Transimpedance Amplifier Gain Control Without Feedback Parasitics

Solution Overview

Problem

Transimpedance amplifiers in optical receivers face challenges in achieving a wide dynamic range due to the introduction of parasitic capacitance from nFETs in the feedback path, which compromises input referred noise and bandwidth, limiting their ability to detect both small and large optical signals without distortion.

Innovation Solution

A transimpedance amplifier circuit with a variable shunt resistance connected to the input node, allowing for controllable variation of open-loop and closed-loop transimpedance gain through a controller, which injects a control current into a circuit branch to adjust the resistance, thereby maintaining a DC voltage across a resistor at zero and enabling high-gain or low-gain modes based on input current indications.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If an nFET is used in the feedback path to achieve automatic gain control, then the dynamic range is improved, but the input referred noise and bandwidth are compromised due to excessive parasitic capacitance

Engineering Contradiction:

Improvedynamic rangeVSAvoidinput referred noise

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The patent extracts the gain control function from the feedback path by removing the nFET and implementing it instead through a variable shunt resistance connected to the inverting input node. This separates the gain control mechanism from the feedback element, eliminating the parasitic capacitance problem while preserving the dynamic range capability through electronic control of the shunt resistance value.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces a variable shunt resistance as an intermediary element to achieve gain control. This shunt resistance, controlled by a separate circuit (such as a digitally controlled resistor or analog switch network), mediates the gain adjustment without being part of the feedback path, thus avoiding the introduction of excessive parasitic capacitance that would occur with an nFET in the feedback element.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Adaptability or versatility

If an nFET is used in the feedback path to achieve automatic gain control, then the dynamic range is improved, but the bandwidth is compromised due to excessive parasitic capacitance

Engineering Contradiction:

Improvedynamic rangeVSAvoidbandwidth

Core Design Contradiction:

Adaptability or versatilityVSSpeed

Solution Approach 1:

The patent extracts the gain control function from the feedback path by removing the nFET and implementing it instead through a variable shunt resistance connected to the inverting input node. This separates the gain control mechanism from the feedback element, eliminating the parasitic capacitance problem while preserving the dynamic range capability through electronic control of the shunt resistance value.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces a variable shunt resistance as an intermediary element to achieve gain control. This shunt resistance, controlled by a separate circuit (such as a digitally controlled resistor or analog switch network), mediates the gain adjustment without being part of the feedback path, thus avoiding the introduction of excessive parasitic capacitance that would occur with an nFET in the feedback element.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Object-affected harmful factors

If the TIA is designed for high gain to detect small signals, then the input referred noise is minimized, but the capability to detect large signals without distortion is reduced

Engineering Contradiction:

Improveinput referred noiseVSAvoidsignal detection capability

Core Design Contradiction:

Object-affected harmful factorsVSAdaptability or versatility

Solution Approach 1:

The patent implements dynamic gain control through a variable shunt resistance that can be electronically adjusted based on the input signal level. This allows the TIA to adapt its gain characteristic in real-time, operating in high-gain mode for small signals to minimize noise impact, and switching to low-gain mode for large signals to prevent saturation and distortion, thus achieving wide dynamic range coverage.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent employs automatic gain control feedback mechanisms where the control circuit monitors the output signal level and adjusts the shunt resistance accordingly. When large signals are detected, the feedback control reduces the gain by adjusting the shunt resistance, preventing overload conditions while maintaining the ability to detect small signals when they occur.

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS20240283419A1Transimpedance gain control

Publication Date: 2024.08.22 SEMTECH CORP

AI summary

This application describes apparatus and method for transimpedance gain control. A transimpedance amplifier circuit is described with a transimpedance amplifier having an input node for receiving an input current. A variable shunt resistance is connected to the input node and a controller is operable to controllably vary an open-loop transimpedance gain of the transimpedance amplifier and also the resistance of the variable shunt resistance so as to vary a closed-loop transimpedance gain of the transimpedance amplifier.