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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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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128 views

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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65 views

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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166 views

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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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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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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207 views

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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117 views

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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286 views

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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60 views

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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244 views

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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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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55 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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184 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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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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385 views

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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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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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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132 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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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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693 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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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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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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207 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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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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793 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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134 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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