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Integrated circuits continue to advance, but many of the highest-performance analog front ends still begin with a discrete JFET. Why? Because when you're trying to capture extremely weak signals, the input stage often determines the performance of the entire system. Engineers continue to choose discrete JFETs for applications requiring: Ultra-low noise High input impedance Low input capacitance Excellent linearity Whether designing photodiode amplifiers, hydrophone preamplifiers, precision sensor interfaces, or instrumentation front ends, these characteristics can make the difference between detecting a signal—or missing it. That's why devices like the LSBF862, LSK170, LSK389, and LSK489 continue to be designed into new systems where signal integrity starts at the input stage. 👉 For free samples or to discuss your application with our engineering team, complete the form HERE!

There are a lot of circuits and applications of discrete and integrated component combinations solving a myriad of tasks defined as solutions to a problem. Dad used to tell me that, "The problem isn't the problem; the definition of the problem is the problem." Dr. Gene Slottow at U of I would say, "When you think you're done with the design, throw it out and start over as your unyielding self-critic until you can't anymore, then proceed." Few first circuit iterations are "the one." This was one of those iterations. Gain control circuits typically use a differential front end and use it as a pivot point as a low-noise front end for an Op-Amp and its inherent massive loop gain, feedback, and convolved transitions of nonlinear elements in between. Compensation is often dicey because of the amount of loop gain being thrown out, as well as the group delay to the feedback node. Audio doesn't need the PPM level of a typical op-amp having >120 dB of gain. Audio requires linearity and acute transient response without overshoot. With that in mind, illustrated here is an often-repeated textbook differential input stage with its transconductance gain controlled by emitter current modulation. Preceding an Op-Amp, the low-noise LS312 bipolar pair input voltage must be less than 14 mV for linear operation before the square-law difference becomes nonlinear and it starts to become a comparator. These collector currents source into a discrete differential transimpedance amplifier with the relatively low loop gain of <60 dB. This particular circuit was originally designed to replace the TA7136 in Boss DS-1 guitar pedals with 9 V battery operation and would work down to about 6.5 V—dead battery mode. This Op-Amp has a similar asymmetric output impedance character to the TA7136, where slew rate going negative is less than the slew rate going positive, producing slight even-order, octave-related harmonics. When the amplifier's midpoint voltage is stabilized, it operates linearly open loop with a gain of about 1000, 1 V/mV, and with very low distortion. The LSK389 JFET front end provides high input impedance and linear gain to the single PNP Gm output stage. This 5-component Op-Amp is ideal for battery audio work with only 1 mA operating current. The VCA operates on 9 V and about 2 mA. Higher input voltages simply require a divider to keep input V <20 mV without emitter degeneration. Differential gain can be modified by a resistor between Q1 and Q2 emitters. There are no sacred cows in Analog design. As usual... — Kirkwood Click HERE For free LSK389, LSK170, LSK189, LSK489, or LSBF862 samples, product information or design assistance.

