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JFET Circuit Design Book

Linear Systems

Jul 1, 2024

Introduction, Audience, Preface, Why JFETs & Applications Structure

JFET Circuit Design: A Schematics Library and Application Notes


Linear Systems, as part of its research and development effort, has a continually growing collection of circuits that can be applied in almost every area of electronic design. In our upcoming book, JFET Circuit Design: A Schematics Library and Application Notes, the design engineer will find a wide variety of amplifiers, comparators, current sources, filters, oscillators, RF circuits, sensors, switches, waveform generators, voltage references, and voltage-controlled resistor circuits, all suitable for every type of Internet of Things (IoT) application. From audio to health and medicine to the most advanced scientific instruments, this book will provide what is needed for the engineer, scientist, inventor, or hobbyist to create innovative analog IoT-based designs. Effectively, this book gives the designer the power to design products across the electromagnetic spectrum (light, thermal, infrared, radio, X-ray radiation), the mechanical spectrum (sound, ultrasonic, seismic, pressure, gas, gravity, vortices, shock, water, and wind waves), and the matter wave spectrum (photons, electrons).


JFETs can be found in numerous applications across the entire electromagnetic spectrum.

JFETs can be found in numerous applications across the entire mechanical wave spectrum (the phonon spectrum).


The book represents part of that collection of circuits. Linear Systems continues to classify, design and analyze circuits as part of its efforts to improve circuit performance and extend the realm of discrete based circuit design in both traditional and emerging markets.


The collection of JFET-based electronic circuits has been designed as a resource guide for all who are interested in basic and advanced JFET circuit design. The professional electronic analog design engineer will find all the basic analog building blocks for the construction of advanced and high-performance analog systems. For each standard analog building block, the professional will find numerous circuit derivatives that can be used to obtain optimum performance characteristics for specific applications and parametric goals. For technicians and musicians, the book provides the fundamental basics for building and understanding workable circuits. This includes fundamental amplifier configurations and advanced circuits that can be used for the construction of electronic lab equipment and musical effects equipment. For the student and professor, the book provides the material needed to devise lab experiments, homework, simulation, and test analysis problems. For the inventor and hobbyists, the book provides useful circuits for household electronics, sensor-based designs, amateur radio circuits, and automation circuits that can be interfaced to current app-based digital designs that can be developed with Raspberry Pi and Arduino electronic hardware interface platforms. These hardware platforms can be especially useful for those wanting to test out sensor signal chains for actual interfacing to the Internet.

The book fulfills the needs of audio design engineers, RF design engineers, test engineers, sensor design engineers, as well as medical electronic designers, smart textile engineers, and electronic instrument and scientific instrument designers. A multitude and variety of functional blocks for designs found in these systems are presented that allow for the quick construction of prototypes that can meet specific product and market requirements.

The compendium provides one of the largest collections of JFET-based circuits. This source book will save those interested in designing high-performance discrete-based designs hundreds of hours of time in research and provide insight into effective analog, mixed-signal, and digital-based JFET design techniques. With the material in this compendium, engineers, researchers, inventors, scientists, technicians, musicians, and hobbyists are given the means to produce innovative designs that can be applied to existing, new, and emerging markets.


Understanding the how and why of design with discrete transistors, such as JFET, has always been considered an art and a science. In one realm, designers use their intuitive sense of design to create designs and improve circuit performance. On the other end of the spectrum, designers develop their intuitive sense through in-depth, highly mathematical circuit analysis. Finally, in the middle dimension are circuit simulators and actual hardware prototyping. Combining these different design approaches, one can indeed become a skilled analog designer.


IIn order to expedite the design process, this ongoing compendium of circuit designs takes a block-based approach to analog circuit design. An understanding of all the common and not so common analog design blocks gives the designer the capability to create complex analog systems from a top-down perspective. Putting blocks together to form systems and then optimizing specific circuit blocks for specific performance characteristics lets designers create new and innovative analog systems quickly.

Each of the descriptions of each of the circuits contained refers to the individual circuit blocks within each design and the individual components that comprise these blocks in regard to performance and functionality.

Why JFETs?

The unique properties of JFETs, ultra-high input impedance (high resistance and low capacitance), ultra-low noise, high gain, low harmonic distortion, and low voltage operation have made them the component of choice for analog, digital, and mixed signal design. These special JFET properties give JFETs a very high place in high-performance design applications. In audio systems, the JFET properties make them the ideal fit for high-fidelity preamplifiers for musical instruments and audio equipment. In electronic instruments, JFETs are also regarded as the component of choice. Their high impedance and low noise make them the ideal device for probes and the front end of almost every type of precision electronic measurement device. These include not only electronics lab equipment but also medical equipment and the most advanced telescopes on earth.


In general, JFETs, because of their properties, are found in any device that includes a sensor. The Internet of Things, which is based on sensors, is one area where JFETs are applied. From smart cities to smart clothes and textiles, JFET preamplifiers are the front-end component to reliably and accurately measure every type of physical phenomena: acceleration, heat, light, pressure, radiation, sound, and temperature. In the medical, health, and fitness realm, JFETs are at the heart of high-performance photo-optical wearable medical and fitness devices. A JFET’s very low-frequency noise (flicker or 1/f noise) and ability to measure signals in the microvolt region make them ideal for photoplethysmography and electrocardiography applications. When configured with an Avalanche diode, JFETs can measure light down to the discrete photon level. In the everyday realm, JFETs are ubiquitous. They are used as current sources, current limiting, voltage limiting, amplifiers, switches, and voltage-controlled resistors. They are aptly applied almost everywhere in analog and mixed signal systems.


JFETs are one of the most difficult discrete components to successfully integrate into ASIC and custom IC designs. The primary reason is that their noise performance degrades and their cost increases as the size of the JFET shrinks. This means it is not economically practical to integrate a JFET if one wants to obtain the noise performance of a discrete JFET. All of this means that in order to measure low voltage signals in the microvolt region, such that noise does not mask the signal, a discrete JFET must be used.

JFETs are a must for low voltage, low frequency measurement applications that are required in sensor applications and high-performance audio applications. Another important consideration with JFETs is that they have a very high input impedance from both the standpoint of high input resistance and low input capacitance. This allows them to interface directly to high impedance sensors and other types of high impedance sources without signal attenuation, all of which mitigates the need to place measurement error correction compensation in a design.

Applications Structure

The book is classified according to functionality and specific end-user applications. Many of the circuits in the applications categories can be applied to different applications. Log amps, for instance, are used in both music and instrumentation applications. Circuit descriptions include an overview of end-user applications that the circuit is often found in.

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