Chopper Amplifier Paths Decoupling Bandwidth and Input Bias Current

Solution Overview

Problem

Increasing the bandwidth of chopper amplifiers to enhance signal strength leads to an undesirable increase in input bias current, causing error voltage in signal paths due to charge-transfer and charge-injection from input chopper switches.

Innovation Solution

A chopper amplifier circuit with a first amplifier path and a second amplifier path, where the first path includes chopper circuitry and a switched-capacitor filter, and the second path includes a feed-forward gain stage, allowing the chopping frequency to be decoupled from a threshold frequency to manage input bias current and bandwidth targets.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If the chopping frequency is increased to increase the bandwidth, then the bandwidth is improved, but the input bias current increases

Engineering Contradiction:

ImprovebandwidthVSAvoidinput bias current

Core Design Contradiction:

SpeedVSQuantity of substance

Solution Approach 1:

The amplifier is divided into two separate paths: a first amplifier path with chopper circuitry for low-frequency signals and a second amplifier path without chopper circuitry for high-frequency signals. This segmentation allows each path to be optimized independently, enabling the chopper amplifier to achieve high bandwidth through the second path while maintaining low input bias current through the first path for its operating range

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the operating parameters by introducing a threshold frequency that separates the operating ranges of the two amplifier paths. Below the threshold frequency, the first path with chopper circuitry operates at a lower effective chopping frequency to maintain low input bias current. Above the threshold frequency, the second path operates to provide high bandwidth response, effectively changing the frequency-dependent parameters of the amplifier

Inventive Principle:

Principle #35Parameter changes

2Speed

If the chopping frequency is increased to increase the bandwidth, then the bandwidth is improved, but the error voltage increases

Engineering Contradiction:

ImprovebandwidthVSAvoiderror voltage

Core Design Contradiction:

SpeedVSObject-generated harmful factors

Solution Approach 1:

The amplifier is divided into two separate paths: a first amplifier path with chopper circuitry for low-frequency signals and a second amplifier path without chopper circuitry for high-frequency signals. This segmentation allows each path to be optimized independently, enabling the chopper amplifier to achieve high bandwidth through the second path while maintaining low input bias current through the first path for its operating range

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention converts the harmful effect of chopper circuitry (which generates error voltage and requires high chopping frequency for bandwidth) into a benefit by using it only where necessary (low-frequency path) while providing a clean amplification path (second path) for high-frequency signals, thus eliminating the harm in the frequency range where it would be most problematic

Inventive Principle:

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

Data Source

PatentUS10756685B2Chopper amplifier with decoupled chopping frequency and threshold frequency

Publication Date: 2020.08.25 TEXAS INSTRUMENTS INC

AI summary

A chopper amplifier circuit includes a first amplifier path with chopper circuitry, a switched-capacitor filter, and multiple gain stages. The chopper amplifier circuit also includes a second amplifier path with a feed-forward gain stage. A chopping frequency of the chopper circuitry is greater than a threshold frequency at which the second amplifier path is used instead of the first amplifier path.