in this lecture I will explain diode equivalent circuits but first we will try to understand what is an equivalent circuit an equivalent circuit is a combination of elements combination of elements like resistor capacitor inductor Etc properly choosen to best represent the actual characteristics the actual characteristics of device in a particular operating regon so in equivalent circuit we choose elements like resistor capacitor inductor to best represent the actual characteristics of the device like diode transistor Etc in a particular operating regon now there is one question why we need equivalent circuit why we need equivalent circuit we cannot use traditional circuit analysis techniques like thin theorem Norton theorem superposition theorem with actual device if you have diode or transistor in your circuit and you want to use the circuit analysis techniques like thin theorem n theorem Etc to find out parameters like voltage current or resistance then you cannot use them with the actual device and once the equivalent circuit is defined we can replace the device symbol with equivalent circuit and Sol Sol it using traditional circuit analysis method so it's very simple if you want to find out parameters like current voltage or resistance or simply if you want to solve the circuit in which there are devices like diode transistors Etc then first you have to replace the device by its equivalent circuit so that you can use the circuit analysis techniques so this is the reason why equivalent circuits are so important and in this lecture we will try to find out equivalent circuits for diode we will study three types of of equivalent circuits we will study three types of equivalent circuits the first one is pie wise linear equivalent circuit the first one is piecewise piece wise linear equivalent circuit the second one the second one is constant voltage drop constant voltage drop or or simplified simplified equivalent circuit and the third one the third one and the last one is the ideal equivalent circuit the third one is the ideal equivalent circuit in the last lecture we have already completed the ideal equivalent circuit and we will start start with piecewise linear equivalent circuit there is only one assumption in pie wise linear model we will assume the curve to be linear even with small nonlinearity I will write this down we will assume the curve to be linear linear even with the small non linearity so this is the only assumption in wise linear model and first I will first I will plot the curve and then we will draw the equivalent circuit the y- AIS is for ID and the x-axis is for VD the voltage across the diode and uh if we consider the forward bias condition then we have something like this this voltage is VB the barrier potential and first we will try to find out the diode resistance Rd the diode resistance is Rd and it is equal to 1 by slope the slope of this curve and you can easily find out why it is equal to 1 by slope we already know slope slope is equal to tan Theta from mathematics we know slope is equal to tan Theta and tan Theta tan Theta is equal to perpendicular by base so we have slope slope equal to id id by VD this is what we have and from ohms law we know that VD is equal to ID Rd so if we rearrange this we will find we will find 1 by slope is equal to VD by I and from here you can see if we divide both the sides by ID this will give us VD by ID equals to Rd so this we can replace by r d and the DI resistance is equal to 1 by slope so by calculating the slope we can easily find out the diode resistance and now we will draw the equivalent circuit in this model we have the barrier potential VB and the diode resistance Rd in the equivalent circuit first I will make the ideal diode in ideal diode both barrier potential and diode resistance are equal to zero and we are using the ideal diode the symbol for ideal diode to represent the direction of current I will write this down we use this to represent the direction of current and then we have the barrier potential VB from the plot you can see we have the barrier potential VB and the barrier potential opposes the flow of current so the polarity the polarity will be like this this will oppose the flow of diode current and there is diode resistance Rd this is the diode resistance Rd this is barrier potential VB and this is the equivalent circuit for diode for diode in case of pie wise linear model and we connect P side to the positive terminal and this side we will connect to the negative terminal and this is the voltage this is the voltage across the diode VD and current through the diode is ID so this is the model or the equivalent circuit for the diode in case of pie wise linear model in this we have the barrier potential VB and we also have the D resist resistance Rd which you can easily calculate by using this slope so let's try to find out the DI resistance I will take one example in which in which at 0.8 volts at 0.8 volts the diode current is 10 mamp and we have to find out the diode resistance Rd it is very simple Rd is equal to 0.8 - 0.7 the barrier potential in case of silicon in case case of silicon is 0.7 volts and in case of germanium it is 0.3 volts so we have 0.8 minus 0.7 divided by current is 10 milliamp at 0.8 volts and it is 0 mampers at 0.7 volts 10 ra to^ minus 3 for milliamp and when you solve this you will find the diode resistance is equal to 10 ohm so this is how you can find out the DAT resistance in case of pie wise linear model and this is all for this first model now we will move to constant voltage drop model or simplified model in electrical circuits resistance of diode is very small as compared to the resistance of other elements so it can be neglected there are two assumptions in this second model in first assumption we have to consider the linear curve instead of a small nonlinearity and and in the second assumption we will consider we will consider the diode resistance Rd equal to zero and when Rd is equal to 0 it means 1 by slope is equal to 0 or we can say that slope is equal to infinity and slope is equal to tan Theta So Tan Theta is equal to Infinity Theta is equal to tan inverse infinity or Theta is simply 90° so in case of in case of constant voltage drop model we have Theta equal to 90° this is Theta and in this case it is equal to 90° so it is very easy to plot the curve ID VD we still have the barrier potential VB but the diod resistance is equal to zero so we have plot like this this is Theta equal to 90° and this shows the D resistance Rd is equal to zero this is VB and it is equal to 0.7 for silicon and 0.3 for germanium so this is how the characteristics looks for constant voltage drop model now we will make the equivalent circuit and it is very simple if we compare the equivalent circuit with the first model we will find there is no diode resistance so everything will remain same and we only have to remove the diode resistance distance so I will draw it quickly first we will make ideal diode to represent the direction of current this is the ideal diode then we have the barrier potential barrier potential VB and diode resistance is equal to zero so there are only two things first one is the ideal diode and second one is the barrier potential we will connect P side to the positive terminal and this side to the negative terminal and voltage across the diode voltage across the diode is equal to VD current through the diode is ID so this is all for the constant voltage drop model now we will move to the third model that is the ideal model and in case of Ideal model both diode resistance Rd and barrier potential are equal to zero this is something we have already covered in the last lecture that the D resistance is equal to zero and barrier potential is also equal to zero in case of Ideal equivalent circuit and it is very easy to plot it it is very easy to plot it in forward bias reason as barrier potential is equal to zero we have plot like this this is IID and this is VD barrier potential VB is equal to zero and as the slope is equal to Infinity the resistance of the diode is also equal to zero and if we make the equivalent circuit then it will look something like this there is only ideal diode barrier potential is equal to zero and the DI resistance is also equal to zero so there is only ideal diode current through the diode is ID and if we compare the three models we will find in the first model in the pie wise linear model we have barrier potential VB we have D resistance Rd but in second model the constant voltage drop model the diode resistance Rd is equal to zero so we only have the barrier potential and in the last Model the ideal model both barrier potential and add resistance are equal to zero so this is all for the three models and if we compare them then we will find the constant voltage drop model is mostly used this model is mostly used when you solve numerical problem you have to use this model and in numerical problem if the diode is forward biased like this if the diode is forward bias this is 5 volts this is 0 volts then you have to replace the diode with its barrier potential 5 volts and 0 volts and if it is given that the diode is silicon diode then the barrier potential is equal to 0.7 volts and now you can easily solve the circuit and if the diode is reverse based like this if the diode is reversed by s this is 0 volts and this is 5 volts P side is connected to the low potential and N side is connected to the high potential so this is the reverse bass condition and in case of reverse bass condition there will be no current through the diode so it will act as open circuit like this these two points are very important and by using them we can easily solve the numerical problems in forward bias condition you have to replace the diode by its barrier potential and in reverse pass condition it will act as open circuit so this is all for this lecture see you in the next one