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Questions tagged [jfet]

Junction Field Effect Transistor - A transistor whose channel is modulated by the application of an electric field that is the depletion region of a reversed biased diode that forms the gate electrode. Characteristic features of the device are low noise and low gate capacitance.

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Simplest JFET front end to A/D converter

This is regarding a cheap Pmod I2S2 ADC and DAC (connected to an STM32F407G-DISC1) that uses a Cirrus CS5343 ADC. Now, as best as I can tell from the CS5343 data sheet, the \$ V_{PP} \$ of the input ...
robert bristow-johnson's user avatar
2 votes
3 answers
423 views

Why do some JFETs oscillate and others don't?

Below is a simple JFET Clapp Oscillator, which works quite well. It starts oscillating at around 40us and gets into clean, stable oscillation by 100us: When I replace the J309 JFET with a more ...
SRobertJames's user avatar
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How can this Clapp JFET oscillate when its gate is tied to ground at the resonance frequency?

How does the circuit below oscillate? It seems to have |gain| < 1 at its oscillation frequency of 2.25 MHz, yet it oscillates there. My analysis: The DC path is through L4 only, allowing a bias ...
SRobertJames's user avatar
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2 votes
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How to stabilize this op-amp circuit?

I have a "JFET enhanced" op-amp circuit shown below: "JFET Enhanced" op-amp circuit Similar to the circuit used in the TI app note: App Note The circuit looks stable in SPICE with ...
Chris192's user avatar
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2 answers
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One point I don't understand about Rs in ac analysis of jfets

In the circuit in this 1st picture, Rs resistance is bridged with C3 capacitor, so Rs is disabled when analyzing ac. but in this 2nd picture there is a capacitor bridging the Rs resistor, but we draw ...
softcalculate's user avatar
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Is there an established way to model this PIR + JFET circuit?

Pictured is a PIR (passive infra-red) sensor hooked up to a JFET. The diagram is from these design notes, which describes the workings of the PIR sensor as: ...
cornelius's user avatar
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3 answers
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Why doesn't my microphone amplifier work?

I designed a microphone amplifier like in the simulation image below. The images of it in real life are also below. The microphone is a JLI2555. The input voltage used in the simulation is 1mV. When I ...
LEXOR AI's user avatar
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Switching circuit with logic 3.3V control on p-channel JFET

The task is to derive sinewave (of higher frequency) chopped by step-function (also periodic but of lower frequency, i.e. several periods of sine wave are placed in that pulse). So I have a sinewave ...
Sombercy's user avatar
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1 answer
61 views

JFET Pinch-off voltage or threshold voltage

the operating condition of JFET in saturation is stated below from this website. Vp stands for pinch-off voltage in the article. But after reading this post shouldn't Vp change to Vthreshold in Vds ...
Jack Huang's user avatar
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What is the input impedance of a JFET common drain amplifier at high frequency?

I have been trying to learn how JFETs work. The first amplifier that I read about was the CS amplifier: I found that for a typical common source amplifier configuration at low frequency, the input ...
Analog's user avatar
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Shockley equation of an E-MOSFET with a B instead of a K

I have a question about the following homework exercise. I have no clue what B means in this equation. I could not find meaning of it in google nor in the books. Could someone link me a resource or ...
T1P0Z's user avatar
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How to filter noise/ripple for a JFET audio amplifier?

In my first realization of a common source JFET preamplifier, I encountered a rather predictable problem but for which I was unable to find any documentation: the power supply ripple and noise are ...
boromyr's user avatar
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1 answer
118 views

Best type of switch for very small and sensitive signals

I have an H-bridge connected to a piezoelectric ceramic transducer which feeds it 40kHz 38V AC. After 3 full oscillations, it is stopped and it starts waiting for an echo, which is probably going to ...
epicMan123's user avatar
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Parameter Interaction Graph of JFET

I am looking at datasheet of the MMBFJ175 P-Channel JFET (Datasheet). There is one graph were I have a really hard time to understand, what it means and how to interpret it. . IDSS doesn't have a ...
Lucas's user avatar
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Resettable integrator resetting to the negative rail not ground

I've recently built a resettable integrator board which uses a low-drift JFET op-amp and a sample and hold chip. The sample and hold amplifier datasheet showed a resettable integrator configuration ...
sam.schimanski's user avatar
2 votes
3 answers
240 views

JFET-BJT amplifier circuit: AoE figure 3.31

I have two questions about the following JFET-BJT amplifier circuit. This circuit is presented and analyzed in paragraph 3.2.3D of the Art of Electronics. The book comes up with the following design ...
Pim Borman's user avatar
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What is happening at the gate of a JFET to cause an electric field?

