Hey friends, welcome to kohiki.com  So, in this article, we will learn about the diode resistance. Now, in the previous article on this diode, we had seen the V-I characteristic of the diode. So, if we consider this diode as an ideal diode, in that case, the forward bias conditions should offer zero resistance. While in the case of the reverse biased condition, it should offer infinite resistance. But the actual diode offers some finite resistance in the forward biased condition and if offers very large resistance in the reversed bias condition diode resistance.

And the diode resistance V-I characteristic of the actual diode will look like this. Now, in this characteristic curve, both positive and the negative axis has a different scale. But if both axes have the same scale, then the curve will look like this. So, as you can see over here, this diode allows the flow of current only in one direction. While it almost blocks the current in the reverse direction. Now, if you observe this curve, here the relationship between the voltage and current is non-linear. While in the case of a resistor, it has a linear relationship. It means that this resistor offers constant resistance for the different operating voltages and currents diode resistance.

## Diode Resistance

Diode Resistance While, in the case of the diode, depending on the operating condition the resistance that is offered by the diode will change.

Diode Resistance So, now depending on the applied input signal, the diode resistance can be categorized into two categories. One is the DC or static resistance and the second is the AC or dynamic resistance.

### Dc Or Static Resistance

Dc Or Static Resistance So, first, let’s talk about the DC resistance. Diode Resistance So, this DC or the static resistance of the diode is the resistance that is offered by the diode in the case when the applied voltage is DC voltage.

And the value Dc Or Static Resistance of this DC resistance can be found by knowing the value of the operating voltage and current.

So, for example, let’s say one diode is operated at let’s say 10mA and 0.8V. So, in this condition, the DC or the static resistance of the diode can be given as 0.8Vdivided by 10 mA.

Dc Or Static Resistance That is equal to 80 ohms. So, just by knowing the operating voltage and current we can easily find the value of this DC resistance. Diode Resistance So, for the given circuit, once the operating voltage and currents are fixed, then the value of this DC resistance will remain constant. And that’s why it is known as static resistance. But whenever the operating voltage and currents are changed, then the value of this DC or the static resistance will change.

Dc Or Static Resistance So, now let’s say, for the given diode, the operating current, and the voltages are 20 mA and 0.9 V respectively. Diode Resistance Dc Or Static Resistance So, in this condition, now the DC or static resistance will be equal to 0.9V divided by20 mA. That is equal to 45 ohms.

So, Dc Or Static Resistance as you can see, as the operating condition changes, the value of this DC resistance will also change. Diode Resistance Dc Or Static Resistance And as we move towards the right-hand side of the curve, then the resistance that is offered by the diode will also reduce.

So, now whenever Dc Or Static Resistance this diode is forward biased in this case, the DC resistance that is offered by the diode is known as the DC forward resistance. Similarly when the diode is operated in the reversed bias condition, then the DC resistance that is offered by the diode is known as the reverse DC resistance.

So, for example, let’s say the diode is operated in the reverse bias condition, and in this condition, Dc Or Static Resistance the applied voltage across the diode is 5V or to be precise -5V, Diode Resistance and the current that is flowing through this diode is 2 micro-ampere.

So, in this condition, the Dc Or Static Resistance that is offered by the diode will be equal to 5Vdivided by 2 micro-ampere. That is equal to 2.5 Mega Ohm.

So, as you can see, in the reversed bias condition, Dc Or Static Resistance the resistance that is offered by the diode is very large. Diode Resistance So, in this way, whenever the Dc Or Static Resistance DC voltage is applied to the diode, just by knowing the operating voltage and current we can easily find the DC or the static resistance. Dc Or Static Resistance And on this diode curve, the operating condition of the diode can be simply represented by this Q-point.

So, this Q-point represents the operating voltage and the current for the given diode. Now, so far we have learned how to find the DC resistance in a case when the applied voltage is DC voltage Dc Or Static Resistance.

### Ac Or Dynamic Resistance

Ac Or Dynamic Resistance So, now similarly let’s find out, how to find the resistance of the diode when the applied voltage is AC voltage.

So, in this condition, Ac Or Dynamic Resistance the resistance of the diode is known as either AC resistance or the dynamic resistance. Diode Resistance Now, here, we are assuming that before applyingAC signal to this diode, some finite DC voltage has been already applied.

So, now this point on the diode curve represents the Q-point corresponds to the applied dc voltage, and on top of this DC voltage, Ac Or Dynamic Resistance some finite AC signal has been applied. Ac Or Dynamic Resistance So, here on this X-axis, you can see the variation in the applied input AC signal. Diode Resistance And correspondingly the current that is flowing through the diode will also change.

So, Ac Or Dynamic Resistance if you trace each and every point for this time-varying voltage signal, Diode Resistance then from this graph we can find the corresponding time-varying current signal.

So, now in the case of this AC signal, the applied voltage will change in a certain band. And similarly, the corresponding current will also change in a certain band. So, here the resistance value will change continuously. So, let’s find out how to find the value of the AC resistance when the AC signal is applied to the diode.

So, Ac Or Dynamic Resistance in this condition, the resistance can be found by drawing the line that is tangent to this Q-point. So, Diode Resistance the slope of this line will give the AC resistance for the diode. Now, here this variation is the variation in the voltage, while this variation is the variation in the current.

