Troubleshooting JFET Circuit in Multisim

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Discussion Overview

The discussion revolves around troubleshooting a JFET circuit in Multisim, specifically focusing on the operation of N-channel JFETs and their behavior when used to drive an LED. Participants explore the effects of gate biasing and the role of circuit components.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes a circuit setup with an N-channel JFET (2N4222) where the gate is unconnected, questioning why the JFET does not conduct when powered.
  • Another participant points out the necessity of gate bias for conduction, asking how conduction can occur without it.
  • A different participant expresses uncertainty about the purpose of a drain resistor intended to discharge gate capacitance, suggesting that gate biasing should be direct to achieve IDSS.
  • A participant shares their attempt to use a JFET (J308) to turn on an LED, noting that the LED does not light up without forward biasing the gate, despite achieving a drain current close to the datasheet specification.
  • One participant provides information about the typical voltage drop across LEDs, which varies by color.
  • The participant using the LED specifies a voltage drop of 2V in their circuit.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and method of gate biasing for JFET operation, indicating that multiple competing perspectives remain without a consensus on the best approach.

Contextual Notes

There are unresolved questions regarding the impact of gate capacitance at different frequencies and the specific conditions under which the JFET operates effectively in the circuit.

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I was just reading about JFETs. I decided to try a circuit in multisim.
Connected a N-channel jfet(2N4222), the source is connected to ground and the drain is connected to +12V.
The gate is not connected. But there is a drain resistor(1K) connected between gate and ground to drain any gate capacitance.
When I turn on the power, the jfet should start conducting. correct?
But it doesn't. can some one please explain why this is happening.
 
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What is the gate for? How can you get any conduction if you don't provide any gate bias?
 
I'm not sure why you're putting this "drain resistor" to drain the gate capacitance. You'll only have to worry about capacitance affecting your circuit at high frequencies.
But with the simple gate bias (gate directly grounded), you should get IDSS. And as you reverse the gate with respect to the source (approaching VGS(off), what's suppose to happen?
Mayb you could provide the schematic?
 
Here's what I am trying to do -
Turn ON a LED using a JFET. I have attached the ckt. I changed the JFET to J308.
Without the LED, the Idss is about 40mA which is close to what the datasheet says.
But when I add the LED between the source terminal and ground, it doesn't light up.
If I forward bias the gate with 1V, the LED lights up.
The LED needs only 20mA to light up.
I have attached the schematic.
 

Attachments

  • fetckt.JPG
    fetckt.JPG
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LED voltage drop is the voltage drop across the LED (typically about 1.7 - 3.3 volts; this varies by the color of the LED)
http://en.wikipedia.org/wiki/LED_circuit"
 
Last edited by a moderator:
In my ckt, LED drop is 2V.
 

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