Cascode TIA Gain Control with Tunable Node Shunting

Solution Overview

Problem

High-speed fiber optic communication systems require trans-impedance amplifiers (TIAs) with variable gain to maintain performance metrics like bandwidth and group delay variation constant across varying transimpedance settings, but existing solutions face issues such as gain-bandwidth product reduction, linearity penalties, and noise increases due to voltage gain variations.

Innovation Solution

A TIA circuit with a cascode topology and tunable elements, including MOS transistors, that variably shunt the cascode node and load resistor to control voltage gain, allowing up to 24 dB transimpedance variation while minimizing impact on noise, linearity, and bandwidth.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If voltage gain of the TIA circuit is varied to control transimpedance gain, then the transimpedance gain can be adjusted across a wide dynamic range, but noise performance deteriorates and linearity penalties increase

Engineering Contradiction:

Improvetransimpedance gain adjustment rangeVSAvoidnoise and linearity penalties

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The patent segments the gain control function into two independent parts: (1) transimpedance gain control via feedback resistor adjustment, and (2) voltage gain stabilization via cascode node shunting. This segmentation allows each part to be optimized independently, enabling wide transimpedance adjustment range while maintaining stable voltage gain to avoid noise and linearity penalties

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces an intermediary circuit element (tunable shunt at the cascode node) that mediates between the feedback resistor adjustment and the voltage gain. This intermediary allows the voltage gain to be decoupled from the transimpedance gain variation, enabling independent control of transimpedance gain while maintaining stable voltage gain for optimal noise and linearity performance

Inventive Principle:

Principle #24Intermediary (Mediator)

2Adaptability or versatility

If feedback resistor value is adjusted to vary transimpedance gain, then transimpedance gain can be controlled, but voltage gain becomes unstable affecting noise and linearity

Engineering Contradiction:

Improvetransimpedance gain controlVSAvoidvoltage gain stability

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The patent introduces an intermediary circuit element (tunable shunt at the cascode node) that mediates between the feedback resistor adjustment and the voltage gain. This intermediary allows the voltage gain to be decoupled from the transimpedance gain variation, enabling independent control of transimpedance gain while maintaining stable voltage gain for optimal noise and linearity performance

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent changes the impedance parameter at the cascode node dynamically to compensate for voltage gain variations. By adjusting the shunt impedance at the cascode node in coordination with feedback resistor changes, the voltage gain is stabilized while allowing transimpedance gain to vary across a wide range

Inventive Principle:

Principle #35Parameter changes

3Adaptability or versatility

If wide transimpedance gain variation is implemented, then adaptability to different signal levels is improved, but performance metrics like bandwidth and group delay become inconsistent

Engineering Contradiction:

Improvesignal level adaptabilityVSAvoidbandwidth and group delay consistency

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The patent segments the gain control function into two independent parts: (1) transimpedance gain control via feedback resistor adjustment, and (2) voltage gain stabilization via cascode node shunting. This segmentation allows each part to be optimized independently, enabling wide transimpedance adjustment range while maintaining stable voltage gain to avoid noise and linearity penalties

Inventive Principle:

Principle #1Segmentation

Data Source

PatentUS20240195373A1TIA with tunable gain

Publication Date: 2024.06.13 NOKIA SOLUTIONS & NETWORKS OY

AI summary

An apparatus, such as a coherent optical receiver, includes a TIA, the TIA including a cascode circuit having a cascode node. A first tunable element is connected to tunably shunt the cascode node to vary a voltage gain of the TIA, e.g., up to a first amount. Implementations of the TIA further include another tunable element connected to vary a load of the cascode circuit to vary the voltage gain, e.g., up to a second amount. A current steering circuit may be provided to vary the voltage gain up to a third amount, each of the amounts being only a fraction of a target voltage gain variation of the TIA.