A Quiet Photodiode TIA with Linear Systems’ JFETs

When it comes to photodiode front ends, noise and stability are the real challenges. Photodiode capacitance can destabilize feedback loops, while input bias current and 1/f noise often dominate performance at low signal levels. Designing a transimpedance amplifier (TIA) that maintains accuracy across temperature and frequency isn’t trivial.

That’s where Linear Systems’ single JFETs—the LSK170 and upcoming BF862 replacement—shine. Both provide ultra-low leakage, low noise, and high input impedance, making them ideal for photodiode buffering in precision TIAs.

Why Use a JFET Front End?

Many TIAs rely solely on op-amps. But at high feedback resistances—1 MΩ and above—input bias current quickly turns into millivolts of error. A JFET buffer solves this by isolating the op-amp from the photodiode capacitance and keeping gate currents in the picoampere range.

Key benefits of using LSK170 or BF862 replacement in photodiode TIAs:

• Ultra-low gate leakage → minimizes dark current and DC errors.

• Low 1/f noise → critical for low-frequency sensor and instrumentation applications.

• High input impedance → supports large-value feedback resistors without loading effects.

• Bootstrapping effect → reduces effective photodiode capacitance, improving stability and bandwidth.

  
  

Example Design – Single JFET-Buffered TIA

A widely used approach is to buffer the photodiode with a single JFET (LSK170 or BF862 replacement) before the op-amp’s inverting input.

• RF = 1 MΩ

• CF = 0.5–2 pF (for stability compensation)

• Photodiode capacitance: 2–10 pF

• Supply: ±12 V to ±15 V

  
  

This configuration provides:

• Gain: ~1 V/µA (1 µA photocurrent → 1 V output)

• Bandwidth: >100 kHz (depending on photodiode capacitance and compensation)

• Noise: dominated by RF’s Johnson noise, with minimal added contribution from the JFET.

  
  

Bench Results

In lab tests with an ~8 pF silicon photodiode and modulated LED source:

• Step response: clean at 100 kHz with <10% peaking.

• Noise: only a few µVrms (20 Hz–100 kHz).

• Offset drift: stable within millivolts from 25 °C to 60 °C.

These results confirm that using a single JFET like the LSK170 or BF862 replacement yields quiet, stable TIAs without added complexity.

Applications

• Optical sensors – spectrometers, fluorescence detectors, LiDAR receivers

• Scientific instrumentation – low-light measurement, precision photometry

• Industrial monitoring – process control, position sensing, safety systems

• Audio front ends – ultra-low-noise preamps for microphones and pickups

Advanced Option – Using the LSK389 Dual

For most single-ended photodiode TIAs, the LSK170 or BF862 replacement is the best choice. However, engineers needing even lower noise or matched pairs can consider the LSK389 dual JFET:

• Use one half as a drop-in replacement for a single JFET.

• Parallel both halves for ~3 dB lower voltage noise (at the cost of ~2× leakage).

• Matched dual geometry ensures excellent thermal tracking, ideal for differential photodiodes or ultra-stable instrumentation.

  
  

Summary

Photodiode TIAs demand a careful balance of speed, noise, and stability. By combining a low-noise op-amp with a Linear Systems JFET buffer, designers can achieve:

• Picoamp-level input bias currents

• Low 1/f noise

• Stable operation with high feedback resistors

Choose your device:

• LSK170 / BF862 replacement → best for single-ended TIAs, cost-sensitive designs, and wide adoption.

• LSK389 dual → premium option for ultra-low drift, differential front ends, or noise-critical systems.

  
  

👉 Explore Linear Systems’ LSK170, BF862 replacement, and LSK389 JFETs for your next optical front-end design.