"In 1979, we teamed up at DEST Corp. to design the first desktop OCR machine for scanning standard letter and legal pages with a linear Reticon stored-charge diode array. Internal to that sensor were two identical 2048-pixel diode strings. Light would deplete the diode pixel reset charge and be read as exposure versus time through a charge integrator. Because the CMOS diode-select shift registers created substantial charge injection noise, the dark string was differentiated with the active string for noise cancellation and then sampled. This sample-and-hold circuit had to be fast and free of artifacts. Unlike many feedback S&H circuits, this is an errorless open-loop stored-voltage sampler with no loop recovery or compensation time. A capacitor, shunt-switched by DMOS, stores the dark voltage value before a scan so differenced pixel voltages are DC referenced. Pixel voltages are then capacitively coupled into a high-speed, high-impedance JFET buffer to the sampling switch. DMOS parts are often substrate-biased to lower inter-terminal capacitance, resulting in reduced charge injection when used as switches. Buffered pixel voltages are switch-sampled through a DMOS part compensated for blow-by capacitance in the off-state, charge injection cancellation, and constant video tracking of source-to-substrate bias. The gate switching voltage of -0.7 V to +5 V also tracks the source voltage for constant charge management. The sampled pixel voltage is then buffered through a JFET-controlled variable gain amplifier for setting an automatic gain control to keep text video at a constant amplitude. Circuits like this use bipolar, JFET, and DMOS parts for their unique and diverse properties. The Linear Systems DMOS used here was originally made by Signetics at the time, the JFETs were from Siliconix, and the bipolar dual, an MP352, was from an earlier John Hall company, Micro Power Systems, before LIS. Overall, the signal-to-noise performance was over 70 dB and, moreover, was very repeatable with medium-tolerance components in manufactured quantities in the thousands. This circuit is just one example of the many uses of FETs as switches, amplifiers, and variable resistors. In other portions of this system, an SD5000 quad DMOS was used for isolation and integrator reset on 5-picocoulomb charge integrators following the Reticon RL2048 outputs. Matching of the quad DMOS on die made charge injection cancellation relatively simple and repeatable. I've now been using these parts for the last 45 years and will continue to do so." - Kirkwood Rough Why This Design Still Matters Today While OCR technology has evolved dramatically since 1979, the fundamental analog design challenges remain the same: maximizing signal integrity, minimizing noise, managing charge injection, and preserving accuracy when processing extremely small signals. Kirkwood's experience highlights an important principle that continues to drive modern circuit design: selecting the right device technology for each function. JFETs remain valuable for their high input impedance and low noise performance. DMOS devices continue to excel in high-speed switching applications where low capacitance and controlled charge injection are critical. Bipolar devices provide precision gain and matching characteristics that complement both technologies. Many of today's advanced systems—including image sensors, medical instrumentation, industrial sensing, scientific equipment, and precision measurement systems—still rely on these same analog fundamentals. At Linear Integrated Systems, we continue to develop and manufacture JFET, DMOS, and bipolar technologies that enable engineers to solve challenging signal chain problems across a wide range of applications. Interested in learning more about our analog portfolio or discussing your application? Contact our engineering team or request samples using the links below.

< Back Testing Capability Linear Systems Mar 17, 2023 New Low-Noise Semiconductor Testing Capability FREMONT, Calif., April 11, 2019 /PRNewswire/ -- Linear Integrated Systems, Inc. (Linear Systems) will be unveiling its large-scale sub-nanovolt parts testing capability at the 23rd Annual Conference and Exhibition for Components for Military & Space Electronics (CMSE) in Los Angeles. This testing capability enables individual screening of small-signal discrete components to levels below a billionth of a volt (nV/√Hz) of noise in quantities up to tens of thousands of parts. Linear Systems created this capability in response to a U.S. defense program requirement and is now providing ultra-low-noise-screened parts to other customers. "Small-signal discrete components can help create the lowest noise signal chains possible and that's what our customers have been demanding," said Timothy S. McCune. "This sub-nanovolt testing capability enables us to provide production-line quantities of parts guaranteed to meet specific noise levels. No other company has this capability." Linear Systems will be available to discuss this capability and its line of precision small-signal discrete components at the 23rd Annual Conference and Exhibition for Components for Military & Space Electronics (CMSE) to be held at the Four Points by Sheraton (LAX), Los Angeles on April 16-18. CMSE is the premier event focused on the design, reliability, and application of electronic components for use in avionics aerospace, military & commercial space systems. The conference provides access to more than 30 technical presentations by industry leaders, focused on advanced packaging of ICs, passive components, and a number of other topics critical to components used in high reliability military and aerospace systems. Linear Systems specializes in the development and manufacture of precision, ultra-low-noise small-signal discrete components. Its parts, such as the LSK389 and LSK489, are used in highly demanding sensor systems ranging from the Large-Scale Synoptic Telescope to piezoelectric devices to sonobuoys. These parts also provide the front-end amplification for high-end test and measurement, medical and audio equipment. Linear Systems is a full-service, privately-held, designer and manufacturer of small-signal discrete semiconductors established in 1987. The Fremont, CA-based company was founded by John H. Hall, co-founder of Intersil and founder of Micro Power Systems. Linear Systems' product line consists of: Ultra-Low-Noise N-Channel and P-Channel Dual and Single JFETs, High-Speed Lateral DMOS Switches, Bipolar Transistors, BIFET Amplifiers, Current-Regulating Diodes, Low-Leakage Diodes, MOSFETs, PhotoFETS, and Voltage Controlled Resistors. Data sheets, applications notes, SPICE models and other information can be downloaded at linearsystems.com . For more information about Linear Systems please contact Ms. Laura Madonna at laura@linearsystems.com . For customer service, please contact Linear Systems at sales@linearsystems.com or call (510) 490-9160. SOURCE Linear Integrated Systems Related Links www.linearsystems.com

