 Assistant Professor, Department of Electronics, Vulture Institute, Solopu. Friends in this video, we are just going to focus on the different applications of FET, which also called as a junction FET, that is JFET. At the end of this video, you should be able to describe the applications of JFET as a voltage variable resistor and another is the JFET as amplifier. And we just go into the first discussion point, which is the JFET's application as the voltage variable resistor. So friend, the term also describe that this is a resistor whose value is depending upon the voltage in the circuit. So friend here we use a FET in this way, that is, it is a diagram that shows you the FET and it has got three terminals, which is called drain, source and the gate. Now we just apply the battery between drain and the source, which is called as the voltage V0, that is the voltage VDS. And whereas we are applying the voltage from gate to source, that is the voltage VGS there. Now friend, we know here, actually when we are going to plot the output characteristics or I can say the drain characteristics for the given JFET by applying this battery from drain to source, which is normally called the voltage VDS, which is normally kept constant and we are going to supply voltage from gate to source in reverse bias direction. So friend, these are the simple circuit I can arrange it here. And we know here the drain characteristics can be drawn for the given JFET having these kind of nature. Friends you are seeing some hash portions are shown here. So I am just marking some of the names on this top. So region from this O to A, it is O to A basically, it is called as the ohmic region. From this A to this I think O dash here I can say this one, it is nothing but a region in which the output is not changing that much sharply. But above this vertical red line, you see that the current remains constant here. And this is normally called as a region of saturation. This also can be called as a pinch of region. So again friend, we are discussing now the VVR as an application of a FET, that is a voltage variable resistance application of a FET here. So here we are going to consider only the graph from this O to A. Because friend, you see that this is a graph between the output current that is current in the drain and the voltage from drain to source here. You see that the graph is simply showing us a straight line with some angle and friend in this region is found that the output current follows the input voltage quite linearly. So friend, this is a region we used to describe the JFETS application as the voltage variable resistance here. As I said earlier, this portion O A is nothing but called as the ohmic region and that is of concentration. Now friend going deep in this application how we make the arrangement here. As I said earlier, you are going to keep this voltage from drain to source, a constant one. It is not changed here. But whereas I am going to have a batteries applied from gate to source so that the gate to source will be made reverse biased here. Friend in the figure I am showing you the end channel JFET and so that in that end channel JFET the gate is made of P type material and so that the gate of that FET will be given to some negative voltage which ensures that the gate to source is reverse biased here. And friend we are going to vary this voltage here which is from gate to source to manage the width of channel in the FET and we know friend here. So this is a voltage variations we are going to have and so that with this voltage change here you are going to vary the channel width of the end channel in the given FET. Friend in fact we say that suppose if I increase the voltage from this gate to source which is a reverse biased voltage here and so that you are going to increase, you are going to increase the depletion layer width here. And friend you are going to have some control in this given circuit here. And I am just chopping here this graph here because this is nothing but a given graph of this interest here. So friend there is one thing that you have to just admire here just see here it is an end channel FET here. So the channel is made of end type material and the voltage is now shown here that we indicate the duration of the arrow here but actually we are applying the negative voltage from gate to source here and that is providing me this graph here. This is nothing but a region by at which we are going to see the FET is working as a voltage variable resistor. If I suppose take one thing here that is this is a battery, friend currently battery now get reversed here because we are going to make this junction reverse biased here because gate is made of P type material. So this reverse biased voltage from gate to source this one is suppose increased so that that depletion width in this end channel is get increased here. And we know here when width of this channel get decreased here resistant offer by this FET is get increased here. And so that there is a change in a current in the given device here ok. So friend again I revise this application here currently we are going to have a constant batteries applied from drain to source that flows some constant drain current in the FET here. But after that we are going to change the reverse voltage from gate to source here and that will vary the width of depletion here in this end channel and which is decreasing the current flowing in the channel here. And friend this is why it is called as a voltage variable resistor. Friend we get one more application of this FET which is called as a JFET as a amplifier. So we know friend here again FET has got three terminals here. So we can configure this JFET in three different type of connections here. One is called common source configuration second is common drain and third is common gate. Friend similarly in the case of BJT we also have some biasing methods we adopt here in the case of JFET amplifiers. That is we get a fixed bias method then the cell bias and the voltage divider biasing these are the popular biasing methods we use in the JFET amplifier connections. Friends why we go for JFET or the FET as amplifier? Friends FET amplifiers has got some benefits over BJT amplifiers. First of all we are getting excellent voltage gain we are also having a higher input impedance which is again a benefit again circuit showing me less power consumption. So friend also we get one more very good benefit is having a very good frequency response compared with the BJT amplifiers. So friend with these advantages we are going for normally the FET amplifiers for practical solutions there. Friend this is a small circuit that describe you a common source amplifier that uses a fixed biasing. And the input is given at a gate the output is taken across the drain to the ground. And friend if you see that the batteries apply from gate to ground here that is from gate to ground so this RG which is confirming that the gate to source junction is a reverse biasing. And moreover this circuit provides me 180 degree phase shift between input signal and the output signal which is same as in the case of BJT amplifier. Friend we can minimize or we can simplify this JFET amplifier by replacing this JFET by its AC current circuit here which consists of a current score current source whose value is nothing but gm into Vgs in parallel with resistance Rd which is called the drain resistance here. So it is a circuit which is now simplified. Now friend we also can find the value of different parameters here. That is we can find the value of input impedance. So by looking into the input port here this one this RG is between this input terminal to the ground here. So nothing but this RG is the input impedance. Now for the output impedance friend here now we look for this output port into the circuit. You find these are their parallel combination of this Rd and Rd what is AC condition or these are DC condition here. So it comes as Rt parallel Rd here but normally friend in the practical cases this AC drain resistance is very high as compared with the Rd we connect in the circuit. So friend normally this value of Zo which is the output resistance or output impedance is coming simply as Rd. And this one more thing is coming here we always can find the value of the output voltage here. So it is a current source. The voltage available in this output is equal to this Rd parallel Rd into the current flowing in this parallel combination here. Now this current now takes in the reverse direction basically because in circuit current flow from top to bottom here but to get this VO as a positive at the top terminal here current has to get reversed here. So I will define this way equal to which is same as the voltage Vgs here is minus gm times Vgs means this current value into the parallel combination of Rd and Rd. But again friend we know here what is the voltage here. The ratio of the voltage Vgs and the voltage on the input side that is voltage Vgs and the voltage Vgs here. And we define this also by the factor mu which is nothing but equal to minus gm into this Rd parallel Rd that is the load of this given circuit here. So friend roughly I can see here this mu can be simply defined as ratio of this Vdc that is Vds to the voltage Vgs here which is simply coming as minus gm into Rd because Rd parallel Rd is nothing but equal to simple Rd. So friend roughly in the circuit here in the practice the voltage gain is simply depending upon the trans conductance of a given transistor and the load value. Friend this is these are some features of this amplifier we see that this amplifier gives me high input impedance it offers me low output impedance it offers us a very good voltage gain this also offers us a very good current gain values. Friend these are my references for discussion I hope this video is helping you to understand some basics of FETs. Thank you for listening my video thanks for watching.