# 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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### GaAs and GaN as low temperature carriers in transistor

I've been looking at HEMT to use at low temperatures. GaAs and GaN seem to be discussed often in the context of low temperature amplification. My questions are twofold. To be suitable for low ...
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### BJT vs FET for VHF LC oscillator

In the ARRL (2011) section 9.3.1 about LC oscillator circuits it says that "[t]he N-channel JFET source follower shown appears to be the most popular choice nowadays", but does not explain the ...
208 views

### Determine transconductance of a JFET given only voltage gain and internal drain resistance

I have a homework question as follows: If a JFET having a specified value of rd=100 kΩ has an ideal voltage gain of Av(FET) = -200, what is the value of gm ? As far as I know, the closest formula for ...
5k views

### Common Source JFET Amplifier Gain

I would be thankful if someone could explain me what is the general formula used to calculate the gain of a common source jfet amplifier. I have the fallowing circuit: Since this circuit was taken ...
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### JFET impact-ionization current

Can someone explain me what impact-ionization current means and when does it happen? As far as I'm concerned, it's about gate current leakage when operated with high Vds and Id. If I'm right, why does ...
85 views

### Help with JFET circuit analysis

Just having trouble analysing the following circuit with the information given. where k=0.16mA/volt^2, Vt = -1.7V and A = 1V. I believe you have to first find an expression for V_gs but the ...
157 views

### Can an N-ch JFET switch be positive-common-source?

I'm considering switching options for an HF (3-15 MHz) tuner network. Pin diodes are interesting but their surrounding control circuitry is somewhat inconvenient, and they require relatively high ...
283 views

### Line regulation of zener diode with jfet

How am I able to calculate line regulation (S=dUout/dUin) for zener diode with current provided by JFET with zero gate - source voltage? Schematic below Lets say it's typical BF256B as JFET and BZX84-...
531 views

### What is small signal. And what is small signal equivalent circuit

I am very new in electronics. And this topic is bothering me. I want to know what is small signal and how to draw a small signal equivalent of jfet with common source.
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### How do I make a simple bidirectional current limiter

I want to connect two separate batteries together and let the highest voltage one charge the lower one not exceeding the desired current. I expect it to be a solid state solution without the need for ...
161 views

### Dealing with JFET parameter spread in voltage controlled resistor configuration

So I need to make a voltage controlled resistor to control a Q factor in RLC resonator, I'm using a P-channel JFET for this and I can't choose a JFET because datasheets show huge IDSS and VGS spread. ...
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### Operation of Junction field effect transistor

Is Vds necessay for Drain current (Id) or we can get Id by Vgs also as in books it is written that Vds is necessary.
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### Schematic Symbol Help Possible Antiquated JFET Symbol Conversion

This is an old schematic, and the MPF102 JFET in the picture is confusing me. Can someone help me identify the Gate, Source, & Drain connections to this symbol, please! My guess is that the ...
477 views

### Measuring Vackar Oscillator Output Amplitude and High Frequency Effects

This is somewhat related to my previous question regarding why I wasn't seeing a clamp biasing effect with my JFET Vackar oscillator. I think I figured it out and I'm wondering if you guys agree. ...
318 views

### Clamp-biasing at JFET Gate in RF Circuits

Reading the book Communications Circuits by Clarke and Hess, I came upon a chapter entitled "Clamp-biased Square Law Device" (Chapter 4 - 4.9). It took me a while to realize that they were ...
289 views

### Choosing an SMD JFET [closed]

I've prototyped a simple circuit for interfacing a ceramic mic to an iPad. It uses a 2N5457 JFET. I just happened to have that component on hand and it worked well. I'd like to implement this ...
1k views

### Why are there no power JFETs?

After doing a search on mouser, I found only 4 JFETs rated for more than 1A drain-source current, compared to almost 3,000 BJT and 11,000 MOSFETs. Is this because there is less of a market for a high ...
887 views

### How susceptible to ESD damage are JFET gates?

Are the gates of JFETs protected against ESD transients? Is it best to include back to back zeners like with MOSFETs?
720 views

### How do I know what power rating pots need?

EE novice here. In this post on reddit, the author wonders if 125mW-rated potentiometers are usable in the MFOS Noise Toaster. The Noise Toaster drives analog audio circuitry off a single 9V battery ...
207 views

### What are the benefits of this type of JFET biasing

The next schemtaic I was found as an input stage of ultra-linear phono amplifier. What are the benefits of that type of biasing? I don't see that 12V zener has 12V voltage drop on it (only 4.2V) and ...
54 views

### The value Of a Voltage Can someone help

So i was working on JFET networks and in the calculation of The output impedance (Solution from the book) I am stuck in the solution My question is How can Vrd = V0 + Vgs ? The solution picture
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### Soft muting small speakers quickly

I'm running a small 3W speaker from an MP3 module, in order to save power I control the module with a MOSFET switch. The module itself can not go into an very effective low current sleep mode so I ...
909 views

### MCU Controlled JFET switch for negative voltage?

I have a JFET that requires negative voltage to the base in order to close and 0V to be open. I have the following power rails: 5V , -12V & GND (0V). What would be the simplest way to switch the ...
290 views

