 In this video lecture, we are going to analyze the emitter follower circuit. Friends, emitter follower also called the common collector configuration is very important in various circuits because of its important properties of impedance matching. Now in this video, we are going to understand means at the end of this video, you are able to describe the working of emitter follower. You are able to analyze different parameters such as voltage gain, current gain, input resistance and the output resistance and whereas you are also able to specify different benefits of emitter follower circuits. Now this is a diagram for emitter follower. Now in this case, we use one ambient transistor. In the circuit, we use a fixed biased condition to bias a transistor and the load is connected to the circuit through the capacitance C2. Friends if you see this element RE that is resistance in the emitter is common for both input and output. In the circuit, the voltage from base to emitter is driving the current IB into the base is input current. So the voltage VB is taken to be input voltage and whereas the output voltage is across resistance RE. So that RE is a common element between input and output. So it is going to offer us the benefit of the feedback or I can say here RE is the element which is common for input and output is serving as but we are more interested in this AC analysis of the emitter follower circuit. Now in this case, we are again replacing all the DC supplies by 0 and we are going to we are going to replace all the capacitors by the short circuit here. Moreover, we are going to replace a transistor by a low-signal transistor circuit which consists of an impedance that is called as HI and the current source that depends upon the input current that is flowing in the base. In the circuit, the input current is IN, HI is the input resistance. The current source is given as HFE into IN and the current is flowing into the output terminal is taken to be I OUT. Now when we simplify all the circuit here, you will see that there is a component that is RE common between base and the emitter that is between input and the output. But this RE is going to carry two different currents, one is output current that is I and we know here I is nothing but the combination of the base current and the emitter current here. So that we simplify the circuit which shown as there is an input signal source that is VIN then there is a resistance RE that is RB from this VB at to the ground here. There is a component called RB into which the current IB is flowing then the current shown here this beta into RB is flowing as output current that is the current IC and there is a parallel combination between RE and RE. Friends from this circuit we are going to analyze some more parameters here which are more important and we call these parameters as the AC voltage gain, the input impedance, the output resistance and so on. Friends there is one more thing to be mentioned here, if you see this circuit carefully you see that the collector is AC grounded and so that that is coming common between input and output that is why this circuit is example of the common collector configuration. Now in the next slide we are going to analyze the voltage gain and so on. Friends it is the circuit that we already discussed before. Now here we are trying to find out here the expression for this AC voltage gain. AC voltage gain is a ratio of Vout by V input here that shown in the circuit. Vout is across the RE and RL parallel combination here. So Vout is defined as the voltage V is nothing but the parallel combination of RE and RL and the current flowing in that terminal that is IE. Now when we simplify this equation we will see that the value of this Vout is expressed as RE parallel RL into beta plus 1 into IB because IE is expressed as 1 plus beta times IB. Similarly the V input voltage is equal to RS plus RB into RE that is input voltage plus the Vout and we simplify this equation as shown here. Now to find out this voltage gain we take the ratio of the Voutput and the V input and the expression comes as shown here. This if you see in the case of denominator we are having two terms here that is RS plus RB. A second term is beta plus 1 into RE parallel RL. Now when we compare the value of this beta plus 1 into RE parallel RL is very much higher as compared with this RS plus RB. So it is normally avoided or neglected. And thereby we are having the value of this voltage gain that is A is always close to 1 but always less than 1. So this is one of the main property of this emitter follower that is it is having a very less voltage gain in fact is just a equal to 1 most in the most of cases but normally is less than 1. Then we also are trying to find out here the value of input impedance. Friends if you see I am showing here one arrow that shows me here the value of the input impedance that is R dash in it is a ratio of VB upon IB both are AC quantities that is VB is a AC voltage and IB is the AC current. Now from diagram we are defining the value of this VB is equal to IB into RB plus the voltage V output and we are going to redefine this equation by the next term that is IB in the bracket RB plus beta plus 1 into RE parallel RL or we can see here we can write this equation for this R dash in that is the ratio of VB upon IB and when we simplify we get that roughly the value of this RE in dash is equal to beta times RE parallel RL. So friends usually the value of beta is very high and also the value of this RE parallel RL is also normally very high. So that the effective input impedance is because of this RE and RL both together is very high that's why we are going to say that for the common collector configuration that is emitter follower the input impedance is very high it is one of the main property of this circuit. Then friends we are going to discuss about the output impedance. The output impedance is defined as is a ratio of the open circuit output voltage that is VOC that is in this case we are going to open the load divided by the short circuit current that is in this case we are going to short the load that is RL is made 0 here. Now from the circuit we know here in the previous slide normally in this case of the case of emitter follower it is having voltage gain equal to 1 so that the voltage at the output without the load is nearly equal to the voltage on the input side that is VIN. So we can say roughly the VOC is equal to the voltage VIN. Next we are going to define the short circuit current ISC is also taken to be the current I is nothing but 1 plus beta into IB and friends when we simplify this one it can be written as beta into VIN divided by RB plus RS and therefore the developed output impedance or resistance is nearly equal to RB plus RS upon beta and we know here the overall output impedance can be found out because R is in parallel with this R dash out so is a parallel combination of REN R dash out and we can make this equation equal to RB plus RS upon beta. Friends the value of beta is always higher and therefore the value of RB plus RS upon beta is normally smaller because RB is smaller and also RS is smaller here and that is why in the common collector configuration that is emitter follower very less output impedance in the case of common collector amplifiers or connections there. Finally friends these are some properties of this emitter follower that is the voltage gain is nearly close to the unity it is only because of there is a negative feedback offered by this RE component it is normally called to be a deep negative feedback because we 100% feedback this voltage back to input here that is why output is nearly same as the input here. Second benefit is the input impedance is very higher and finally the output impedance is smaller and friends the final two are very important here friends these are my references I hope you understand the different parameters we discussed in the video and