Unlocking Ultra-Low Noise Amplifier Performance with the LSK389 Dual
- JC - Linear Systems
- Oct 29
- 3 min read
At Linear Systems, we’ve long been committed to helping designers push the limits of analog performance — especially in low-noise, high-impedance applications such as sensor front-ends, microphone preamps, instrumentation amplifiers and more. That’s why we are proud to present the application note on our dual monolithic JFET, the LSK389.In this article, we’ll walk you through what makes the LSK389 a standout device, how it works in real-world circuits, and how you can integrate it to unlock superior performance in your designs.
Why the LSK389 matters
The LSK389 is billed as “the industry’s lowest noise Dual N-Channel JFET,” fully noise-tested and guaranteed to meet both low-frequency (1/f) noise and broadband noise specifications.
Key specifications include:
Input noise of ~1.3 nV/√Hz at 1 kHz (ID = 2 mA)
Input noise of ~1.5 nV/√Hz at 10 Hz (ID = 2 mA)
Transconductance (gm) ~14 mS at ID = 2 mA
Device typical capacitances: CISS ~25 pF, CRSS ~5.5 pF
Breakdown voltage ≥ 40 V
Four grades based on IDSS (A-D) to give flexibility in biasing and device selection.
What this means in practice is that you get a JFET device optimized for ultra-low noise (both wideband and 1/f), excellent matching (because it’s a dual monolithic pair), and high input impedance — making it an ideal front-end building block for very demanding analog systems.
Understanding the fundamentals
The application note provides a useful refresher of JFET basics: the drain current equation ID=β(VGS−VT)2I_D = \beta (V_{GS} - V_T)^2ID=β(VGS−VT)2 (for VDS > Vt) and how transconductance gm relates to device bias (gm ≈ 2 √(β × ID)). It then dives into the major noise sources in JFETs:
Thermal channel noise
Gate-current shot noise
1/f noise
Generation-recombination noise
Impact ionization noise
The takeaway: in a well-designed, modern process (such as ours), the first two noise sources can dominate — and by maximizing gm (via sufficient bias current) while minimizing parasitic resistances and capacitances, you can push input-referred noise down. Indeed, for one measured LSK389 device at 2 mA, the thermal noise calculation gives ~0.9 nV/√Hz.
Circuit implementation examples
The application note gives several practical circuit topologies including:
Single-ended amplifier with parallel JFETs to reduce noise by ~3 dB.
Differential pair input stage using the LSK389, noting that a differential configuration imposes a ~3 dB noise penalty vs. a single-ended stage (all else equal) because both halves contribute.
Cascoded and bootstrapped cascode amplifiers to improve output impedance, reduce Miller effect, and allow high-voltage operation.
A very high-performance, ultra-low noise folded-cascode amplifier (with four parallel differential pairs of LSK389 devices) achieving ~0.7 nV/√Hz input-referred noise in a differential amplifier.
Also covered: substrate bias considerations (since the dual monolithic pair shares a substrate, and substrate diodes exist between the gates and the substrate).
Design tips for optimal use
Based on the note and our internal best practice:
Bias the LSK389 at ~2–4 mA for a good trade-off of noise vs. power dissipation (note: at 8 mA the device runs hotter, raising noise slightly)
When paralleling devices for lower noise, ensure each has its own tail current source (in the case of differential inputs) to avoid instability or bias mismatches.
Use low-noise current sources and keep source-degeneration resistors minimal, since they degrade noise performance.
Consider cascode configurations if you need higher voltage tolerance or better bandwidth but verify stability via simulation due to potential HF anomalies in bootstrapped/ driven-cascode arrangements.
When you have very high-impedance sources (for example, piezoelectric sensors, condenser mics, or GΩ ranges), gate leakage/ current shot noise and substrate diode leakage become increasingly important — so good device matching and low leakage are critical.
Why it matters for your system
Because we at Linear Systems focus on analog performance, small-signal discrete semiconductors and legacy designs with modern relevance (including switching, modulation and low-noise front-ends), the LSK389 perfectly fits into applications where every nanovolt counts. Whether you’re designing:
A phonograph (MM/MC) preamp for an audiophile system
A high-impedance sensor preamp for piezoelectric or pyroelectric devices
A front-end for a condenser microphone
An electrometer or ultra-low current instrumentation amplifier
— the LSK389 gives you a strong foundation.
Conclusion
With the LSK389, you get a dual monolithic matched JFET pair that delivers best-in-class low noise, excellent matching, low capacitance and high transconductance. The application note by renowned analog designer Bob Cordell provides the theory, design examples and tips you need to put that device to work. If you’re designing low-noise analog front-ends, we invite you to download the note and consider the LSK389 in your next iteration.
Need evaluation samples? Contact our team — we’re happy to support your prototype and production needs. Call (510) 490-9160 or email support@linearsytems.com.
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