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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572 views

Common Source JFET amplifier input voltage

I want to compare to different formulas which stand for same parameter/value. Here is common source JFET amplifier circuit schematic: Here is the first formula which should apply in general to all ...
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2k views

Common drain JFET output resistance “problem”

The output resistance of small signal model for common drain JFET amplifier equals $$ \frac{1}{g_\text{m}} || R_{\text{ds(on)}} \parallel R_\text{S} $$ and since $$ R_{\text{ds(on)}} =\frac{1}{g_\text{...
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4k views

Common Source JFET small signal equivalent circuit

This circuit is equivalent to small-signal common source JFET amplifier: If you look at the input and output impedance you can see that Zin = R1 || R2 and Zout = rd || Rd || Rl. It can be seen from ...
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362 views

I/O Voltage of JFET Common Source with source degeneration

For common source JFET (n-channel): Uin = Ugs (approximation for simplicity), but really Uin is a little bit lower because of input resistance, like so and Uout equals something like this: But how ...
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5k views

Bypass source capacitor JFET

I know there were some changes applied to voltage amplification formula of BJT when "Ce" (emitter bypass capacitor) was connected to the circuit. How is it with JFET (MOSFET too) and its bypass ...
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What's the purpose of the diode at the JFET gate in this Hartley oscillator?

This is something of a follow-up question to one I asked a few days back (Original JFET Hartley Question). Having made the changes suggested in the comments (i.e., changing Rd/R3 to 270 ohms, instead ...
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279 views

Problem distinguishing BJT from JFET

I was testing few types of transistors on my newly acquired peak atlas pro and came across some N-channel depletion mode JFETs marked 2n3819. To my surprise the device indicated they are NPN BJTs. ...
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637 views

JFET transconductance vs. MOSFET transconductance

JFET's n-channel highest transconductance (from datasheets) is marked as In theory, the same value is defined as that applies for n-channel JFET. I haven't found equation which defines MOSFET's ...
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881 views

JFET Different Output Characteristics for different Vgs,off

Hi guys, I've been trying to design a simple amplifier with some gain. My first question is, why are there multiple output characteristic graphs (2 shown) for a single JFET? My first step in designing ...
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1answer
524 views

JFET Creating DC Offset In All-Pass Filter Output?

Full disclosure, this is a circuit I've constructed as part of a lab at school, however this isn't a specific question I need to answer in the report, this is something I've observed and can't explain ...
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1k views

Matching JFETs for differential amplifier application

How to choose a properly matched pair of JFETs for differential amplifier? Is it enough to measure their Idss?
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356 views

Photodiode JFET amplifier stage in pinch off?

In the circuit below, it seems that the JFET is biased in a regime with very low current (pinched off?). With respect to most designs I came across, this one uses rather large source and drain ...
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726 views

How to measure / determine a FET's Vto (Vgs_off)?

I have a bunch of FETs and i would like to select them / choose only those with -0.7 Vto. Is it an easy way to measure this parameter? Thanks
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818 views

NC JFET switch with only one voltage source [closed]

The below circuit functions as a NC switch. At Vgs = 0V current flows through the channel of J1 (the JFET). When S is closed J1 will open and no current flows through the channel anymore. Right now ...
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5answers
169 views

Simple, low power way to buffer a 500mV threshold signal to an open collector output?

I have a signal that is 0-500mV when low and 600-900mV when high. I'd like to buffer this signal to an open collector output. The output must withstand a maximum of 3.3V be capable of sinking 60uA. ...
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100 views

Diode with an enabler function

I need to implement a diode, which only operates, when enabled by external signal. Specifications: When the enabling signal is off, there is no current irrespective of voltage (positive or negative, ...
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790 views

Shockley's Equation for JFET transistor in Proteus

I'm trying to do a simulation using Proteus for the NJFET Transistor ... But when simulating the Shockley's Transfer equation i get a wrong curve And here is graph's properties
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Common Source JFET Amplifier

I am trying to build a simple JFET common source amplifier to get about 5-10 times gain for a signal I have coming from a microphone with an amplifier already in it, from ADA Fruit, here I am working ...
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136 views

Is it possible to create a (depletion mode) MOSFET using only single type of semiconductor?

If I understood JFET diagram correctly, the p-n junction is only used to isolate gate. Using same principle, would it be possible to construct a MOSFET with no junction at all, i. e. a single ...
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2answers
700 views

Understanding an N-JFET

I am currently learning about FET transistors, and there is some things that I do not understand about an n-type JFET. I have two question about this transistor. Why is Vgs important and how do we ...
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1k views

How to correctly wire LF356 JFET Opamp as amplifier?

I have a circuit in which a square wave with smaller amplitude should be amplified with an opamp. Before, I used a MAX492 which has G/B-product of 500 kHz. With this circuit I manage to easily amplify ...
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1answer
461 views

colpitts oscillator with jFET not working

I decided to build a colpitts oscillator using a jFET (2N3819): It seems to work fine @ http://www.falstad.com/circuit/ but when i try to build this circuit it does not work, any ideas? Thank you ...
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444 views

Voltage follower with a pair of matched FETs

I am studying for an exam and as a part of a question I have to explain biasing and operation of the voltage follower that uses two matched JFETs in the following arrangement: I am not sure that I ...
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1answer
911 views

Having trouble designing a JFET input stage in a Multistage Transistor Amp

I've been given the task to design a multistage transistor amp. Specs given are: Overall voltage gain: 80 (min) to 100 (max) Input resistance no less than 1Mohm Voltage supplies: +-10V Achieve max ...
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1answer
619 views

Can this VCA bring output volume to zero?