### What is the op-amp in this JFET-matcher circuit?

In the circuit diagram on this page, there appears to be an unspecified op-amp. Could anyone guess at a component I could use for this and/or explain why there is no specification? Background: I am ...
331 views

### Multisim virtual JFET saturation current

How I can set virtual JFET saturation current in Multisim. I can't find any parameter with appropriate name in model settings of virtual JFET?
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### Question about JFET resistor connection

In the picture is a Photovoltaic mosfet driver. So I understand that the JFET is used to discharge the MOSFET gate to source capacitance to decrease its turn of time. Whats the function of the 2M ...
335 views

### JFET Q-point for use it as resistor

I'm trying to use JFET as resistor. For use it as a resistor DS voltage need to be less than Vp. Vp from datasheet of 2n4391 is equal to -VgsOff and equal to 4.0V. How I can to set scheme to view ...
1k views

### VGS(off) for JFET in AGC circuit

I want to use a well known AGC circuit (see below) with 3.3V instead of 5V. The description of the circuit can be found here, and state the following about the JFET: Other JFETs with VGS(OFF) of 5V ...
784 views

### For the same JFET why are there two curves in a Transfer characteristics graph?

I am trying to learn about n-channel JFETs from David. A. Bell's book. In the transfer characteristics graph, there seem to be two curves as shown. Note that VGS(off) = Vp As you can see, one curve ...
1k views

### What is the difference in performance of a JFET current source to a 741 Op Amp current source?

It is possible to build a circuit that provides a steady current source via these two methods: Screenshot of op amp circuit Screenshot of self-biasing JFET current source In the circuit using an op ...
774 views

### JFET Burn Out in Wien Bridge Oscillator

I have a simple Wien Bridge Oscillator (with JFET gain control) circuit pictured below. This circuit works as expected at lower voltages (+/-9 V), but I am running into some problems with higher ...
529 views

### Joule Thief - Why only single celled?

According to this Wikipedia article: It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged ...
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### FET Practical DC Analysis Problems

I just finished with theory understanding of FET transistor principles, etc. Right after that I tried to make simple circuit Common-Source with FET and MOSFET the problems occurred one by another. I ...
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### Need a switching circuit that is closed when Vgs is negative and open when Vgs positive, any ideas please?

I need a circuit that will be closed when Vgs is negative and open when Vgs positive. Basically the inverse of Enhancement mode MOSFETs or JFETs. I appreciate your help with this. Thank you!
663 views

### Phase Modulator with JFET

At the university we have seen this circuit as a Phase Modulator: simulate this circuit – Schematic created using CircuitLab where $m(t)$ is the message at audio frequency and $v_p(t)$ is ...
584 views

### How does the Miller effect affect a JFET-amplifier stage with tuned-circuit input?

I'm trying to build a simple JFET amplifier of MW signals (500kHz-1.5MHz) using a ferrite antenna and a variable capacitor to tune the desired station. The schematics is pretty obvious, as seen in the ...
2k views

### Which one is the pinch-off voltage?

According to my book and teacher : It is the Gate voltage at which the depletion region closes the channel. According to some people on the internet and Youtube videos: It is the Drain to ...
554 views

### Why does this JFET attenuator distort the signal this way?

I have been trying to build a simple VGA using an op-amp and JFET for gain control. I get good results over a small range of voltages (like factor of 10) but as soon as I try to improve this, the ...
2k views

### JFET - transconductance, gain(mu) and internal resistance at the drain

Hello guys, here is such assignment: to determine gm, internal resistance at the drain and maximum gain (mu). 1) If Transconductance gm can be found from the formulas and graph. 2) How to determine ...
1k views

### Calculate the upper cutoff frequency of a common gate

JFET: I am trying to calculate the upper cut-off frequency of this circuit. Gain gave me 5, as it did in the teacher's solutions. But I'm applying the expression to the condensers and it's giving me ...
566 views

### JFET Transistor Output Impedance

The output impedance ($R_{out}$) of this circuit is $R_{S3}$? New circuit:
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### Analogue multiplexer using JFET for small signals

I'm working on a circuit with a non-inverting amp and I would like to switch between two reference voltages, both with low output impedance. So in effect i want something like this: simulate this ...
2k views

### About BJT, JFET & MOSFET transconductance

For BJT, the term "gm" refers to base-emitter conductance. But when we are talking about FETs and MOSFETs, where is this "gm"? Between gate-source junction for JFET? Drain to source conductance for ...
4k views

### How to identify JFET terminals with multimeter?

If i have a JFET besides using datasheet , is there any way to use multimeter or ohm meter to find which terminals are gat,e drain, source of the JFET?
476 views

### precision current sink for battery discharge

I'm trying to build a precision computer-controlled current sink to discharge a battery. The maximum discharge current is to be 2mA (at the moment; I might go to larger currents later) and the battery ...
2k views

### FET Push-Pull Amplifier

I'm curious about JFET/MOSFET circuit B class amplifier. When BJT is used we only have those (+/-)0.7V limiting the transistors and causing crossover distortion. But with adding FETs or MOSFETs to the ...