Ultra-Low-Noise JFETs Explained

A Design Guide to the LSK389 Dual and LSK170 Single JFETs

When designing ultra-low-noise analog front ends, device selection matters. Whether you're building instrumentation, audio, or sensing systems, the transistor you choose can directly impact signal integrity, stability, and overall system performance.

Two of the most trusted solutions for engineers worldwide are the LSK389 dual JFET and LSK170 single JFET — precision components engineered for applications where noise, matching, and reliability cannot be compromised.

Why Engineers Still Choose JFET Front Ends

JFETs continue to dominate critical signal paths because they offer:

• extremely low noise floors

• ultra-high input impedance

• predictable biasing behavior

• excellent linearity

For sensitive measurement or audio circuits, these characteristics are often essential — not optional.

LSK389 — Matched Dual JFET on a Single Die

The LSK389 eliminates one of analog design’s biggest challenges: matching discrete transistors.

Because both JFETs are fabricated on the same piece of silicon, designers get:

• tight electrical matching

• excellent thermal tracking

• long-term stability

Key Performance Highlights

• 1.6 nV/√Hz noise (typical @ 1 kHz)

• 20 mS transconductance (typical)

• 15 mV max VGS offset matching

• 40 V breakdown voltage

  
  

This monolithic architecture provides tighter performance than hand-matching discrete devices while reducing design time and cost.

LSK170 — Benchmark Single Ultra-Low-Noise JFET

The LSK170 is designed for applications that require the lowest possible noise from a single device.

Key Performance Highlights

• 0.9 nV/√Hz noise (typical @ 1 kHz)

• ~20 GΩ input impedance

• 22 mS transconductance (typical)

• ≤22 pF capacitance

  
  

It’s also a direct replacement for the classic 2SK170, making it ideal for legacy upgrades or modern redesigns.

When to Use Each Device

Choose LSK389 if your design needs:

• differential input stages

• matched pairs

• precision instrumentation

Choose LSK170 if your design needs:

• lowest single-device noise

• ultra-high impedance inputs

• compact layouts

Typical Applications

These devices are widely used in:

• microphone preamps

• sensor interfaces

• instrumentation amplifiers

• hydrophones and acoustic detection

• medical electronics

• test and measurement systems

Design Insight Engineers Appreciate

Narrow IDSS grading means less biasing guesswork and more predictable circuit performance, especially in low-voltage or battery-powered systems.

That translates directly into:

✔ faster prototypes✔ fewer revisions✔ more consistent production results

Need Help Choosing the Right Grade?

Linear Integrated Systems offers custom screening options and technical support to help you select the optimal device for your design.

📩 Contact our engineering team: support@linearsystems.com