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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31 views

SIPMOS as current source

I read this question and I have the same issue: acquire and protect an input with 5-24V range. I wonder if nowadays I can use a BSS126 as current source transistor. It's a depletion mode N-channel, I ...
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125 views

Simple MM Phono Preamp Design

I'v found myself in need of a new preamp for my turntable (the one built into my power-amp went out with a bang and tripped the breaker!). I'm using a moving magnet type cartridge with output around ...
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82 views

Using an N-channel JFET as a VCR

Hi i am trying to find a way to determine the range of the drain source resistance (RDS) for a N-JFET only with the parameters given on its datasheet. I know that the drain source resistance is ...
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1k views

How to get an analog signal 0 to 12 V DC based on reference 0 to 5 V DC?

I have an Arduino which is creating an analog voltage from 0 to 5 V (I used external DAC for it) and I need to use it for the regulation of "big" circuit with 12 V DC. So I am trying to get ...
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176 views

Design of an Automatic Gain Control Using a JFET

simulate this circuit – Schematic created using CircuitLab I have implemented the above circuit which is supposed to amplify a negative 200kHz sinusoid to constant amplitude even if the ...
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119 views

Long-tailed pair - gain and bandwidth

I'm trying to understand this schematic of this differential probe: I marked some blocks. I have some questions about the block I marked "Long-tailed pair" (LTP). I have no experience with LTPs so ...
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83 views

My very basic voltage-controlled amplifier gives an asymmetric waveform?

I want to build a voltage-controlled amplifier. The Wikipedia pages on voltage-controlled amplifiers and voltage-controlled resistors suggested using the following approach: first I would simply build ...
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40 views

Confusion regarding Fixed-Biased configuration of JFET

This is the picture of a fixed-biased JFET, taken from a book :- This is written in the book, relating to the above pic :- I am unable to understand why \$ V_{GS} \$ has a different polarity than \$ ...
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JFET: Find ID (Drain) and VD

Hello everyone as the title says I need to find: Id (Drain) and Vd of the circuit. If use the below type for above circuit i will take correct result? Id = Idss(1 - Vgs/ Vp)^2 Vd = Vcc - Id* R45 ...
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2answers
87 views

JFET Vgs Calculation Equation

Does anyone know the formula to calculate the Vgs of a JFET used as a variable resistor? Say, I have a Vds of 24v and I require a current of 20mA (just arbitrary values) I would need a 1.2k resistance,...
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58 views

Would leakage through a JFET create thermal noise

The impedance through the gate of a JFET is very high and leakage current very small. Would this impedance act like a large resistor creating thermal noise? Would this be a low noise way of ...
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136 views

Can I use a JFET like this?

I'm trying to bias the inputs of an op amp, I can't simply use a resistor because there is a tiny cap at the input and it forms a voltage divider and I get no signal through. Can I use a JFET like ...
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159 views

JFET stabilised wien bridge component ratios

I find myself in need of a sine wave oscillator and have decided on the wien bridge. I have followed all the tutorials but have a few questions that do not seem to be answered elsewhere. Firstly, I ...
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61 views

Quad voltage controlled resistance

In order to have a quad volume control, I thought of having a single log pot between a DC negative source and 4 gates of 4 matched JFET's as shown above. We're talking audio here, I haven't calculate ...
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59 views

Simple N-JFET circuit with PWM

I have the following N-JFET DC circuit for which I must find the output voltage \$V_{out}\ = V_D\$ (the drain voltage with respect to ground) when a PWM signal from \$-10V\$ to \$0V\$ is applied to ...
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50 views

Why does the signal generator voltage change in this circuit?

In this circuit, when I measure with an oscilloscope the square wave I get this: (The switch S is open in both measurements) But when I change the R2 resistor to 15 Ohm, I get this: Why does that ...
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218 views

How current is steady after pinch off voltage?

