Troubleshooting JFET Circuit in Multisim

In summary, the conversation discusses the use of a N-channel JFET (2N4222) in a circuit in multisim. The JFET is connected with the source to ground and the drain to +12V, with a drain resistor (1K) between the gate and ground to drain any gate capacitance. However, when the power is turned on, the JFET does not conduct. The purpose of the gate and how to achieve conduction without providing gate bias are questioned. The conversation then moves onto using a JFET to turn on a LED, but the LED does not light up when connected to the source terminal and ground. It is discovered that the LED only requires 20mA to light up and
  • #1
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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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  • #2
What is the gate for? How can you get any conduction if you don't provide any gate bias?
 
  • #3
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?
 
  • #4
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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  • #5
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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  • #6
In my ckt, LED drop is 2V.
 

Related to Troubleshooting JFET Circuit in Multisim

1. What is JFET and how does it work?

JFET stands for Junction Field Effect Transistor. It is a type of semiconductor device used for amplifying or switching electronic signals. The JFET works by controlling the flow of current between the source and drain terminals through a voltage applied to the gate terminal.

2. What are the common issues that can occur in a JFET circuit?

Some common issues in a JFET circuit include incorrect biasing, improper connections, damaged components, and incorrect component values. These issues can cause the circuit to malfunction or not work at all.

3. How can I troubleshoot a JFET circuit in Multisim?

To troubleshoot a JFET circuit in Multisim, first check for any errors or warnings in the simulation results. Then, make sure all the components are properly connected and have the correct values. If the circuit is still not working, try changing the biasing or component values to see if that solves the issue. You can also use the built-in debugging tools in Multisim to analyze the circuit and identify any potential issues.

4. What are some common techniques for troubleshooting JFET circuits?

Some common techniques for troubleshooting JFET circuits include checking for proper biasing, using a multimeter to measure voltages and currents at different points in the circuit, and using a signal generator to test different parts of the circuit. It is also helpful to refer to the datasheet of the JFET and other components used in the circuit to ensure they are being used correctly.

5. What should I do if I am unable to troubleshoot my JFET circuit?

If you are unable to troubleshoot your JFET circuit, it may be helpful to seek assistance from a more experienced engineer or consult online resources and forums for troubleshooting tips. You can also try simulating the circuit in a different software or breadboarding it in real life to see if the issue persists. In some cases, the JFET or other components may be faulty and need to be replaced.

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