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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. 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 .

In our last blog, we revisited Ray Marston’s classic Op-Amp Cookbook – Part 1  and explored how its timeless design principles continue to resonate with today’s engineers. Now, we’re diving into Part 2 of the series , originally published in Nuts & Volts Magazine  (August 2001). This installment moves from basic op-amp operation into practical design territory—linear amplifiers and active filters. Although the circuits featured in the article are designed around the classic 741 op-amp, the principles remain directly relevant to modern designs, especially when paired with the low-noise, high-performance devices available today. Key Takeaways from Part 2 1. Inverting and Non-Inverting Amplifiers Marston details classic configurations for both DC and AC amplification. Input/output impedance control and offset nulling are emphasized for achieving stability and precision. These remain foundational building blocks for analog designers. 2. Voltage Followers The voltage follower (unity-gain buffer) provides high input impedance and low output impedance—crucial in isolating circuit stages. While the 741 was the reference device in the article, today’s applications demand lower noise, higher bandwidth, and stability at unity gain. 3. Adders and Subtractors Summing amplifiers and differential subtractors are shown as tools for audio mixing, precision measurement, and signal conditioning. These circuits illustrate how op-amps bring flexibility to multi-input and comparison-based designs. 4. Active Filters Marston demonstrates how op-amps can be used to implement Butterworth low-pass, high-pass, and band-pass filters. These “active filters” reject unwanted frequencies without bulky inductors, paving the way for compact analog solutions that are still widely applied. Where Linear Systems Fits In At Linear Integrated Systems, we design low-noise, high-precision discrete semiconductors  that give these foundational circuits new life in demanding modern applications. For example: Our LSK389 dual JFET  can be paired with op-amp filter stages to achieve ultra-low-noise performance in audio and instrumentation. The LSK170/LSJ74  devices provide excellent input stage options for non-inverting high-impedance amplifiers. Precision current sources like the J202 and J113 families  complement voltage follower and filter designs that require tight current control. By substituting these devices into Marston’s foundational topologies, designers can push performance beyond the limitations of legacy 741 op-amps—achieving lower distortion, wider bandwidth, and quieter operation. Why It Still Matters Analog isn’t going away—it’s evolving. These foundational op-amp circuits continue to form the backbone of countless modern designs, from audio preamps to sensor front-ends and precision measurement equipment. Pairing the timeless principles from Op-Amp Cookbook Part 2  with the performance of Linear Systems devices helps bridge the gap between theory and modern application. So what do you think— is this “old news,” or is it still the analog engineer’s playbook? 👉 Explore our product portfolio at www.linearsystems.com  and see how our devices can bring new precision to proven circuit concepts.

Analog Performance Without Compromise In today’s world of high-resolution audio, sensitive instrumentation, and precision sensing, one component can make all the difference between a good design and a great one. The LSK170 Series  from Linear Systems delivers ultra-low noise performance , exceptional linearity, and the flexibility engineers need to push their designs further. Why Engineers Choose LSK170 Ultra-Low Noise Floor  – Just 1 nV/√Hz at 1 kHz ensures pristine signal fidelity. Wide IDSS Selection  – Multiple bins for easier circuit biasing and consistent results. High Input Impedance & Low Capacitance  – Perfect for high-impedance sources and sensitive measurement systems. Pin-for-Pin Compatibility  – Direct replacement for Toshiba 2SK170, with enhanced performance. Flexible Packaging  – Available in TO-92, SOT-23, SOT-89, and bare die (die form)  for maximum design versatility. Built for the Most Demanding Applications Whether it’s audio preamps, hydrophones, CT scanners, vibration sensors, or electrometers , the LSK170 thrives in environments where every nanovolt matters. Its proven reliability, U.S. manufacturing, and virtually zero popcorn noise make it the choice of professionals and DIY enthusiasts alike. Why LSK170 Matters Today Classic JFET performance meets modern manufacturing. By combining high transconductance, robust breakdown voltage, and industry-leading noise performance , the LSK170 keeps analog design relevant—and powerful—in today’s high-tech world. Want to See the LSK170 in Action? We’re offering free samples  to qualified engineers who want to evaluate the LSK170—or other Linear Systems parts—in their own designs. 👉 Request Your Sample Here Have technical questions, need a quote, or looking for more information? Our engineers are here to help: 📞 Call us at (510) 490-9160 or 📧 Email support@linearsystems.com