I was studying the JFET. In the books and other material it is mentioned that when the voltage between the drain and source is increased then drain current is also increased. After a certain point (...
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79 views

Circuit to save one battery when another battery is present

I have a battery powered system and I want to preserve the test batteries during setup and calibration. I want the system to use the setup batteries if they are present and use the test batteries if ...
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238 views

Single MOSFET current source (JFET like)

Is it possible to build MOSFET current source similar to JFET current source like this: simulate this circuit – Schematic created using CircuitLab With MOSFET it should be something like this: ...
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44 views

TO-72 package FET Case pin

I have 2N3823 JFETs in TO-72 package, which has 4 leads. No problems with Gate, Drain, and Source, but there is also a Case pin that I have been ignoring. Now I am putting together a PCB for a ...
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84 views

How can an electret microphone have a more or less flat response?

I've got two concerns. The first is that the voltage across the electret capacitor is proportional to \$\frac{1}{d^2}\$. For the voltage to be proportional to the pressure difference between both ...
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478 views

Why are the advantages of JFET over MOSFET, or why are JFET still used?

I saw JFET in an electret mic circuit, and it is quite recent, so I'm wondering why. Since we are looking for the highest gate impedance, MOSFET would seem to be a better choice. I've also read that ...
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284 views

What's the catch with these JFETs?

I'm designing a low-noise, high-bandwidth (100 MHz) pre-amp using JFETs. The choice for JFETs was because it needs to operate with large source impedances (>10 kΩ). I spent a while looking for a ...
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1k views

What kind of transistor turns on with 0.2 volts?

I need a circuit to turn on a LED with 0.2 volts. I want to do it with a transistor as a switch. What kind of transistor will turn on at this voltage?
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74 views

Why does the output voltage of my linear gain amplifier depend on the JFET gate voltage?

I am trying to build a linear gain voltage controlled amplifier using an N-channel JFET as a voltage variable resistor (see circuit below). As the circuit is set op now, I would expect Vout to be ...
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108 views

Voltage controlled amplitude problem - JFET circuit

I work on a function generator project. It generates waveforms such as sine, triangle, rectangle with PWM regulation, saw. I would like to digitally adjust the signal output amplitude. Input signals ...
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85 views

How does the biasing work in a JFET based triangle to sinewave converter?

To those who haven't see this cool little circuit it employs the non-linearity and symmetry of the JEFET to clip a triangular signal to make it more sinusoidal Can someone please explain to me how ...
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435 views

Origin of increased noise in BF862/OP27 charge amplifier

For the amplification of piezoelectric sensor signals I am currently building a charge amplifier with a low noise JFET frontend utilizing an NXP BF862. The amplifier design is based on application ...
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100 views

Converting a non-electret condenser microphone circuit to electret?

I'm trying to convert an MXL 603s circuit, which has a non-electret capsule, to an electret version. I'm pretty certain that the only real difference between the two would be the fact that the non-...
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354 views

JFET colpitts oscillator - very unstable

I've designed, simulated and build this simple JFET colpitts oscillator and I couldn't get it to work with real parts. I didn't have BF256B jfet model so I simulated it using some generic one. The ...
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100 views

JFET Voltage Divider design question

I've been trying to get this question done for a couple of days now and no matter how much I try I've been unable to get it done. Your help in this question is greatly appreciated.
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66 views

Why is the source voltage 1/4 of the supply voltage applied to the drain in a voltage-divider JFET circuit?

