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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How does JFET cascode amplifier reduce input capacitance?

I read that a JFET cascode amplifier has a reduced input capacitance thus lowering the Miller capacitance and will obtain wider bandwidth. In order to obtain lower Miller cap, the voltage gain must ...
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48 views

How VDB JFET circuit reverse-biases the gate-to-source voltage?

I already know how Gate-bias, Self-bias, and Current-source bias works in a JFET configuration. However, I am still stuck in voltage-divider-bias JFET circuit. (circuit below) In a JFET circuit, the ...
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166 views

What would happen if I left the gate open in a self-biased JFET circuit?

In a self-biased JFET circuit, the gate voltage must be approximately zero so that the reverse voltage at the gate-to-source will be equal(but negative) to the voltage across the source resistance. ...
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103 views

Using a JFET as a voltage limiter

I have a circuit that runs on 1V up to 15V DC. I want to attach a low-power battery-level (1.5V) device to this supply rail, and have it work over the broadest input range possible (yes, it will cut ...
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49 views

Fine points about the nature of bias current in oamps and experiments

In order to build a sensitive application, I am struggling to understand the behavior of the bias (and offset) current in oamps. I always thought unconsciously that the input terminals of the oamp act ...
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89 views

What is pinched off current?

I know what pinch off voltage is.It is the gate-source voltage at which drain current is cut off completely (pinched off) is called the pinch-off voltage. But what is pinched off current?
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259 views

Describing the process of pinching off of a n channel in JFET

The cross sectional diagram above shows an N-type semiconductor channel with a P-type region called the Gate diffused into the N-type channel forming a reverse biased PN-junction and it is this ...
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43 views

Quick JFET Datasheet Comparison/Analysis

In a previous question I asked for a replacement for the J402/LS3954 dual JFET that has gone out of production and increasingly expensive. (Mouser has them for $23 right now, what a deal) and I came ...
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120 views

Determining JFET substitution parameters

I have a design (see page 3) that uses a JFET to linearize the charge current for a capacitor used as an integrator. I've been using the BSR56, but that's been discontinued. I am used to looking at ...
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166 views

Replacement for E402/J402/LS3954 JFET?

I've come across old synthesizer schematics for an oscillator which makes use of a couple E402s. I am looking for a suitable replacement, and I don't think the specs have to be spot on at all, but I ...
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43 views

Can anyone properly explain me the characteristic curve of the p-type JFET?

I know how a MOSFET works. But recently I wanted to know details about JFET. When V(GS) = 0 volts there will obviously a current flowing through the p-type channel. What confuses me is how the ...
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66 views

jfet series switch isn't pulling the output to 0V

I gave a square wave [-10, 0] across gate-source and the input is dc 10V. When the square wave is -10V, I thought the jfet cutsoff and the output would be 0V. But I'm getting ~3.1V at the output. Why ...
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In jfet, how can VGS(off) be found with just IDSS and gm0?

Transconductance curve of jfet is a parabola. Given a point (0, IDSS) and gm0, which is the slope at VGS=0, how is it possible to find the VGSOFF ? One point and slope give a straight line, not ...
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59 views

In jfet voltage divider bias how can Vgs be negative with out knowing the resistor values?

With out referring to particular values of the resistors, my textbook states that Vgs in below circuit is negative. How is this possible? If I increase R2, it increases Vgs, right?
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In jfet, why Vgs is half the cutoff voltage when the source resistor is same as the ohmic resistance of the jfet?

\$R_{DS}\$ is the resistance of jfet in the ohmic region. When the source resistor is matched to this value, my textbook seems to say that \$V_{GS}\$ will be half the cutoff/pinchoff voltage. Why is ...
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98 views

Micropower voltage regulator with discrete components

Overview I have an unregulated power source (photodiodes BPW34) from which I want to get a regulated voltage output 1V. My load consumes less than 10 microamps of current at 1 V. Constraints: I'm ...
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572 views

How does adding a current source and extra resistor to source follower improve its performance?

This question is similar to FET - Source follower utilizing current source to lower voltage offset, although I do not entirely understand the explanations, which is why I am posting my own question. ...
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92 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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750 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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264 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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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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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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769 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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2answers
568 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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49 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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523 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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90 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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1answer
160 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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321 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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75 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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128 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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59 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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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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91 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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1answer
794 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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82 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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138 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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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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376 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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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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204 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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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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849 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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209 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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954 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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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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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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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 ...