Looking for a smart way to keep your signal levels steady in radar or drone systems? This clever AGC (Automatic Gain Control) design uses a JFET as a voltage-controlled resistor to dynamically manage amplifier gain. At the heart of the circuit is a non-inverting op-amp whose gain is controlled by the resistance between the JFET's drain and source. The op-amp output feeds a full-wave rectifier, which then drives an integrator. Acting as a low-pass filter, the integrator smooths out ripple for lower distortion and more consistent performance. The op-amp + diode configuration delivers a rectified DC voltage proportional to the input signal. This voltage adjusts the JFET’s resistance, which in turn fine-tunes the amplifier’s gain to maintain a steady 0.2V peak-to-peak output—ideal for close-range detection. Originally designed for radar-seeking systems, this robust AGC circuit is just as relevant today in autonomous vehicles and drones, where real-time responsiveness and stability are key. 📚 Source: C. Marco, “Automatic Gain Control Operates Over Two Decades,”   Electronics , Aug. 16, 1973; reprinted in Circuits for Electronics Engineers , 1977.

When you're measuring the faintest electrical signals produced by the brain and heart, any noise, leakage, or delay can corrupt the diagnostic data. That’s why precision diagnostics demand clean analog switching , with speed and signal integrity that traditional switches simply can’t deliver. ⚡ Why Speed and Leakage Matter in EEG/EKG Design High-Speed Matters : With switching speeds under 1 ns, you avoid temporal distortion. Fast switching means less glitching when you're sampling at high rates—critical for capturing accurate waveforms for both EEG (0.5–100 Hz) and EKG (up to 150 Hz) signals. Ultra-Low Leakage (Nanoamps) : In high-impedance front-ends, leakage currents—even just a few nanoamps—can introduce bias errors or drift, compromising measurement precision. Low leakage ensures your signal chain remains true to the physiology. ( analog.com ) Minimal Capacitance & Charge Injection : Low capacitance reduces filtering effects at high frequencies, while low charge injection prevents voltage spikes during switching, keeping analog fidelity intact. ( analog.com ) 🔍 Introducing the SST210: Built for Precision Our SST210  analog switch hits all the marks: Feature Spec Why It Matters Switch-on time < 1 ns Preserves waveform fidelity Reverse capacitance ~0.2 pF Minimizes signal distortion ( mouser.com , electronicspecifier.com ) Leakage current Nanoamp-level Avoids bias and drift in high-impedance EEG/EKG inputs Low ON-resistance Guaranteed Maintains consistent gain and low signal loss Single-supply operation Simple + drive requirements Keeps your front-end compact and reliable This makes the SST210 ideal for sensitive front-end paths like sample-and-hold circuits, measurement multiplexers, deglitch circuits in DACs, and EEG/EKG monitoring front-ends. 🧠 The Role of Clean Switching in EEG/EKG Accurate Amplification: EEG and EKG signals typically sit in the microvolt to millivolt range. Any offset current—especially in the nanoamp range—can shift baselines or distort peaks, skewing diagnostic readings. High-Resolution Sampling: Bio-signal pipelines use anti-aliasing filters tuned just above signal bandwidth. Fast, glitch-free switching ensures these filters work as intended, without introducing jitter or settling delay ( mouser.com ). Reliable Multiplexing: Systems often share ADCs across multiple channels. Clean switching is essential when sampling multiple leads/channels tightly—for instance, when recording dozens of EEG electrodes or precordial EKG leads. 🛠️ How to Integrate the SST210 Front-End Placement : Use the SST210 directly before the input buffer/amplifier. Its low leakage protects against bias creep. Drive Supply : A single 5 V rail is all that’s needed—no complex gate drivers required. Mounting : The SOT‑143 package is compact and easy to route on high-density PCB layouts. Testing Protocols : Evaluate the switch in your actual signal chain with known microvolt-level inputs to measure added noise, offset, and settling behavior. 🎁 Try It for Yourself We’re offering free SST210 samples to qualified engineers  so you can test its performance in your EEG/EKG designs without cost or risk. 👉Complete our samples request form:  https://www.linearsystems.com/about-2 Don’t settle for analog switching that compromises your diagnostic signal. Let the SST210 give your biomedical systems the signal clarity they deserve— down to the nanoamp . Precision diagnostics start with precision switching. Claim your free sample and elevate your design today!

