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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Problem using JFET to trigger 555 in monostable mode

Desired function: create a 555 timer in monostable mode, triggered on by a JFET shorting the 555's pin 2 to ground Schematic: simulate this circuit – Schematic created using CircuitLab Input: ...
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Using an NPN transistor as a signal switch

I'm using this NPN switch to turn on and off a clean guitar signal (as part of a much bigger circuit not seen.) The base is connected to the switch, which gives the base 9V through a 1 megaohm ...
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Amplitude modulation with JFET

I have found a circuit that does AM modulation with a JFET transistor(2N5485.) I modified it slightly and ended up with the circuit below. A sample output is also given. I used Microcap 12 for the ...
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41 views

JFET circuit analysis

Below is the circuit in question: Below is my attempt, however, I feel there is something wrong because the value of Rd is not fixed.
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Can we short JFET source and gate junctions as shown in the circuit?

Is the below circuit accurate? The way the circuit is drawn in the question it looks like the source current is entering the gate, which is not possible since ig = 0 (excluding very low leakage). ...
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40 views

JFET series switch

The picture shows a series JFET switch. Is r_d itself the load or does the load have to be attached at the port v_out? If the second one is true then what is the purpose of r_d? Can't we replace r_d ...
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52 views

A JFET based light sensor

I am working on a LDR based JFET project which would turn on automatically when it is sufficiently dark. Accordingly, I have designed this circuit. Is this practical? Also when the LDR value reaches ...
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35 views

JFET biasing question

as we all know in order to work the JFET correctly, we need to have a Voltage across R_s which should be greater than the voltage across R_2. now I can make the voltage across R_s of my choice by ...
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47 views

JFET self biasing

I'm learning JFET self biasing. what I've understood so far is the resistor R_s is used to create a bias voltage as shown. since no gate current flows that means no current in R_G so the potential of ...
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What makes some commercial JFETs asymmetric?

In a MOSFET, the obvious cause of asymmetry is the fact that the source is tied to the body. But in a JFET, no such obvious answer exists, as the body, gate, and drain are all the same piece of ...
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407 views

JFET as an amplifier

I'm working on this circuit below: What is the functionality of R2 here? Stabilizing the input voltage? (If it was, it would've been connected in series, right?)
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Conditions for a JFET transistor to saturate

I have come across the relations: vDS > vGS – VP for n-channel JFET vDS < vGS – VP for P-channel JFET The two are given as the conditions for JFET to be in saturation. How can I arrive at these ...
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33 views

Relay to electronic switch change

I would like to change relay functionality in electronic switch. As per the concept, relay pole and NC position is short even there is no power applied to it. I need complete exact relay functionality ...
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Confusing datasheet for JFET 113: Which value for IDSS applies?

I am trying to build a small amplifier with a JFET J113 to get a high input impedance. I have a problem with the datasheet: On page 3 it gives a value of IDSS=2mA for J113. However, on the following ...
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55 views

Implementing negative feedback on a JFET amplifier

I have started designing and simulating a simple Jfet amplifier using LTSpice. The circuit I have below seems fine, apart from too much gain for my needs (about 20x to much gain). To fix this, I ...
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36 views

Self-biased JFET - Midpoint bias: Why set the drain voltage to one-half of it?

To set a midpoint bias, I understand that ID = IDSS/2 (and by equation, VGS = VGS(off)/3.4). For calculating resistor values, why set VD = VDD/2?
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Controlling PWM with an external oscillator and a JFET

I'd like to control the pulse with modulation of a VCO with an external oscillator (triangle wave). The PWM circuit is a comparator fed with the triangle wave output of the VCO. The VCO schematic is a ...
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74 views

What are the pros and cons of using different transistor types in a Colpitts oscillator circuit?

When I do a generic search for "Colpitts oscillator circuit" I get lots of results with circuit examples that use an NPN transistor in a common emitter or common collector configuration as ...
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46 views

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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39 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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222 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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70 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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1answer
43 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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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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110 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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71 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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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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162 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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66 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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281 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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40 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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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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161 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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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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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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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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583 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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32 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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83 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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69 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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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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56 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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129 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
39 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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84 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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97 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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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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59 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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38 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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