I am reading "Practical Electronics for Inventers" 4th edition and came across this circuit. As I understand, for a JFET, current will flow from drain to source for a n-channel (opposite ...
Qubit's user avatar
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2 votes
1 answer
196 views

Closed-loop differential-input JFET amplifier

I am unsure how a differential-input JFET amplifier in a closed-loop can be built. A very simple amplifier with differential inputs and differential outputs can look like this: As far as I understand,...
Charly's user avatar
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1 vote
2 answers
257 views

How to write the equation for the input side of a JFET?

I have been reading about the voltage gain of JFET, but I am unable to understand how to write the equation for the input side. Can somebody explain the method to write the equation for the input ...
Preet's user avatar
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Calculating JFET stats for a constant current source [duplicate]

I need to find a constant current source for a variable LM317 current regulator (schematic below), and I would like to know which JFETs to use. Can somebody let me know how much current I need and ...
Klumpy7's user avatar
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Why don't these JFET simulations agree?

I was solving for poles and zeros of this JFET amplifier with an active load, and found an unexpected zero far off in the response when using the SPICE model. The large-signal and small-signal models ...
Sam Gallagher's user avatar
1 vote
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120 views

JFET + Bipolar source follower: with VGS constant, how Vout can be an AC signal?

Id is the sum of IB and IR1 with IB small. We can approximatively say Id equals IR1. BJT give an voltage across R1 equals to VBE. VR1 = VBE which is limiting the current flowing across R1. In ...
Acapulco's user avatar
2 votes
1 answer
144 views

JFET Source follower input impedance [duplicate]

I want to derive the input impedance of this JFET circuit : simulate this circuit – Schematic created using CircuitLab What is the method in order to derive the input impedance of this circuit? ...
exolune's user avatar
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495 views

What JFETS are used in electret mics?

Electret mics supposedly use current of 0.5mA to 1.5 mA. In all JFETs the current at VGS = 0V is around 10mA. What JFET is then used in electret mics to be able to be powered by a laptop for example ...
LEXOR AI's user avatar
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6 votes
2 answers
172 views

Alternative to P-JFET design to ground load in case of low supply voltage

I'm working on a device and I'm encountering some problems. Basically the device pushes varying currents through a variable load using a current source and an H-bridge. For safety purposes, the load ...
Jonieboy95's user avatar
6 votes
1 answer
173 views

Two JFET circuit

I have come across this circuit with two JFETs as shown below. Does anyone know what is the purpose of these two JFETs?
user42423's user avatar
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4 votes
3 answers
1k views

Overvoltage protection with ultra low leakage current for 3.3 V [duplicate]

I am in search of a method to implement voltage clamping or overvoltage protection for a sense line operating at 3.3V, with an ultra-low leakage current of ideally less than 1 nA. If achieving less ...
Olgidos's user avatar
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2 votes
1 answer
98 views

How to calculate I_D and V_GS in a JFET with load resistance

When \$V_\text{DS} = V_\text{P}\$, thus \$I_\text{D}\$ will achieve maximum value hence equal to \$I_\text{DSS}=10\text{mA}\$. By substituting \$I_D\$ into \$I_\text{D}=I_\text{DSS}\left(1-V_\text{GS}/...
Chee Yi Luay's user avatar
1 vote
1 answer
214 views

JFET or MOSFET for controlling the speed of a compressor (via resistance modulation)

I've recently purchased a 12v compressor with a control unit attached (QDZH35G - usually meant for refrigerators or A/C units, but this is for a DIY project). The speed of the compressor can be ...
Justin's user avatar
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1 answer
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Simulation of JFET Amplifier in LTspice

I followed a guide by FesZ Electronics on biasing a JFET amplifier and came up with the following circuit: simulate this circuit – Schematic created using CircuitLab I then hooked up to a 0....
Phönix 64's user avatar
3 votes
2 answers
275 views

Protecting op amp from high voltages with leakage <= 1 nA

I need to protect a op amp of a TIA from too high voltages/currents From its datasheet, the LMP7721 support a maximum differential VIN Differential of –0.3 - 0.3 V. I cant use general diodes, due to ...
Olgidos's user avatar
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2 votes
1 answer
177 views

Why do JFET use buried channels?

All sketches of JFET construction that provide an actual 3D impression suggest that the channel is buried, e.g. here. I imagine that the formation of such a channel geometry requires at least two ...
tobalt's user avatar
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2 votes
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JFET mismatch between mathematical model and SPICE

As a sequel of this question, I built a mathematical model for the Id current of the following (same) JFET circuit: For a simplified model, I took LAMBDA=0 and set ...
edwillys's user avatar
3 votes
1 answer
133 views

JFET Id calculation mismatch in SPICE

I'm simulating the following circuit in PySpice: simulate this circuit – Schematic created using CircuitLab In this configuration, the N-JFET is always in saturation mode, as you can see by the ...
edwillys's user avatar
0 votes
1 answer
207 views

