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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Why does the resistance of this JFET VCR change?

I'm trying to build an automatic gain control (compressor) for my audio amplifier. For this I'm using a 2N5458 JFET as a voltage controlled resistor. The circuit looks like this in LTSpice: The drain-...
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24 views

JFET RDS(on) Matching

Quick question here - I have a circuit that originally used a 2SK30ATM-GR JFET, with an RDS(on) maximum of 800 ohms. Most of the cheap JFETs I can find have a significantly smaller RDS value, ...
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81 views

Gate leakage current for a JFET?

I'm really confused about how JFETs work. JFETS are usually significantly higher doped donor xor acceptor atoms. My professor mentioned that MOSFETs usually have an oxide in between the the metal and ...
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67 views

Multiple way to enable regulator

I want to know how can I enable the regulator in three different ways in correct way through the hardware. There is two PCBs in below picture. PCB1: 24V input plug (user plugged) 16V battery Charging ...
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40 views

JFETs question about pinch-off voltage

Can the pinch-off voltage be equal to zero? In my opinion no, because the JEFT would be off all the time, isn't it?
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38 views

Series current Feedback

I was tryig to build this circuit ("Series-feedback pair") from "Art of electronics" book (p. 150). Since I do not have the exact components I tried to model it in LTSpice with ...
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66 views

6-channel piezo buffer jfet circuit

Even though I'm a total beginner at electronics, I've decided to build a 6-channel piezo pickup buffer/mixer unit, for a stringed instrument. It should fit into a typically sized electronics cavity, ...
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57 views

Iellegible JFET name, seems to be HEPxxxxx

I have this poor scanned schematic on which there is a Motorola P-channel JFET, labeled as Q5, and I can't read the name. It seems to me that the 2nd note is written "Q5 is HEP*1036" where * ...
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87 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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43 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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39 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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130 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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37 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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32 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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78 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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37 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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84 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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35 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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249 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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1answer
28 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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51 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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49 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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39 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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48 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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72 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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34 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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53 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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56 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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38 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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50 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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27 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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45 views

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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66 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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357 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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42 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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366 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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140 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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809 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
429 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
259 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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43 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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179 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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72 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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147 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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224 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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66 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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91 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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