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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Troubleshooting simple JFET preamplifier design

The circuit shown below (with actual measured values) simulates correctly in as much that it provides some gain. When I have built the circuit connected it to a signal generator using a 1mVp-p zero dc ...
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I don't understand JFET/this diagram Associate CET study guide

0 I need help understanding JFET amplifier circuit I have a lot of questions that maybe basic questions I'm sorry. This is a N-channel Jfet so it has negative voltage but what does that even mean “...
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Calculate JFET resistance

I'm exploring JFETs at the moment; I'd like to use them as a variable resistor and set the resistance with a control voltage. I'm using it to control a 1Vpp sine wave through ds. I'm using a J201 JFET....
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Why does the polarity of voltage Vds have to be reversed for p-Channel JFET in comparison to n-Channel JFET?

I am trying to replicate the characteristics of n-channel and p-channel JFET, but I don't understand why voltage Vds has to be reversed in the p-Channel JFET for current to flow. I have these ...
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LTspice JFET Voltage Divider Bias Configuration

I want to simulate this circuit in LTspice program and find Idss,Vp and Idsq,Vdsq but cannot. How can i simulate transfer characteristic?
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RC filters in JFET negative feedback

I saw a useful answer about JEFT negative feedback here and so I thought I'd ask for some help too!! I'm attempting to create a JFET gain stage that uses a filter in a negative feedback loop to boost ...
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What Does This JFET Do If Vgs is Always 0V

I'm trying to understand this reference circuit that I found in the datasheet for a piezo accelerometer. I don't understand what the purpose is if the gate and source are common. My best guess after ...
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JFET self bias, choosing source resistor so that the operating point is (1/2, 1/4)

When \$R_s=R_{DS} = \frac{-V_{GS(OFF)}}{I_{DSS}}\$, my textbook seems to suggest that \$I_{D}\$ equals \$\frac{I_{DSS}}{4}\$, but I'm not able to derive this. Any help? My work: \$I_D = I_{DSS}(1-\...
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What is the purpose of gate resistor in gate bias?

The input driving below circuit has only two states: \$-10V\$ for OFF and \$0V\$ for ON. Why can't we apply this input directly to the gate? I mean removing the resistor \$R_G\$ doesn't seem to ...
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Why are regular diodes designed with ideality factors \$n\gg 1\$

(The question is at the end of the text.) In have a recurring application, where I want to clamp ESD strikes into a sensitive high-impedance into a guard node using antiparallel diodes. simulate this ...
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What is the best way to find an RF FET device for a design?

I'm searching for a jfet or mosfet for a receiver front end, but there doesn't seem to be many options. Everything I find seems to be obsolete. The closest thing I've found to a component that matches ...
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JFET- circuit. Uds and Id dependency?

For a lab assigment I did measurements with the oszilloscope for the circuit below containung JFET BF245A. I obtained those curves for Id (drain) in relation to Ugs (gate-source) for the Uds (drain-...
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Electret microphone: convert sound to current or resistance?

See this picture (from Wikipedia): The electret microphone consists of a capacitor with a static charge and a JFET. I wonder how the electrical signal relates to the sound signal. I understand that ...
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Why are JFETs used in voltage-controlled resistors instead of MOSFETs?

Voltage-controlled resistors (VCRs) are usually implemented using JFETs. JFETs are less common than MOSFETs, so, why not use MOSFETs in VCRs instead? They both feature an Ohmic region. Is this because ...
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Is the BS170 MOSFET the right candidate for the task?

I have 2 FETs in my arsenal, the BF256 and the BS170 and I want to reset an opamp integrator. Is the BS170 the right part for the task? The datasheet doesn't tell a lot, except it seems to have "...
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Do the depletion regions meet during pinch off in JFET?

In an N-channel JFET, when the voltage across gate and source terminals is 0V and the voltage across drain and source terminals is increased, at a certain voltage (pinch off voltage), the depletion ...
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Adjustable voltage, 1 MHz capacitive biasing

I'm a budding physicist with little electronic design experience and am interested in driving an alternating E-field of adjustable strength within capacitive loads. Buying a several thousand dollar ...
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Is my summary of transistor behaviour (on or off) in saturation mode correct?