JFET datasheet theoretical vs practical aspects

I am re-learning electronics basics (starting with the JFET). After going through several JFET datasheets (2N548x, 2N545x, J11x, J310 etc) I found it hard to map the theoretical aspects to the the ...
tinkerbeast's user avatar
1 vote
2 answers
697 views

JFET self bias-configuration quiescent point calculation

I am learning JFET biasing configurations. For self biasing configuration (circuit below), the quiescent point is determined as the intersection of the following equations(1) - ...
tinkerbeast's user avatar
2 votes
1 answer
127 views

Why is there current flowing into my JFET? [closed]

When I connect my JFET to a 9 volt battery and then short circuit the gate to the source via an ammmeter, it reads 6 mA of current. When I try biasing the JFET (BFW 11) with a 1 MΩ resistor the ...
LEXOR AI's user avatar
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3 votes
1 answer
344 views

Why is drain-source parasitic capacitance(Cds) omitted in JFET datasheets?

I've checked several datasheets on JFETs and I cannot find drain-source parasitic capacitance. They either include only Cgs and Cdg or they only include Ciss or Crss. Here are a few high frequency ...
Phill Donn's user avatar
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1 answer
254 views

Have I connected the JLI2555 microphone capsule correctly in my circuit?

I am working on a DIY Project from this YouTube video. I have bought the JLI2555 microphone capsule but I haven't managed to get it to work. I connected a JFET with the gate of the capsule and source ...
LEXOR AI's user avatar
  • 346
5 votes
1 answer
841 views

How does one set the Q-point for a JFET cascode amplifier, and what are the correct "Idss" requirements for cascoded JFETs?

I am trying to teach myself electronics, and am currently attempting to learn about cascodes with the intent of building an RF amplifier that will amplify a microvolt signal up to line level with ...
normal chemist's user avatar
0 votes
2 answers
226 views

Design an AGC circuit for microphone using JFET

At the beginning of the circuit, I designed a 20Hz to 20kHz second-order Butterworth bandpass filter circuit, and then I tried to pass the filtered signal through an AGC circuit to achieve a smooth ...
Jeremy Wang's user avatar
2 votes
1 answer
206 views

Design of a band-pass filter with automatic gain control using a JFET

I have implemented this circuit which is supposed to do AGC and filter out the signal between 20 Hz-20 kHz, but it seems have some problems. The bandpass filter works well, but the agc circuit doesn't ...
Jeremy Wang's user avatar
7 votes
5 answers
1k views

Why does the FET input stage of op-amps still have an input current limit?

AFAIK, Igs of MOSFET/JFET is around 0. That means that in MOSFET/JFET almost no current flows from gate to drain. Modern FET input stage: I read some op-amp datasheets, those op-amps have FET inputs, ...
curlywei's user avatar
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1 vote
2 answers
112 views

Can this switching schematic be simplified with digital control?

This is a portion of the schematic of an old (80s) audio synthesizer/effect (Roland Juno 6). The two inputs are triangle waves (opposite phase). The circuit shorted by TR8 and TR17 is not shown for ...
Dimitri Petrucci's user avatar
3 votes
0 answers
68 views

Active load and current source purpose on NE5532

I'm trying to do some reverse engineering on the NE5532 schematic, but I can't understand how is designed the current source for the first stage. What I want to analyze now is in yellow on the picture....
tac's user avatar
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0 votes
1 answer
111 views

Gain of 0 dB for a common drain using JFET

I need to design a circuit that has a gain of 0 dB that allows a voltage attack on a 100 Ω load. My first thought was to make a common-drain circuit since I had to use only JFETs. First I only used 1 ...
TTiM3R's user avatar
  • 51
3 votes
1 answer
294 views

JFETing op-amp inputs

My goal is to be able to use a high-speed, low-distortion, BJT-input op-amp having a much lower input current - that's why I'm trying to JFET the inputs. I don't want to cut DC with input cap, I want ...
cubix's user avatar
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5 votes
2 answers
334 views

Schmitt trigger using JFET

I want to make a Schmitt trigger using a JFET instead of a bipolar transistor. I started by trying to understand how this circuit worked using a BJT and then moved on to try and replace them with ...
TTiM3R's user avatar
  • 51
4 votes
1 answer
164 views

Why do JFETs have such wide parameter spread?

I've noticed that in almost every JFET datasheet I've looked at, the values for Idss and Vto (also known as Vgs-off vary hugely. A J111, for example, could have a Vto of anywhere from −3 to −10 volts ...
user avatar
1 vote
2 answers
139 views

FET Amplifier Output Distortion

I am new to electronics and still in learning phase. I have made an FET pre-amplifier (using n-Channel JFET) for electret microphone. The output distorts when input signal crosses 200 mV. The top of ...
Awais Mushtaq's user avatar

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