 for direct coupled amplifier. At the end of session students will be able to explain the operation of direct coupled amplifier and analyze its various parameters. These are the contents. So where there is need of direct coupled amplifier as its name suggests that there is no any coupling element directly we are providing the output of 4 stage to the input of next stage. Therefore there is no any coupling device which allows us to get the DC current to the base of the next stage and here due to that it affects the biasing conditions ok. So we are providing the different biasing conditions for the each stage where there is need of this direct coupled amplifier because as we have seen for the low frequency applications. When frequency value is low impedance is greater at that time that coupling capacitor as well as the input capacitor will provide the impedance. To over that we have taken the next kind of coupling that is direct coupling and therefore we have this direct coupled amplifier. So as R and C are not used frequency response is good for the direct coupled amplifiers. It is not suitable for the higher frequencies because at higher frequencies strict capacitance that is the inter electrode capacitance between the terminals of the transistor will give will reduce the value of voltage gain. But again when I say I am not using any coupling element over there and I am just simply connecting with the help of wire to the next stage I am having the greater effect due to the temperature. As VB and beta varies with the temperature which leads to the change in the value of collector current and therefore there will be unwanted drift in the output. But so because as temperature varies VB value will change beta value will change and therefore IC will change and that IC is again further given to the next stage of the amplifier which again causes the further change in the biasing conditions. Therefore we cannot avoid any kind of loss at the greater frequencies over there. So this is the circuit diagram for direct coupled transistor amplifier. Voltage divider biasing is used here no any coupling capacitor or bypass capacitor or input capacitor is used. Therefore weak signal is applied at the base of transistor which is amplified and obtained at the collector distance and that same is given to the output of next stage. And due to that we get the amplified signal at the output of next stage. So we get the amplification of signal with this direct coupled amplifier. So keep in mind while analyzing always we are converting our circuit into the equivalent form. So as I do not have any capacitor over there I will just replace my transistor by its equivalent form and that equivalent form consists of R1, R2 parallel combination and this is my base terminal, imitator terminal and the collector terminal. As there is no element causing any effect on the frequency I am just using this as a resistive element rather than representing it in the form of HRE or HOE etcetera or HI over there. So this is my RE1 that is imiter resistor. This is my equivalent resistor that is beta 1 RE1 dash plus RE1. So this is my equivalent imiter resistor. This is the load of the first stage and which is parallel to the input impedance of the next stage. So in this way I can calculate the value of voltage gain in terms of resistor but before to that you must know what is RAC. RAC1 is equal to what the effective load resistance provided by the first stage. So RAC1 upon RE1 dash plus RE1 which is equal to RC1 parallel with beta 2 RE2 dash RE2. So what is this RC1? RC1 is the load at the first stage which is in parallel with the input impedance of next stage. Therefore I am getting this two quantities in parallel and RE1 plus RE1 dash plus RE1. So this is my input stages or imiter resistance at the first stage. So in the same fashion I will get the value of AV2 and multiplication of these two will give you the overall voltage gain. But while calculating AV2 I will take the parallel combination of RC2 and RL. If I am having more than two stages in my direct coupled amplifier I will get AVn that is last stage gain equal to RCm that is last stage collector resistance in parallel with the load resistance divided by RE2 dash plus RE2 nothing but REn dash plus REn. So in this way I will calculate the voltage gain for the last stage and then to the first stage. This is my frequency response which is giving the flat frequency response or excellent frequency response at the low stages. As I can say for 0 frequency value also I will be getting the large gain. So I can use this kind of amplifier for amplification of DC signal or DC value of the input signal. So as no-bypercent coupling capacitors are used we have the flat frequency response over here and up to FH. We will get that flat frequency response up to FH. But after that what happens the stray wiring capacitance and the internal transistor capacitance that is inter electrode wires will reduce the value of gain. Therefore it is not suitable for the high frequency signal. Now think and select the correct option. The question is the number of stages that can be directly coupled is limited. Why it is so? See the number of stages coupled in a direct coupled amplifier is limited because these are my three options which one is correct over here. So options are changes in the temperature, cause thermal instability, circuit becomes heavy and costly and it becomes difficult for the biasing. So here I can say correct option is first one yes because due to the temperature as I had told you in the first slide only that due to the temperature VBE and beta value will change and which causes the further instability because I am providing the collector value of that first stage to the next stage and that next stage to the further next stage and that is why I will get the thermal instability. So at one particular point my biasing conditions will collapse and I will not able to couple the further stages. Advantages are these are used for the very low frequency signal and it gives the flat frequency response that is very good sign for the using this for the audio frequencies and circuit is very simple in the arrangement and it is very cheap as no any additional coupling element is needed over there but it is not suitable for the higher frequency because the state capacitance will give you the reduction in the value of gain. It has the poor temperature stability so the applications. So as the excellent frequency response is there I can use this in the pulse amplifiers then in the differential amplifiers in the regulator circuits for the electronic power supply in the computer circuit in the jet engine control and in the electronic instruments. So this kind of direct coupled amplifier is useful in the amplification for the computer circuits or for the regulator circuits because it is a flat frequency response. These are the references used. Thank you.