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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2answers
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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-...
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2answers
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 ...
0
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2answers
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{...
3
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3answers
1k views

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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3answers
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 ...
0
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1answer
1k views

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 ...
11
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1answer
330 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 ...
9
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4answers
27k views

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 ...
4
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2answers
1k views

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 ...
2
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1answer
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.
3
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3answers
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How can I make a simple voltage-controlled attenuator?

Having discovered just how awful the AC ripple is on some of my 5VDC wall-adaptors, I decided to make an Arduino program to calculate the RMS AC component and DC component of an adaptor under varying ...
2
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2answers
108 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. ...
20
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6answers
5k views

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 ...
5
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1answer
1k views

Is operating point calculation the same for P-JFET and N-JFET with voltage-divider bias?

I've seen this calculation of operating point for N-JFET: Together with the relation \$I_D = I_{DSS}(1-V_{GS}/U_P)^2\$, it is possible to find \$I_D\$ and \$V_{SD}\$. This method doesn't take into ...
4
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1answer
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 ...
2
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1answer
3k views

ERROR(ORPSIM-16492)

I am new to PSPICE as well as the community and i'm trying some circuits given by my professor for an assignment. I've completed most of them which helped me in understanding how the software works ...
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0answers
315 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 ...
0
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1answer
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 ...
2
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2answers
1k views

How current is steady after pinch off voltage?

I was studying the JFET. In the books and other material it is mentioned that when the voltage between the drain and source is increased then drain current is also increased. After a certain point (...
2
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3answers
2k views

Bipolar Op Amp w/ Near Zero Offset?

My understanding is that if source resistance is low, a bipolar op amp has 10dB less noise than a JFET. I prefer the JFET op amps because the input offset is so low that you can eliminate coupling ...
0
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1answer
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 ...
0
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1answer
2k views

P-channel JFET gate voltage

DC Circuit I drew this to try and understand why the gate has voltage on it when SW1 is closed. All I'm trying to accomplish is that when the gate has 0V, the 14V supply is connected to the 10k ...