There are three types of amplifiers that are most often used in the design of sensor signal chains. Two of the more popular include the transresistance (transimpedance) amplifier and the charge amplifier. Additionally, there is the voltage amplifier which is often used with sensors that incorporate their own JFET. The transimpedance amplifier is used to convert a current to a voltage. Specifically, its gain can be specified in volts/A. A charge amplifier on the other hand is used to convert a charge to a voltage. A charge amplifier invariable uses a feedback capacitor to configure the amplifier as an integrator. The integrator effectively produces an output voltage that is proportional to the time rate of change of charge.   Transimpedance amplifiers, for the most part, are used in light or electromagnetic based sensor applications. Specifically, the current from a photodiode or a photodetector (induced from the light that strikes it) is converted to a voltage. On the other hand, charge amplifiers, are most often used with piezoelectric based sensors such as piezoelectric bases accelerometers, shock sensors, pressure sensors and hydrophones. However, it is still not uncommon to see piezoelectric sensors used with standard common source amplifiers. Photodiodes are also used with charge amplifiers. Often the selection of the amplifier is based on the characteristics of the sensor or detector used and the performance requirements of the system. Manufacturers will often integrate JFETs into their detectors or sensors to simplify the amplifier interface process. However, for high end performance applications, sophisticated JFET based preamps will be used to supplement the on-board JFET or the sensor will be purchased without the JFET. TRANSIMPEDANCE AMPLIFIER SENSOR APPLICATIONS   The transimpedance amplifier is most often used in photodiode (photodetector) applications often used in a photo optical system. They are used across the electromagnetic frequency spectrum to determine the electromagnetic or light intensity that strikes a surface of the photodetector. This includes light in the gamma ray, X-ray, ultraviolet, visible light, near infrared, mid infrared and far infrared ranges.   The most simplistic topology of the transimpedance amplifier is shown in Figure 1. A current source provides the current that flows through the feedback resistor of the op-amp. This results in an output voltage that is directly proportional to the input current. The feedback resistor determines the transimpedance or transresistance gain, V/I.   A more real-life circuit is shown in Figure 2. It contains a feedback capacitor to prevent oscillations that are a result of parasitic capacitances. The feedback capacitor will affect the AC voltage gain and the stability of the circuit. Oscillations must be designed out.   Explore more about these fascinating concepts and discover industry-leading FETs at Linear Systems: www.linearsystems.com . Stay tuned for our insightful FET Friday posts by following us on LinkedIn !

At Linear Integrated Systems, we know that performance, reliability, and precision are non-negotiable when it comes to analog front-end design. That’s why we developed the LSK389 , a monolithic dual JFET that offers best-in-class low noise, tight gain matching, and outstanding thermal stability. What Makes the LSK389 Unique? The LSK389 is built on a single piece of silicon , giving it superior symmetry and performance consistency over temperature. This eliminates the mismatch and drift often seen with discrete JFET pairs, making it a go-to solution in audio, medical, instrumentation, and defense applications . Key Performance Advantages: Ultra-Low Noise : Ideal for low-level signal amplification in sensitive analog circuits. Precision Matching : Designed for applications requiring tight differential performance. Thermal Stability : Minimal drift ensures long-term reliability across wide temperature ranges. Whether you're working on high-end audio preamps, precision medical instrumentation, or low-drift sensor interfaces , the LSK389 offers the performance edge you need. Want to See the LSK389 in Action? We're offering free samples  to qualified engineers who want to evaluate the LSK389 in their designs. 👉 Request Your Sample Here About Linear Integrated Systems With over 35 years of semiconductor innovation, Linear Systems designs and manufactures high-performance discrete components with a focus on JFETs, bipolar transistors, DMOS switches, and more. Learn more at www.linearsystems.com .

