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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72 views

Working of JFET as amplifier

Can someone please explain the purpose of using the JFET in the configuration attached in the image? I understand the JFETs are for amplification purpose but I am unable to understand the working of ...
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32 views

Calculation of drain current in JFET

simulate this circuit – Schematic created using CircuitLab If IDSS= 10mA and Vp= -5V, when VDS=Vp, what is the value of ID and VGS? Lets say I apply KVL on drain-source loop. Then I get VDD-IDRD-...
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34 views

JFET resistance and transconductance

The book says the rds of the 2N8461 JFET can be varied from 100 ohm to 10Meg. I went on and tried to verify this using the equation and plug values in it from the datasheet using the minimum values ...
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65 views

Source Resistor of JFET Source-Follower

I am designing a simple Buffer to get the voltage of a high-impedanze source to an ADC (60k Impedance). For that I choosed an JFET in the source-follower configuration. simulate this circuit – ...
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36 views

What is the current in a FET as gate voltage is increased towards large voltages?

I know that beyond a threshold voltage VGS, increasing the gate-source voltage VGS in a field-effect transistor (FET) increases the drain-source current IDS quadratically. I've looked on the web and ...
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31 views

JFET switch circuit

I want to discharge capacitor when battery pack is disconnected. Will this work? In my project I use power source selector IC. Problem is, this IC does not recognize battery reconnection until ...
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46 views

How would you determine the Miller input and output capacitance for the amplifier in the figure?

When I calculate the Miller input and output capacitance I get; Cin(Miller) as 6.38pF and Cout(Miller) as 5.66pF. However, these values are incorrect.
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27 views

How to choose Drain current for a JFET Differential Amp

I have to design a JFET differential amplifier with a current mirror. I am unsure where to start and what to set the biasing current to. Any help will be appreciated to point me in the right direction....
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81 views

Common drain small signal analysis and unresolved question for fet

question: There is an amplifier circuit obtained between JFET and MOSFET. For the FET used in the circuit, IDSS = 8 mA, VGSoff = -3V and working point current is 1.42 mA. Use MOSFET parameters as ...
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23 views

JFET Constant Current Source (CRD alternative)

Today I have build constant current source based on J111. To drain I connected VCC it's 24VDC from laboratory power supply, then I connected resitor 470Ohm between gate and source. To source pin i ...
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4answers
131 views

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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19 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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46 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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44 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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38 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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40 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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58 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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1answer
33 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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43 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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33 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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36 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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48 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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24 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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58 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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134 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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35 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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293 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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113 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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538 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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1answer
291 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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1answer
178 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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1answer
42 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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85 views

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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135 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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1answer
71 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
146 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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198 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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65 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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80 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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1answer
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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2answers
463 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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83 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
440 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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52 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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100 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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631 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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1answer
310 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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