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.
Questions tagged [jfet]
299 questions
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What's the difference between field-effect transistors (FETs) marketed as switches vs. amplifiers?
For instance, the J108 JFET is listed as "N-Channel Switch", and the datasheet mentions the RDS on resistance, while the J201 JFET is listed as "N-Channel General Purpose Amplifier" (and the on-resistance would have to be deduced from the IDS…
endolith
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I don't quite understand this FET-BJT preamp circuit
I see this circuit a lot on electret microphone preamps, but I don't quite understand it. The FET is operated as a common source amplifier, so it has gain, inverts, and has relatively high output impedance. So it would make sense to follow it by a…
endolith
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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.
As part of an electronics lab, I was asked to…
corgiworld
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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 silicon with no material differences between them (as…
Hearth
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Is there a modern discrete component alternative to the JFET as preamplifier?
Please consider this schematic of an electret microphone capsule:
(Source: Wikipedia)
While considering something similar for signal conditioning of a 20 mVpp, 3k impedance output from a piezo bender, I found that discrete JFETs seem to have…
Anindo Ghosh
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Why JFET is called a voltage controlled device while BJT is called a current controlled device?
Why are JFET/FET called voltage controlled devices while BJTs are called current controlled devices. Both require a voltage to operate properly. In both an electric field is produced so what is the difference?
Now the question is that current is…
Ali Khan
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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 that has me perplexed. It's shown on the left in the…
Dave H.
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How to do a simple overcurrent protection/circuit breaker circuit for 12V 1-2A?
We're developing an I/O shield for Arduino (sort of) and having four FET controlled output connectors with approximated 12V 1-2A load each. I need all these four outputs to be short-circuit protected and such incidents detected by another input pin…
speakman
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About JFET polarity
I often see the internal schema of a JFET transistor and it always look symmetrical. N-type channel that connect the drain and the source and P-type semiconductor on the gate for a N-JFET. I know that JFET have a polarity but since the schema is…
mathk
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Would this JFET+op-amp circuit work?
This is Figure 9-3(b) from Motchenbacher - Low-Noise Electronic System Design, so I would expect it to work, but I don't understand it:
The addition of a discrete common-source stage at the input can offer lower noise. Overall negative feedback to…
endolith
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VCA Using JFET and Op Amp
Just messing around with LTSpice I came up with this VCA that simulates shockingly good for what it is:
I don't recall ever seeing the use of an op amp to mix the signal and control voltage in an isolated way like this so I just thought I would…
squarewav
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Why does the FET input stage of op-amps still have an input current limit?
AFAIK, Igs of MOSFET/JFET is around 0.
That means that in MOSFET/JFET almost no current flows from gate to drain.
Modern FET input stage:
I read some op-amp datasheets, those op-amps have FET inputs, but they still have a max. value for the input…
curlywei
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Why are JFET transistors used in circuits with high input impedances?
I read an introductory textbook to understand this but it doesn't tell the reason behind it. Is that because the JFETs require much less voltage to operate comparing to BJTs and one needs to drop voltage at the input? I'm a bit confused.
user16307
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Why are the advantages of JFET over MOSFET, or why are JFET still used?
I saw JFET in an electret mic circuit, and it is quite recent, so I'm wondering why.
Since we are looking for the highest gate impedance, MOSFET would seem to be a better choice.
I've also read that JFET have got a flatter response because of a…
Jonas Daverio
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Advantages of depletion mode FET devices over enhancement MOSFETs
I've grown the impression that JFETs and depletion mode MOSFETs are not used very much nowadays, especially in new designs.
I know that they need different bias networks, especially as discrete components, so I guess there might be some advantages…
LorenzoDonati4Ukraine-OnStrike
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