Looking to push the limits of low-noise performance? This application note explores advanced amplifier techniques using Linear Systems’ LSK389 , a monolithic dual N-channel JFET known for its industry-leading noise specs and precision matching. 📄 Dive into design details and circuit techniques that make the LSK389 ideal for audio, instrumentation, and sensor applications: https://www.linearsystems.com/applicationnotes/lsk389-app-note #FETFriday #LSK389 #JFET #LowNoise #AnalogDesign #LinearSystems

At Linear Integrated Systems, we’re passionate about crafting the world’s highest performance discrete semiconductors. Today, we’re excited to spotlight an advanced design that exemplifies the exceptional performance of our JFET products: the All-JFET Differential Amplifier .   ⚙️ The Heart of the Design This amplifier circuit, built entirely around JFETs, demonstrates how carefully selected and matched components can create an ultra-low-noise, highly linear amplification stage. Let’s take a closer look at the core building blocks:   🔹 Differential Input Stage The amplifier begins with a balanced differential pair: Q1A/Q1B (LSK489) : Monolithic dual N-channel JFETs Q2A/Q2B (LSJ689) : Complementary dual P-channel JFETs These matched pairs ensure symmetrical operation, significantly reducing offset voltages and maximizing common-mode noise rejection (CMRR).   🔹 Biasing and Current Mirrors Using precision current sources and mirrors, the amplifier achieves rock-solid biasing. This stability is key to delivering consistent gain, minimal distortion, and a clean audio path across temperature swings.   🔹 Push-Pull Output Stage The output section features pairs of LSK170B and LSJ74A JFETs. These push-pull stages combine to provide: Excellent linearity Low output impedance Ability to drive demanding loads, while maintaining the sonic signature of JFETs   🔹 LED Biasing and Decoupling LEDs are cleverly used to stabilize bias points while keeping the amplifier quiet and responsive. Careful capacitor decoupling throughout the circuit ensures that no unwanted noise seeps into the signal path.   🎧 Why Use an All-JFET Amplifier? ✅ Ultra-Low Noise Our JFETs have some of the lowest noise figures in the industry—ideal for high-end audio, test & measurement, and sensor applications.   ✅ High Linearity JFETs’ square-law characteristics deliver sweet, natural-sounding linearity that’s prized in audiophile-grade equipment.   ✅ Common-Mode Noise Rejection Thanks to the differential topology, this amplifier can reject noise that affects both input lines equally—perfect for instrumentation and precision audio.   ✅ Reliability & Thermal Stability Matched pairs like the LSK489 and LSJ689 maintain consistent performance across temperature variations, ensuring your design stays rock-solid.   🔬 Applications in the Real World From audio preamplifiers  in recording studios to sensor front-ends  in medical equipment and aerospace systems, this all-JFET differential amplifier topology ensures performance that professionals rely on.   This diagram showcases how Linear Integrated Systems’ JFETs—like the LSK489, LSJ689, LSJ74A, and LSK170B—work together to create an amplifier that excels in sonic purity and technical performance.   💡 Ready to Build or Learn More? If you’re designing a precision amplifier or want to see how our JFETs can elevate your next project, get in touch! Visit www.linearsystems.com to stay updated on the latest application notes, product releases, and technical insights.

Creating unique pedal effects doesn’t always require complex circuitry. In fact, many classic sounds are built by simply overdriving a JFET amplifier, tweaking input signals, or using clever circuit designs to shift filter center frequencies. For instance, a single JFET amplifier  can produce a clean, natural-sounding signal— as long as the input voltage remains within the normal operating range  of the amplifier. This depends on the JFET’s gain and the supply voltage you’re using. Once you start pushing the input beyond these typical limits, that’s when the magic of overdrive and harmonic color begins to emerge. Check out the circuit diagram  and simulation data  below to see how this basic building block can become the heart of your next creative pedal design! Figure 1 - A common source amplifier, based on a JFET, is the basis for the design of distortion pedal effect circuit. Figure 2 - The output of the JFET amplifier shows little distortion Figure 3 - Examination of the FFT, one can see that the 2nd and 3rd harmonics are all below 20 dB of the 1st harmonic (300 Hz) In order to introduce some overdrive distortion, the input signal is increased to 3 Volts peak to peak. The resultant output waveform is given below. As can be seen, it is asymmetrically clipped at the top. Figure 4 - Applying a 3 Volt peak-to-peak input signal to the JFET amplifier results in significant distortion. In this case the top of the waveform is clipped.  The Fast Fourier Transform, below, indicates that the harmonics generated at 600 Hz, 900 Hz and 1200 Hz are significant (near 0 dB).  When the distorted and non-distorted waveforms are listened to on Microsoft’s Media Player you can discern the difference. The bass sound has been muted significantly. Figure 5 - Overdriving the JFET amplifier generates 2nd and 3rd harmonics that overpower the 1st harmonic Instead of overdriving the circuit, you can lower the power supply to produce a softer tone. The figure below is the waveform generated (Vout1) when the power supply was lowered to 5 V and the source resistor increased to 1000 Ohm. Figure 6 - A softer tone (Vout1) is produced with a 0.5 Volt peak-to-peak input signal and 5 Volt supply voltage Comparing the FFT of the 5 Volt with the 15 Volt circuit, one sees that the 600 Hz harmonic has been reduced to around -40 dB. When comparing the actual audio of the two circuits, one can hear the difference. The audio of the 5 V circuit is much more subdued than the 15 Volt overdriven circuit.   Figure 7 - Lowering the supply voltage to 5 Volts reduces the level of the 2nd harmonic of the output signal (Vout1) For a more detailed discussion of distortion circuits, see the article by R. G. Keen, A Musical Distortion Primer

