# 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 calculate it?
• How can we calculate the pinch-off voltage without the datasheet?

Thanks in advance!

• Vgs is what you apply to the device and not what you calculate. Commented Nov 2, 2016 at 17:37
• @Andyaka then what's the difference between Vgate and Vgs?
– Mu3
Commented Nov 2, 2016 at 17:44
• A voltage is not a point measurement, it is a difference between two points hence Vgate is meaningless without a statement (implied or stated) about where the reference point is. Vgs implies voltage between gate and source. Commented Nov 2, 2016 at 17:47
• Vgs is voltage measured from gate to source. Vgate could be voltage at gate measured with respect to something else (but is in some circuits taken to mean gate to source). Commented Nov 2, 2016 at 17:48

## 2 Answers

Why is Vgs important and how do we calculate it?

As written in the answers and comments: Vgs is not something you calculate. Vgs is an external property (a voltage) you apply. Vgs is important because it determines the behaviour of the device, i.e. how easily it will conduct current.

Compare Vgs to the gas pedal in a car. It determines the speed of a car. But it's not a one-to-one relationship. A car with an empty gas tank will not move. A car with a strong headwind or up a hill will move slower.

How can we calculate the pinch-off voltage without the datasheet?

We cannot. You could measure it though. Apply a certain Vds and a certain Vgs. Then vary Vgs and write down the value when the current from the Vds source becomes zero.

In the car analogy it is like asking how far you need to push the gas pedal in to drive up a hill at 10 meter/second without you knowing anything about the car.

N-JFET is a kind of transistor that just like other transistors has a transconductance (Gain), the comparison of output to the input; in this case is the comparison of Id (Ampere)/Vgs (Volt). From the transconductance, one can see that the amount of Id is controlled by Vgs. Please see the link to the graph below, Transfer characteristic of JFET

For N-JFET to control the Id the Vgs must be a negative voltage referred to the Gnd (the source); it will conduct the biggest Id currents when Vgs is 0 Volt. The more negative the given Vgs the smaller the saturation Ids and it will stop conducting Id current when the Vgs reached the pinch-off voltage.

To know the pinch-off voltage without a datasheet one must do a lab work to find it by oneself or one can use or run the electronic simulator to obtain it.