor coupled to the first terminal of the resistor or the second terminal of the resistor. The transistor can be coupled to a bias current and additional capacitors. A switch can be coupled between the transistor and the first terminal of the resistor. A switch can be coupled between the transistor and the second terminal of the resistor. The filter circuitry can be configured to provide a low-pass filter response with a reduced in-band droop when the switch and the bias current are activated.