J111 SERIES LOW LEAKAGE, N-CHANNEL JFET SWITCH < Back J/SST111 Series J111 SERIES LOW LEAKAGE, N-CHANNEL JFET SWITCH The J/SST111 Series Low Leakage, N-Channel JFET Switch is a direct replacement for the Siliconix-Vishay equivalent part. It is ideal for fast switching, low leakage, switching applications. Available in the TO-92 3L RoHS and SOT-23 3L RoHS package, as well as in die from. Advanced screening options are available for our diverse product lineup, featuring JFETS, Bipolar transistors, MOSFETs, current regulators, and Diodes. Our special screening covers all the parameters listed in the standard datasheet, including comprehensive package pin-out. Connect with our experienced technical team to discuss your specific needs and tailor your requirements—email us at support@linearsystems.com or call (510) 490-9160. MOQ applies to these specialized services. Ordering Information: Below are the options you have when ordering this part series: J111 TO-92 3L RoHS J112 TO-92 3L RoHS J113 TO-92 3L RoHS SST111 SOT-23 3L RoHS SST112 SOT-23 3L RoHS SST113 SOT-23 3L RoHS J111 Die J112 Die J113 Die Datasheet Spice Model Application Notes

< Back LSK489 App Note Introduction For circuits designed to work with high impedance sources, ranging from electrometers to microphone preamplifiers, the use of a low-noise, high-impedance device between the input and the op amp is needed in order to optimize performance. At first glance, one of Linear Systems’ most popular parts, the LSK389 ultra-low-noise dual JFET would appear to be a good choice for such an application. The part’s high input impedance (1 TΩ) and low noise (1 nV/√Hz at 1kHz and 2mA drain current) enables power transfer while adding almost no noise to the signal. But further examination of the LSK389’s specification shows an input capacitance of over 20pF. This will cause intermodulation distortion as the circuit’s input signal increases in frequency if the source impedance is high. This is because the JFET junction capacitances are nonlinear. This will be especially the case where common source amplifier arrangements allow the Miller effect to multiply the effective value of the gate-drain capacitance. Further, the LSK389’s input impedance will fall to a lower value as the frequency increases relative to a part with lower input capacitance. A better design choice is Linear Systems’ new offering, the LSK489. Though the LSK489 has slightly higher noise (1.5 nV/√Hz vs. 1.0 nV/√Hz) its much lower input capacitance of only 4pF means that it will maintain its high input impedance as the frequency of the input signal rises. More importantly, using the lower-capacitance LSK489 will create a circuit that is much less susceptible to intermodulation distortion than one using the LSK389. The LSK489’s lower gate-to-drain capacitance enables more effective, elegant audio circuit designs. The relatively high capacitance of the LSK389 often requires designers to use a cascode circuit to provide the ability to handle higher bandwidths without intermodulation distortion. The cascode does this by eliminating the Miller effect that can multiply the effective gate-drain capacitance and its associated nonlinear effects. However, the cascode adds complexity and noise contributed by the cascode transistors. The LSK489 is an N-channel dual low-noise, low-capacitance, tightly matched monolithic field effect transistor. It features: • 3ms transconductance at 2mA drain current • 1000 GΩ input impedance • 60V breakdown voltage • gate-drain capacitance of only 1.5pF • 4pF input capacitance • 1.5 nV/√Hz noise at 1kHz • best low-noise/low-capacitance combination in the industry • lowest input capacitance per unit gate length in the industry • lowest noise for a given gate length in the industry • tight Vgs matching at operating bias Read More