Ever streamed a 4K video from halfway across the world? That picture starts as a faint flicker racing through glass fiber. The first chip that hears that flicker and turns it into a strong, clean electrical signal ultimately playing the video in your laptop is a TIA — basically a microphone for light.

High-speed communications -- AI data centers, coherent optics, pluggables, and short-reach mmWave – depend on analog bandwidth and signal integrity. At the optical–electrical boundary, the Transimpedance Amplifier (TIA) sets the receiver front-end bandwidth – with low input impedance and a phase-flat response—and delivers a clean, ultra-broadband voltage that sustains 100+ Gb/s performance.

In this part, we’ll only explore the need for ultra-wideband TIAs, why they are relevant in optical communication and its function.

What is a TIA? (No math, just the idea)

TIA is the first chip after a light sensor (photodiode) shown in figure 1.

It takes the tiny electrical current that the photodiode produces when light hits it and turns that into a clean, usable voltage signal for the rest of the electronics. The photocurrent is very small (10-100uA) and only appears when any light is present creating a PRBS signal which is the input of the TIA. TIA amplifies the signal for further processing.

The Transimpedance Bandwidth of the amplified output of the TIA is directly related to the optical communication data rate.

Data rate, R_(b ) ≈ B_TIA / 0.7​

The higher the transimpedance bandwidth, the greater the data rate.

3) Why a TIA is Needed (the physics in one breath)

Photodiodes (PD) behave like current sources with parallel capacitance. A voltage amplifier “looks” into that capacitance and loses the fast edges.

TIA holds the PD node near a constant voltage (virtual ground). That keeps the PD’s reverse bias steady, so its capacitance and responsivity don’t swing with signal level.

Other amplifiers don’t offer this, instead the high i/p impedance exacerbates the signal. That’s why every optical receiver—NRZ, PAM-4, coherent—begins with a TIA.

4) AI, Optical Communication & TIA -

AI Data Centers – Problems faced in AI data centers:

• Rack-to rack traffic of AI data centers cause copper wire to hit the wall
• Attenuation rises in high frequency due to loss & skin effect
• Bandwidth decreases dramatically for rack-to-rack transmission
• Lossy copper can burn and prohibit power as well as add latency
• Optical links can fix these issues - Massive bandwidth density (> 200 Gbps) & far lower channel loss over distance ( 0.2 dB/Km for 1550 nm light), immunity to EMI - lower power & less latency.

TIA & High-Speed Optical Communication –

TIAs are the doorway from photons to bits. That’s why TIA & optical communication go hand-in-hand:

• Sensitivity: TIAs offer lower i/p referred noise which enables detect smaller PD currents
• Ultra-broadband: Wide TIA bandwidth translates to higher optical rate, low group delay & baud rate while keeping the “eye” open
• Impedance control: Low effective input impedance keeps the PD node fast despite PD capacitance, even if PD voltage varies - o/p isn’t affected due to feedback resistor

Without a great TIA, the “optical advantage” is squandered before the ADC even sees the signal.

5) Why Ultra-Broadband is Our Obsession

At Neural Semiconductor Limited, we’re building for the next jump, not the last. Our goal is a state-of-the-art, ultra-wideband TIA engineered to support optical links up to 180 Gb/s with a phase-honest front-end. Delivering that kind of bandwidth at the receiver front end is how we intend to leave a distinct mark in the ever-evolving AI and high-speed communications landscape.

In the coming parts, we’ll see how we can actually Design an Ultra-Wideband TIA. Stay tuned.