So, let me just zoom this portion. Ac Or Dynamic Resistance And if I zoom it then it will look like this. So, here just by drawing the tangent to this Q-point, Diode Resistance we can find the value of the AC resistance.

Ac Or Dynamic Resistance Because the slope of this line will give the resistance for the diode. That is equal to the change in the voltage divided by the change in the current. Ac Or Dynamic Resistance So, Diode Resistance now so far the method that we have discussed is the graphical way by which we can find the AC resistance. Ac Or Dynamic Resistance But there is one more way by which we can also find the AC resistance.

So, let’s look into it. Ac Or Dynamic Resistance So, now here this expression is the expression for the diode. And this expression defines the relationship between the diode current and the voltage that appears across the diode. So, now if we simply take the differentiation of this expression then we can also find the value of this AC resistance. Diode Resistance Ac Or Dynamic Resistance So, let’s see this method, and let’s also derive the expression for this AC resistance.

Ac Or Dynamic Resistance So, now in this expression, the Is represents the reverse saturation current for the diode. So, basically, it is the current that is flowing through the diode in the reverse bias condition. Ac Or Dynamic Resistance Then this Vt represents the thermal voltage for the diode. While Vd is the forward voltage for the given diode.

And this η is the ideality factor. Diode Resistance So, for the diode, this ideality factor used to be in the range of one and two.

But here we are assuming that the diode has an ideality factor of one. Now, this thermal voltage is defined by this expression.

(Ac Or Dynamic Resistance)That is equal to kT divided by q. Where k is the Boltzmann constant and T is the temperature of the Diode Resistance. While q is the charge of the electron. So, at the room temperature of 27 degrees, or T is equal to 300 K, the value of this thermal voltage will be roughly around 26 mV.

• So, here we will assume that the diode is operated at room temperature and we will assume the value of this thermal voltage as 26 mV.
• Now, in this expression, if you observe this term is much larger than 1.So, approximately we can write this expression as Id is equal to Is times e to the power of (Vd/Vt).
• Because here, we are assuming that the ideality factor of the diode is 1.So, now let’s just differentiate this expression with respect to diode voltage.
• So, if we differentiate it then we can write (dId /DVD), that is equal to (Is/Vt) x e^(Vd/Vt)Or simply we can write this expression as (Id +Is)/Vt.Because we know that Id is equal to Is*e^(Vd/Vt)- Is.
• So, this particular expression can be represented like this.
• Now, here this current Id is much larger than this reverse saturation current.
• So, simply we can neglect the second term. So, we can write this expression as (dId/dVd) which is approximately equal to Id/Vt.
• So, if we take the inverse of it then we will get the resistance. So, we can say that the AC resistance will be equal to this thermal voltage Vt divided by the diode current Id.
• Now, here this diode current Id is the DC current that is flowing through the diode.So, for the given diode, if we know the Q-point then simply we can find the value of this AC resistance. So, this is another way we can also find the AC resistance.

Now, so far in our discussion, we have assumed that the applied AC signal is very small. But suppose if the applied AC signal has a very large voltage swing then, in that case, the resistance that is offered by the diode is known as the average AC resistance.

#### Average Ac Resistance

So, let’s say the AC signal that is applied to the diode is a large signal. And because of that, we are getting this voltage swing. So, if this AC signal is applied to the diode then the variation in this voltage is equal to delta Vd and the variation in the current is equal to delta Id. So, in this case, diode resistance can be found by connecting the two extreme points of the operation.

And by connecting it we can find the slope of the line. And the inverse of this slope will represent the value of the average AC resistance. So, we can say that the average AC resistance will be equal to change in voltage divided by change in current. So, let’s say, a very large AC signal has been applied to this diode. And because of that, the variation in the voltage is from 0.8V to the 1V.

While the variation in the current is from let’s say 10 mA to 30 mA. So, from this, we can say that the change in the current is equal to 20 mA, while the change in the voltage is equal to 0.2V.

So, the average AC resistance will be equal to 0.2V divided by 20mA. And that will be equal to 10 ohms. So, in this way, when a very large AC swing is applied to the diode then we can find the value of this average AC resistance

### Diode Resistance

Diode Resistance Now, whenever I model this diode, the total resistance of the diode will be the summation of this AC or DC resistance plus Bulk resistance. Now, as we have discussed in the previous article, the bulk resistance is the resistance that is offered by the semiconductor material which is used for the construction of this diode.

Now, the value of this bulk resistance used to be very small, and for most practical cases, the value of this bulk resistance can be negligible. So, let me just summarise what we have seen so far.

### Last Word About Diode Resistance/Summary

So, we have seen that the Diode Resistance of the diode can be categorized based on the applied input signal. So, if the input that is applied to this diode is the DC signal, in that case, the resistance that is offered by the diode is DC resistance.

And it can be easily found from the operating voltage and current. Then we have seen that whenever the AC signal is applied to the diode, in that case, the diode resistance can be found by drawing the line that is tangent to the Q-point. And it can also be found in this expression.

That is equal to thermal voltage divided diode current. Then after we have seen that if the applied AC signal has a very large voltage swing, Dc Or Static Resistance then the resistance of the diode can be found by drawing the line between the two extreme points of operation. And that resistance value is known as the average AC resistance.

So, I hope in this article you understood about the diode resistance. In This, We Also Learned About Dc Or Static Resistance Or Ac Dynamic Resistance. If You Have Any Dought About Dc Or Static Resistance Or Ac Then Ask Me.

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