Operating regions of JFET

In the last post, we studied the difference between JFET and BJT there I had explained why we prefer JFET over BJT. I hope you understood all the concepts of JFET which we had studied up till now.

In this particular post, we are going to see the region of operation of JFET.

So similar to BJT, JFET can be operated in 3 different regions although there are 4 regions but we neglect the 4th region i.e. breakdown region and I hope you know why we neglect it. If you don't know the reason then watch the video given below, the video is of BJT but still, you will get a clear idea that why we neglect it.

Video:-

Since we had studied drain-source characteristics in great depth so based on that we concluded that JFET operates in 3 different regions 

1) Cutoff region

2) Active/Saturation region

3) Ohmic region

Now we will discuss all the 3 regions in great depth.

1) Cutoff region (Vgs < Vp):-

As you can see in the above figure that when Vgs was made more negative the depletion region keep on increasing and the count of electrons which was moving from source to drain also decreases and Id almost becomes zero, also Vds was minimum or almost equals to zero. So the output is negligible and therefore we can say that JFET is operating in the cutoff region. Thus in this particular region, JFET behaves as an open switch. So in short we can maintain the cutoff region of JFET by making Vgs<Vp.

2) Saturation region (Vds > Vgs - Vp):-

You can clearly see above that when Vgs is set to a certain value and drain to source voltage is greater, then electrons can easily move from source to drain as depletion region is less. Due to which maximum current flows from drain to source. In this region, the drain current Id entirely depends on the Vgs and not a function of Vds thus JFET is operated as a closed switch and as an amplifier in this region. 

3) Ohmic region (Vds>0 and Vds<Vp):-

In this region, the JFET behaves as a variable resistor. Since this region behaves as a variable resistor thus the ohmic region of JFET is used in many voltage regulation applications. If JFET operating as voltage control variable resistance it biased in the ohmic region. The control voltage is VGS and it finds resistance by changing the Q-point. For JFET biasing in the ohmic region, the dc load line should intersect the JFET characteristic curve in the ohmic region as shown in the below figure.

CONCLUSION:-

In the end, I would like to conclude the whole working of the JFET with the help of one video. At the end of this video, I had inserted the working of JFET with the help of one small animation and I hope it will help you to boost up your imagination skills.

VIDEO:-

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