Bridging Analog Foundations with Precision: Op-Amp Cookbook Meets LSK389 Excellence

For our FET Friday post we are revisiting the timeless Op‑Amp Cookbook, Part 1 by Ray Marston, which reminds us how fundamental op‑amp principles remain the bedrock of analog design, even in today’s high-performance landscape. If you're up for a refresher, check it out here: Op‑Amp Cookbook — Part 1.

At Linear Systems, we believe in building on those fundamentals—using precision components like the LSK389—to bring classic designs into the modern era.

Insights from Part 1 of the Op-Amp Cookbook

Marston lays the groundwork with clear block diagrams and practical circuit examples. He describes the op‑amp as a high‑gain, direct‑coupled amplifier featuring inverting and non‑inverting inputs with offset‑compensation and output stages powered by both ± and common rails.

He then walks through fundamental topologies:

• Inverting Amplifier: Offers predictable gain via resistor ratio; bandwidth shrinks as gain increases.

• AC‑Coupled Inverting Amplifier: Introduces coupling caps for DC rejection, removing the need for offset nulling.

• Non‑Inverting Amplifier / Voltage Follower: Provides high input impedance and unity gain buffering.

• Bootstrapping & High‑Impedance Techniques: Expands input impedance into megaohm ranges using feedback or guard rings.

These circuits highlight essential truths—bandwidth constraints (via gain-bandwidth product), offset considerations, and impedance management—that remain relevant in high-precision design.

Elevating Classic Circuits with the LSK389

Our LSK389 ultra‑low‑noise monolithic dual JFET dramatically improves performance when integrated into these classic op‑amp stages:

• The LSK389 is renowned as perhaps the lowest-noise dual JFET in its class—rated at ~1.9 nV/√Hz at 1 kHz, with 100% noise testing to eliminate burst/popcorn noise. (Linear Systems)

• It features tight matching, excellent thermal tracking, low capacitance (~25 pF), and high breakdown voltage, making it ideal for sensitive front-end designs. (Linear Systems)

How It Enhances Key Configurations:

Original Circuit	Typical Challenge	How LSK389 Helps
Inverting / Non‑Inverting	Input noise, bias instability	Ultra‑low noise, low bias current, high input impedance
Differential / Instrumentation	Matching and drift	Monolithic, tightly matched pair ensures low offset and drift
Bootstrapped / High‑Z	Leakage and loading	Low capacitance preserves high impedance integrity
Sensor / Audio Preamps	Weak signal fidelity	Lowers noise floor dramatically for sensitive analog paths
Cascode / Bias Circuits	Stability and drift	Matches and thermally tracks for stable, precise biasing

The integrated dual JFET structure means excellent matching for differential and instrumentation stages. Its low input capacitance supports high-speed operation without undue loading.

Real-World Application: Discrete JFET Front-End

Consider placing an LSK389 input stage before a standard op-amp:

• You get ultra-low-noise buffering using the LSK389, followed by the op-amp’s gain stage.

• The result? A cleaner, quieter overall signal chain, ideal for audio, instrumentation, or sensor systems where signal fidelity matters.

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By pairing Marston’s Op-Amp Cookbook, Part 1 foundational designs with the precision capabilities of the LSK389, you get the best of both worlds: classic analog theory operationalized through high-performance hardware.

So… what do you think? Old news, or still deeply relevant?

For samples of the LSK389, or any Linear Systems parts, complete our request form, or email support@linearsystems.com.