I'm working on a synthesizer and trying to keep the VCA simple. I'm also trying to avoid LM13700s, partly because I want this to run on one 9V batter. So I'm trying to make this design work: My CV ...
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746 views

Jfet as HV resistor

I want a voltage controlled resistor for high voltages for a project I'm working on and using a common emitter/source transistor to shunt current into ground in order to get a desired voltage is not ...
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660 views

make a buffer balanced

i am still new to electronic and i probably have a stupid question.. i am trying to build a buffer for a piezo microphone following the project on this site: http://www.scotthelmke.com/Mint-box-...
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767 views

What is the most commonly used transistor?

Just a question about transistors from a rookie. I was wondering what kind of transistor is most commonly used? Bipolar transistors? JFET? MOSFET?
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570 views

JFET - Ceramic Resonator - Oscillator Phenomonom going in and out of oscillation

An oscillator using a ceramic resonator is going in and out of oscillation. I have an oscillator circuit with a peculiar phenomonon. The oscillator does work (mostly). The “crystal” I am using ...
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1answer
225 views

JFET Amplifier Exersice

I'm having a little confusion with a question on JFET amplifiers. This is the Q. You are required to design an audio amplifier using an n-channel JFET which has a pinch off voltage of -16V and a ...
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1k views

Transistor Based Wien Bridge Oscillator

I've been studying the schematic for this RLC Bridge from Heathkit (IB-5281). The full pdf can be found here (search for the part number): http://www.vintage-radio.info/heathkit. The circuit ...
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1answer
147 views

Currents confusion in a JFET datasheet

I am analyzing a JFET. The datasheet has both information about the currents Igss and the maximum of Ig. I know that the Igss is a the reverse current in the PN junction but I can't explain myself ...
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99 views

The characteristics of a JFET

I understand that when Vgs = 0, and there is a steady current Vds, there is a pinching shaped depletion region and so on. But what I have a confusion with is when Vgs > 0 or Vgs < 0. The book ...
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6k views

Transfer characteristic of JFET

I read that in transfer characteristics of JFET, Vds is kept constant and Vgs is varied. But when Vp for any device is constant and Vds is also constant then even Vgs should be constant.
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636 views

In an n-channel JFET, why isn't the formation of depletion layer symmetrical?

When a reverse bias voltage is applied between the gate and the source, depletion layer is formed between the p-type and n-type materials in the JFET. My question is why isn't it symmetrical? Why does ...
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838 views

Understanding p JFET

I would like to make sure that I understood p JFET (https://electronics.stackexchange.com/a/97350/78253) right: With SWitch1 in its current position, there is voltage at the gate of the J1 p JFET, ...
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142 views

Actively biased JFET Amplifier

simulate this circuit – Schematic created using CircuitLab Its just the basic diagram (the actual circuit is a low noise amplifier at 5.5GHz).It does work very correctly.In this circuit the BJT ...
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1k views

JFET Audio switch, does not work in simulation?

This is a circuit I've found online and apparently it is taken from the very rare audio book by National semi. The circuit is used to switch a signal on and off at a transition time that the user sets ...
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706 views

Depletion P-channel JFET that's saturated at \$V_{GS}=0V\$

Is there any depletion-mode P-channel JFET that is saturated at \$V_{GS} = 0V\$?
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Transfer characteristic of p-channel JFET

Considering the output characteristic of the n channel JFET, can we somehow obtain the same characteristic only for the p-channel type of this transistor? In R.Jaeger's book about microelectronics ...
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3k views

JFET's Understanding - Voltage Controlled Resistor

If the JFET is supplied from a constant current source (5ma flowing Source-Drain) Will changing the Vgs voltage do anything? This Image for reference: Another graph of a 2N5457 taken for the ...
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567 views

FETs transfer characteristics in 4 quadrants

Recently I was given a following problem on the test: Usually, transfer characteristic (Id(Vgs)@Vds) of JFET and MOSFET is plotted in the 2 quadrants. Plot it in all 4. After the test we were not ...
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586 views

JFET in place of digipot for gain control?

I'm hoping to build a sound sensor using this schematic (http://cdn.sparkfun.com/datasheets/Sensors/Sound/sound-detector.pdf) To programmatically adjust the gain, I could use a digipot. My (naive) ...
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1answer
2k views

Computing K parameter for Mosfet/JFet/

I am writing a simple simulator for Mosfet and JFet. The equations governing these involve the important parameter "K". But, the data sheets normally do not provide the value of parameter K. So, I ...
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3answers
841 views

jfet preamp output attenuated and clipped on bottom rail when connected to codec

I am currently working on an embedded audio processing system for electric guitar. I have constructed two JFET preamps (left and right channels) based on the j201 transistor using the common source ...
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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 ...
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cirrus/wolfson wm8731 - Connecting Analog Input to Codec

I am currently working on an embedded audio processor project based on the stm32f4discovery board and the wm8731 codec from cirrus/wolfson. I was wondering if anyone here is familiar with the codec ...
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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 ...
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Need help finding the drain current of a jfet

I have this simple dc jfet circuit (homework): First of all I had to find the values of the drain and source resistors given the data above, this was pretty simple and I got: $$R_D=4k\Omega $$ $$R_S=...
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JFET voltage divider bias calculations

Hi im having some trouble with knowing how to perform calculations for \$I_D\$ in a simple JFET where it is biased by means of a voltage divider. I understand that use of the equation: $$I_D = I_{...