According to a Junction Field Effect Transistor (JFET) article in ElectronicsTutorial, the voltage across the source resistance is equal to 1/4 the supply voltage applied to the drain/gate for a class ...
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1k views

ERROR(ORPSIM-16492)

I am new to PSPICE as well as the community and i'm trying some circuits given by my professor for an assignment. I've completed most of them which helped me in understanding how the software works ...
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Can a JFET replace a IGFET? (A 3N139 IGFET)

it is my first question so please forgive my errors. I try to build the People Detector from Popular Electronics http://www.swtpc.com/mholley/PopularElectronics/Jun1968/PE_Jun_1968_pg28.jpg. The 3N139 ...
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Choosing a suitable FET for a design project

I'm currently working on a project to produce a high speed and high voltage pulse. I'm trying to find a suitable FET. I intended to supply a voltage of about 50v to circuit. And using the FET to go ...
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35 views

GaAs and GaN as low temperature carriers in transistor

I've been looking at HEMT to use at low temperatures. GaAs and GaN seem to be discussed often in the context of low temperature amplification. My questions are twofold. To be suitable for low ...
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910 views

BJT vs FET for VHF LC oscillator

In the ARRL (2011) section 9.3.1 about LC oscillator circuits it says that "[t]he N-channel JFET source follower shown appears to be the most popular choice nowadays", but does not explain the ...
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130 views

Determine transconductance of a JFET given only voltage gain and internal drain resistance

I have a homework question as follows: If a JFET having a specified value of rd=100 kΩ has an ideal voltage gain of Av(FET) = -200, what is the value of gm ? As far as I know, the closest formula for ...
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2k views

Common Source JFET Amplifier Gain

I would be thankful if someone could explain me what is the general formula used to calculate the gain of a common source jfet amplifier. I have the fallowing circuit: Since this circuit was taken ...
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45 views

JFET impact-ionization current

Can someone explain me what impact-ionization current means and when does it happen? As far as I'm concerned, it's about gate current leakage when operated with high Vds and Id. If I'm right, why does ...
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78 views

Help with JFET circuit analysis

Just having trouble analysing the following circuit with the information given. where k=0.16mA/volt^2, Vt = -1.7V and A = 1V. I believe you have to first find an expression for V_gs but the ...
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98 views

Can an N-ch JFET switch be positive-common-source?

I'm considering switching options for an HF (3-15 MHz) tuner network. Pin diodes are interesting but their surrounding control circuitry is somewhat inconvenient, and they require relatively high ...
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174 views

Line regulation of zener diode with jfet

How am I able to calculate line regulation (S=dUout/dUin) for zener diode with current provided by JFET with zero gate - source voltage? Schematic below Lets say it's typical BF256B as JFET and BZX84-...
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338 views

What is small signal. And what is small signal equivalent circuit

I am very new in electronics. And this topic is bothering me. I want to know what is small signal and how to draw a small signal equivalent of jfet with common source.
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224 views

How do I make a simple bidirectional current limiter

I want to connect two separate batteries together and let the highest voltage one charge the lower one not exceeding the desired current. I expect it to be a solid state solution without the need for ...
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123 views

Dealing with JFET parameter spread in voltage controlled resistor configuration

So I need to make a voltage controlled resistor to control a Q factor in RLC resonator, I'm using a P-channel JFET for this and I can't choose a JFET because datasheets show huge IDSS and VGS spread. ...
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52 views

Operation of Junction field effect transistor

Is Vds necessay for Drain current (Id) or we can get Id by Vgs also as in books it is written that Vds is necessary.
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361 views

Schematic Symbol Help Possible Antiquated JFET Symbol Conversion

This is an old schematic, and the MPF102 JFET in the picture is confusing me. Can someone help me identify the Gate, Source, & Drain connections to this symbol, please! My guess is that the ...
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281 views

Measuring Vackar Oscillator Output Amplitude and High Frequency Effects

This is somewhat related to my previous question regarding why I wasn't seeing a clamp biasing effect with my JFET Vackar oscillator. I think I figured it out and I'm wondering if you guys agree. ...
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230 views

Clamp-biasing at JFET Gate in RF Circuits

Reading the book Communications Circuits by Clarke and Hess, I came upon a chapter entitled "Clamp-biased Square Law Device" (Chapter 4 - 4.9). It took me a while to realize that they were ...