Troubleshooting JFET Circuit in Multisim

AI Thread Summary
The discussion revolves around troubleshooting a JFET circuit in Multisim, specifically using an N-channel JFET (2N4222) and later a J308 to drive an LED. The user initially connects the source to ground and the drain to +12V without gate bias, leading to confusion about conduction. It is clarified that without proper gate bias, the JFET will not conduct, and a drain resistor is unnecessary for low-frequency applications. When the user applies a forward bias of 1V to the gate, the LED lights up, indicating that sufficient gate voltage is essential for operation. The LED requires about 20mA to function, and the voltage drop across it is noted to be around 2V.
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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.
 

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  • 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"
 
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In my ckt, LED drop is 2V.
 
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