At Linear Systems, we're continually inspired by the creative ways engineers use discrete components like JFETs to build elegant analog solutions. This Friday FET Design  features a voltage-integrating light meter  that leverages a Siemens BPX 63 photodiode  for exceptional low-light sensitivity, combined with a clever JFET-based feedback network to control integration timing and minimize distortion. 📐 Circuit Overview This design ensures that only useful light  is used to affect aperture control—excluding noise and ambient artifacts. It also offers fast recovery from light bursts , critical when operating in unpredictable lighting environments such as astrophotography or low-light film capture. ⚙️ Key Features: Photodiode BPX 63 : Delivers 10 nA/lux sensitivity. JFET T1 & T2 : Manage fast recovery and low-noise gate control. High-Gain Amplification : Amplification exceeds 3000 when S1 and S2 open. C1 Capacitor Integration : Produces a linear voltage output  based on incident light. T3 Feedback Switch : Ends exposure when output reaches 1V, improving timing accuracy. +/-3V Supply : Ideal for compact, portable designs. 🧠 How It Works: When the camera shutter is closed, S1 and S2 are closed , shorting the op-amp’s feedback loop through T1, preventing unnecessary charge accumulation. As the shutter opens , S1 and S2 open , and the amplifier gain increases. The integrating capacitor C1  starts charging with the photocurrent. When voltage reaches 1V , T3 (BCW 60)  conducts, signaling the end of exposure by discharging the load path and resetting the system. This buffered design improves upon classic drain-to-gate feedback by taking global feedback from the op-amp’s output (U1A), reducing distortion and enabling operation at higher frequencies. The use of discrete JFETs  in key signal paths ensures minimal noise and excellent control characteristics. 📊 Applications: Light-integrating exposure meters Scientific photometry Low-light measurement instrumentation 🔍 Learn more about high-performance JFETs and analog design at: www.linearsystems.com 💡 Follow us for more design insights every #FETFriday!

In precision analog applications, minimizing distortion while maintaining frequency response is essential when implementing a voltage-controlled resistor (VCR) using an N-Channel JFET. One effective method to achieve this is through buffered global feedback . In this design, the feedback path to the JFET gate is derived from the output of an op-amp (U1A) rather than using a traditional drain-to-gate feedback resistor. The feedback network, composed of R3 and R4, connects the op-amp output directly to the gate, allowing for: Significantly Reduced Signal Distortion:  Buffered feedback isolates the gate drive from the drain voltage, improving linearity across the operating range. Improved High-Frequency Response:  Because the feedback is sourced from the op-amp, R3 and R4 values can be lower, resulting in minimized parasitic capacitance effects and broader bandwidth operation. This architecture removes the need for a direct feedback resistor across the JFET and enables the VCR to perform effectively at higher frequencies compared to conventional approaches. For design engineers focused on optimizing low-noise and high-linearity circuits, this method offers a robust solution using N-Channel JFETs. Reference: EDN - A Guide to Using FETs for Voltage-Controlled Circuits, Part 2 Learn more about Linear Systems' ultra-low-noise, precision JFET solutions at www.linearsystems.com . #LinearSystems #VCRDesign #JFET #LowNoiseDesign #AnalogEngineering #HighFrequencyCircuits #FETDesign