In the following Table, I made a summary of my understanding of the configurations in which the various sorts of field-effect transistors are on or off, in saturation mode. Is my summary of transistor ...
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Is the power consumption of field effect transistors lower than that of bipolar transistors, and why? [duplicate]

Is the power consumption of field effect transistors lower than that of bipolar transistors, and why?
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What is a suitable equivalent to 2N5459 JFET? [closed]

I am attempting to build an audio limiter circuit which has a JFET as part of the BOM. This JFET is obsolete and no longer manufacturered. Can someone recommend an equivalent JFET or a way of ...
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In a JFET, is the gate *always* connected to the bulk/substrate?

In a JFET, is the gate always connected to the bulk/substrate ? Or is there "no rule" (sometimes it is, sometimes it is not)? And what about the case of the MOSFET ?
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What happens when an audio JFET amplifier receives signals from two microphones?

I have seen this JFET amplifier circuit in my notebook. It is very possible that it isn't a practical amplifier and that they just tried to teach me about JFET applications, but I have two questions ...
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Proving that this JFET + Bipolar source follower give back a linear output

I was reading about this source follower and wondered how the bipolar transistor makes Vgs somewhat stable. I believe that what happens here is that when Vin is high more current flows through the ...
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3 answers
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Are there two sorts of *geometry* of JFET?

In some schematics of JFET, we see in the geometry that the source and drains are perpendicular to the position of the gate. Example : https://en.wikipedia.org/wiki/JFET In some schematics of JFET, ...
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Where can I find the symbolic symbols of various bipolar, JFET, MOSFET in a *unified* style?

Where can I find the symbolic symbols of various bipolar, JFET, MOSFET in a unified style ?
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Resistance of JFET when used as a voltage controlled resistor

We have designed a circuit for a bidirectional current limiter using an N-channel JFET as below: This limiter is connected to the end of electrode used for EEG measurement. The requirement for the ...
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Is the answer to this JFET amplifier excercise right?

The problem is the following: *TYU 4.14 For the circuit shown in Figure 4.59, the transistor parameters are: IDSS = 6 mA, |VP| = 2 V, and λ = 0. (a) Calculate the quiescent drain current and drain-to-...
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7 answers
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What is the advantage of gate resistor (R3) in this JFET voltage divider biasing circuit?

I've been studying and building JFET circuits to serve as a high-impedance input to a DIY guitar amp. In my research on voltage divider biasing, I've come across a circuit in Nuts & Volts Magazine ...
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Replacement for low resistance JFET current limiter?

A classic way to protect opamps from excessive input current and still preserve a low source resistance (for noise reasons) is to use dual JFETs in combination with rail-clamps internal to the op-amp. ...
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How can I measure CR2032 RTC battery voltage in a microprocessor system?

I need a reliable low battery early warning system. I'm using Epson RX8900SA RTC and CR2032 battery. I want to measure battery voltage (probably every 24 hour or more). How can I do this without ...
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2 votes
1 answer
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How to determine drain and source resistor values for common source JFET?

I am designing a simple common source JFET circuit as the front end to a guitar amplifier. It looks like this: I arrived at the RD and RS values experimentally. I would prefer to determine them using ...
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Design of a Frequency Mixer

I am quite new to this subject. I want to design a frequency mixer to mix a carrier frequency of 100 MHz with a 20 kHz signal. I found the attached circuit. It seems quite a simple circuit to multiply ...
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JFET question about operation

I have a question about the N-JFET.Assume it operates at the linear region and it acts like a voltage controlled resistor. Can we say that if Vdg(drain-gate voltage) is increased then the resistance ...
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Reason behind the naming convention for the non-gate terminals of JFETs?

Why, for an N-channel JFET, is the source terminal defined by convention as the non-gate terminal with the lowest voltage? Why does this convention swap for P-channel?
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n-channel JFET: importance of gate drain voltage (Vgd) and validity of negative drain source voltage (Vds)

We always consider the gate source voltage as this determines the Vgs,off. But how come the Vgd doesn't have the same effect? Following from this (and more importantly) why can't we have a negative ...
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Polarity issue in JFET transistor

I'm wondering what determines the polarity conditions in a JFET - is it that this transistor can only operate when the p-n junction is in reverse biased polarization? If so, what about the states of ...
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1 answer
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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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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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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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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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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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1 answer
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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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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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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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0 answers
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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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2 votes
1 answer
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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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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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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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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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