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Engineering Insights from Linear Systems

For nearly four decades, Linear Systems has specialized in high-performance discrete semiconductors for demanding analog applications. Today, we're putting that experience to work beyond the devices themselves by sharing practical technical resources for the engineering community.

Explore the Linear Systems Learning Series, JFET circuit and design spotlights, application insights, technical articles, new product releases, and semiconductor industry updates—all focused on helping engineers solve real-world analog design challenges.

You'll also find the latest on our ultra-low-noise JFETs, lateral DMOS switches, bipolar transistors, BIFETs, current regulators, low-leakage diodes, and other precision semiconductor technologies.

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THE HIDDEN DECISION IN EVERY JFET CIRCUIT: SETTING THE OPERATING POINT

The same JFET can deliver excellent performance in one circuit—and disappoint in another. The difference may not be the device itself. It may be the operating point. A JFET’s datasheet describes what the device can do under specified conditions. Biasing determines the drain current, terminal voltages and signal headroom it will actually have in the circuit. That operating point influences nearly every important aspect of performance: Gain Noise Transconductance Linearity Outp

Oct 23

SAME JFET, DIFFERENT JOB: SOURCE FOLLOWER VS. COMMON-SOURCE AMPLIFIERS

A JFET can serve very different purposes depending on how it is connected. Place the output at the drain, and the device can provide voltage gain. Take the output from the source, and the same JFET can buffer a signal while presenting a lower impedance to the next stage. These two fundamental configurations—the common-source amplifier and the source follower—appear throughout low-noise preamplifiers, sensor interfaces, audio circuits, instrumentation and other precision analo

Oct 16

JFET Design Spotlight: Keeping It Simple Whenever We Can

Sometimes the most effective analog solutions are also the simplest. This design demonstrates how a low-noise, high-gain differential amplifier can deliver excellent linearity, wide bandwidth, and fast transient response without feedback. “The sensing and manipulation of differential signals can take on a myriad of configurations using all types of semiconductor types and vacuum tubes. Inputs have all ranges of impedances, all ranges of source loading conditions, and what b

Oct 14

Linear Systems Learning Series: Understanding Gate Leakage

When engineers evaluate a JFET, specifications such as noise, transconductance, input capacitance, and IDSS often receive most of the attention. But in high-impedance circuits, another specification can become extremely important: gate leakage current. JFET gate current may be extraordinarily small, often measured in picoamps or nanoamps. But when that tiny current interacts with a very high source impedance, it can create a measurable error. Understanding gate leakage, and t

Oct 6

BEYOND THE DATASHEET: HOW JFET NOISE IS MEASURED—AND WHY TESTING MATTERS

A JFET datasheet may list an impressively low voltage-noise number—but how was that number measured, at what frequency, and does it represent a typical value or a guaranteed maximum? Those details matter. At nanovolt-per-root-hertz levels, the measurement system, test conditions and specification method can significantly influence the result. Understanding what sits behind the published number helps engineers make more meaningful comparisons between devices. WHAT DOES A JFET

Sep 25

JFET Design Spotlight: Negative Resistance in a Semiconductor

JFETs are commonly used in amplifiers, current sources, variable resistors, and oscillators, but sometimes the most interesting designs come from using them in ways you wouldn’t normally expect. This week’s Design Spotlight explores one of those “odd uses”: creating a negative resistance effect with complementary N-channel and P-channel JFETs. “Use of JFETs in typical applications range from amplifiers to variable resistors and oscillators, among many other applications, bu

Sep 24

Linear Systems Learning Series: Understanding VGS(off) - What JFET Cutoff Voltage Really Means

Last week, we looked at IDSS—the drain current a JFET conducts when VGS = 0 V under specified conditions. This week, we're looking at the other end of the JFET's transfer characteristic: VGS(off), or gate-to-source cutoff voltage. If IDSS helps tell us where the JFET starts at zero gate bias, VGS(off) helps define the point at which the channel is essentially turned off. Understanding both gives engineers a much better picture of how a JFET responds to gate voltage—and how to

Sep 2

Linear Systems Learning Series Understanding: IDSS - What Drain Current Tells You About a JFET

Last week, we took a closer look at transconductance (gfs) and what it tells engineers about how strongly a JFET responds to a change in gate voltage. This week, we're continuing our look at key JFET datasheet parameters with another specification you'll see frequently: IDSS — Drain-to-Source Saturation Current. IDSS is one of the fundamental characteristics of a JFET, and understanding it is important when selecting a device and establishing its operating point. Put simply:

Aug 26

Linear Systems Learning Series: Understanding Transconductance: What gfs Tells You About a JFET

Last week, we looked at how to read a JFET datasheet and the key specifications engineers should evaluate when selecting a device. This week, we're taking a closer look at one of those parameters: transconductance (gfs). Transconductance is an important JFET characteristic because it tells us how effectively the device converts a change in gate-to-source voltage into a change in drain current. Put simply: How strongly does the JFET respond to a change at its input? What Is Tr

Aug 18

Beyond Low Noise: Why Input Capacitance Matters

Last week we discussed why engineers continue to choose discrete JFETs for precision analog designs. But selecting the right JFET involves more than comparing noise specifications. For high-impedance applications such as photodiodes, hydrophones, piezoelectric sensors, condenser microphones, and precision instrumentation, input capacitance can have a significant impact on circuit performance. Higher input capacitance can reduce bandwidth, increase signal loading, and make com

Jul 16

Improving Bandwidth in Photodiode Amplifiers with the LSK489 Ultra-Low Noise, Low Capacitance Dual JFET

Designing high-speed photodiode and sensor front ends often comes down to one limiting factor: input capacitance. Even when noise performance is optimized, excess capacitance at the input can restrict bandwidth, slow response time, and introduce distortion. For engineers working with precision optical or sensor signals, this creates a constant tradeoff between speed, stability, and complexity. In photodiode and high-impedance sensor circuits, total input capacitance directly

Apr 15

Struggling with Obsolete Parts? How Engineers Are Solving It

Every engineer has faced it at some point: a critical component in your design is suddenly obsolete, unavailable, or impossible to source reliably. And unlike digital systems, analog designs don’t always give you the luxury of a quick swap. So what do you do when a key part disappears? Why Obsolescence Hits Analog Designs Harder In digital design, you can often work around a part change with firmware or minor adjustments. In analog circuits, even small parameter differences c

Apr 6