 Majid Gundale, Assistant Professor, Department of Electronics, Balch instead of Solapur. Friends in this video, we are going to looking for the small single hybrid model for transistor. Now at the end of this video, we shall be able to describe the hybrid model for BJT and you should be able to draw the hybrid model circuit for different transistor configurations. Friends, we are just going to utilize the 2 port network theory for the hybrid model for the transistor there. Let us just revise in brief what is the meaning of 2 port network. So friends on the figure on the right hand side, it is having a black box here and we supply this black box by a voltage V1 that forces a current I1 in the black box and the voltage V2 is on the other side that sends the current I2 in the black box. So friends we call this circuit as a 2 port network. It has got 2 ports, one is on the input side that is the port on the V1 side is called inputs port and the port which is on the output side is named by this voltage V2. Now again question come why we only go for a 2 port network to replace a transistor. Now we get some reasons behind this. As we know the BJT is a 3 terminal device. Then he uses 2 batteries for providing the biasing. In fact we use battery one is on the input side other can be on the output side. Friends this is the same situation that we get in the case of the 2 port networks. So we expect that this 2 port network theory will be properly implemented to this BJT. So that we normally use a 2 port network to replace a transistor in the circuit. Friends again we know the number of 2 port network are available here and that define different type of parameters in the practice such as Z-parameters, Y-parameters and H-parameters there. Now friend we use only H-parameters in this one because from this H-parameters we can be able to find the value of voltage gain, the current gain, the input resistance and the output resistance of a circuit. And friend these are the very important parameters in the case of transistor amplifiers. And friends with this fact we understand that or we just expect that this H-parameter model is suitable for my transistors replacement here. Now friend again just one step I want to just tell you why we use this H model. Because transistor is not a linear device understand. If you see the characteristics of these transistors here they are not linear. So when you analyze circuit with these transistors it is quite difficult to write any equation for the given circuit here. So we are going to make it simple. So we are trying to replace a transistor by some components which are linear in nature. So friend this hybrid model is one of this approach here by which we can simplify a circuit during analysis. So friend this is a 2 port network and we describe this network performance by these two equations here. So these are basically the two basic equations of this hybrid model. It is defined as V1 is equal to H11 I1 plus H12 V2. And we define this output current I2 equal to H21 I1 plus H22 V2. Friend we use these basic equations to find the value of this hybrid parameters here. Because friend in this equation the two parameters that is I1 and the V2 that we can control. So friend in my first finding of this parameter here I am just going to make this voltage V2 is 0 here. So friend this is nothing but we can achieve this condition by short circuit in the output port in the 2 port network and so that the voltage V2 is made 0 here. So friend I will replace this condition in this above equation. So I will get the voltage V1 can be defined simply as H11 I1 and this I2 can be defined as H21 I1 and from these two equations I can find the values of H11. Now which is ratio of this voltage V1 by I1 and the H21 is nothing but ratio of the current I2 and current I1. From the same line friend here we can find the value of this H12 and this H22 by making the current I10. So this condition can be achieved here by open circuiting the input port. So by this condition of this I1 equal to 0 we can define the values of this H parameters by these equations that is H12 is taken as ratio of V1 by V2 and H22 is ratio of I2 by V2. Friend if I suppose say conventionally so H11 is ratio of V1 by I1. So we know it is ratio of input voltage by input current so it is normally called the input resistance there. So I will use a special law later called as HI to denote this H11 in the case of transistors. Then H21 is a ratio of output current that is I2 current and the current I1. So that is the ratio of output current to the input current and we define this H21 parameter by HF parameter here. Then H21 is ratio of V1 by V2 ratio of input voltage to the output voltage is normally called as a reverse voltage gain and we call this parameter H21 by one more parameter called as HR. And finally friend we get one more parameter which we define is H22 is ratio of I2 by V2 and this is nothing but the output admittance and will show this parameter by this H0 later I can say. So friend when we write these equations the basic H equations to the BJT we will replace the different parameters I use by some new parameters. So V1 is replaced by VI, I1 is nothing but current II, I2 is current I0 and the V2 is voltage V0 here. So we can write this equation for transistor simply as this VI is equal to HI into I1 plus HR into V0 then second equation comes as I0 equal to HF into I1 plus HO into V2. So friend from this equation I can simply build this circuit here because we know this V1 is sum of this I1 into HF plus HR into V0 that justifies the first equation and then from the output side here we can define this output current that is I0 is equal to that is HFE that is HF into I1 plus HO into I0 because HO is the output admittance here. So I am just taking the product of this IO and this HO friends I can simply modify this simple circuit here for different type of configurations we use in the practice here. Let us see for the common emitter connections here. So friend now just I will write this VI equal to the voltage VEB then the current II is nothing but the current IE then next is current I0 is current IC then the voltage V0 can be taken as voltage VCB. So when this is a circuit that is drawn for the common base configuration so the equations can be modified as the VEB is equal to HIB into IE plus HRB into VCB and second equation is coming as the output current IC equal to HFB into IE plus HOB into VCE. Friend again we can simply modify this equation for the common emitter so in this VI is coming as voltage VBE current II is nothing but the current IB current in the output side is nothing but the current in the collector that is IC and VU is nothing but the voltage VCE. Now going for this common collector so friend this becomes very easy isn't it because we simply replace the different input voltage terms and the input current term by corresponding term for this common conditions configuration there. So friend in this way we are going to replace a transistor by this simple circuit which consists of a single voltage source and one current source and this consists of the input resistance and the output admittance here now friend before we close here why this circuit is called as a small signal transistor model first normally we use this kind of approach for a circuit that deals with a small signal on the input side means you are trying to amplify a weak signal and you design some amplifier so suddenly in that case of amplifier you have to use a transistor having more value of current gain that is HFE and moreover we are using this circuit for AC conditions here so friend in that circuit I can replace my transistor by that simple part circuit and that's why we call this circuit as a small signal transistor model so friend every way every way when you use H model to replace a transistor this is normally called as a small signal transistor model friend these are my references for this discussion I hope this video is helping you to understand the basic concepts of H model and we are able to describe the H model for different type of configurations thank you friend for listening thanks