Diode Circuit Analysis | Diode Circuit Problems

Circuit 1:Calculate Vo and current in the given diode circuit.

Vo=Vdd-V(diode)=10-0.7

Vo=9.3V

Apply KVL for current

-10+0.7+I*1000=0

I=9.3mA

Circuit 2: Calculate Vo and current in the given diode circuit.

 

Apply KVL

-10+1000I+0.7+1000I=0

2000I=9.3

I=4.65mA

Use Ohm's Law for Vo

Vo=I*1000

Vo=4.65V

Circuit 3:

Calculate Vo

 

Circuit 4: Calculate iD.

A simple diode circuit is given. Solve it with the help of Kirchoff's voltage law. Apply KVL to first loop. 

Apply KVL to the second loop. 

Solve for i1 and i2. Add equation 1 and equation 2, to get the value of i2.

Finally substitute i2 in equation 1 and figure out the value of i1. 

i1-i2 is the current flowing through the diode that is iD. So, iD = 0.96A

Circuit 5:

Calculate voltage across 20k resistor

 

Load Line Analysis ( Semiconductor Diodes)

In this tutorial, the following concepts are covered.

Outline:

1. What is a load line of a PN junction diode? How to draw a load line?
2. What is the characteristic curve? How to find the characteristic curve?
3. What is Q point? How do find out its value?

Load line is a straight line. From this line, we can find exact values of a semiconductor diode current and voltages. By changing the value of resistance and applied voltage value (these are the circuit parameters) we can move the Q point up and down along the y-axis, This straight line is called as called it the load line.
If the load changes, the slope of the load line changes.

The characteristic curve or V-I curve or voltage current graph shows the diode current and voltage relationship. The voltage applied is independent variable and is along x-axis and the current is a dependent variable and is on y-axis. 

Below is the V-I graph of a semiconductor diode. The curve is the diode current which increases rapidly as the voltage approaches to 0.7V. the straight line is the load line. The operating point is also marked.

Q point/Operating point

Look at point A:
At this point the current is maximum and the voltage is zero. This point is called 'Saturation'.
Look at point B:
At this point the voltage is maximum and the current is zero. This point is called 'Cut off'.

The intersection of the load line and characteristic curve will define the Operating / bias / Q point of a network.

There are two methods of finding Q point.

Graphical Method 

Iterative Method 

Here is a video that cover all the step; draw a load line, draw the characteristic curve and then the Q point. Have a look at this.

 
      

Diode Q Point Analysis Using OrCAD

Getting Started With OrCAD 9.2

How To Add Libraries in OrCAD 9.2

If You want to simulate a circuit, then you have to use that libraries only that are present in pspice folder. Here is the video watch it.

How To Install OrCAD 9.2 Lite Edition

Graphical Method of Finding Operating point/ Q-point

Fig 1 Circuit

Fig 2 Characteristic curve

Now draw load line: Load line comes from KVL equation

   I_D = (V1-V_D) / R1 ...equation 1

  put  I_D=0   in equation 1

equation 1  becomes V_D=V1=10 (eqA)

Now put V_D=0 in equation 1

  equation 1 becomes  I_D= V1/1000

  I_D = 10mA ( eqB)

Mark  I_D=10mA  and  V_D=10V  on the characteristic curve and join these two points.

Fid 3 Q-point at the intersection of Load Line and Characteristic Curve

        

Diode Internal Resistances

Diode Internal Resistances:

Resistance is the ratio of voltage and  current. For a resistor this value is constant. Because it's voltage current relationship is linear or when we plot IV curve of a resistor, there is a straight line. However, when we look at a diode's IV curve, we notice that it is not linear.

Every semiconductor device has some resistance due to which it is unable to exhibit ideal characteristics. Diode also has two internal resistances i.e. DC and AC resistances.

DC or static resistance:

Depends on shape of the curve
It Depends on the current through diode the higher the current the lower will be the resistance

AC or dynamic or incremental resistance:

It is small signal resistance. this resistance varies inversely with current.
      
 
 
          
 
   

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PN Junction Diodes - Forward, Reversed Biased & Unbiased Diodes

What is an Ordinary Silicon Diode:

It is simply a pn junction. One side of a diode is doped with a p-type impurity and the other with an n-type impurity. The symbol looks like an arrow that shows the direction of the current in the circuit. i.e. the current flows from a positive to negative direction.

fig 1 Diode Circuit Symbol

Unbiased Diode:

It is nothing but a piece of a semiconductor material doped with p-type material on one side and n-type material on the other side. The junction is the border where both regions meet. There are many free electrons in the n-type region because of doping and in the same manner, the p-region has many holes because of doping. The holes near the junction diffuse across the junction into the n-type region. Whereas, electrons near the junction diffuse across the junction in the p-type region. The n-type region loses some free electrons because of diffusion across the junction. Due to this, a layer of positive charges is formed near the junction. Similarly, a p-type junction loses some holes because of diffusion across the junction. Due to this, a layer of negative charges formed across the junction.

 

Fig 3 PN-Junction before diffusion

Figure 4 PN-Junction After Diffusion

Figure 4 shows the pn-junction in equilibrium. At the equilibrium the negative charges in the depletion region (due to potential barrier see figure 4) repels the remaining charges for further diffusion, similarly, positive charges or holes oppose further diffusion. So at equilibrium, the depletion region acts as a potential barrier and hence no further movement of charges.

MODES OF OPERATION Of PN JUNCTION:

When we apply forward bias, the equilibrium disturbs and the potential barriers reduce and start conducting the current if the applied potential is greater than the barrier potential.

Forward Bias: 

Applying positive potential at the p-type and negative potential at the n-type.

Fig 5 Forward Bias

The application of positive potential V will Pressure electrons in the n-type material and holes in the p-type material to recombine with the ions near the boundary and reduce the width of the depletion region. The width of the depletion region decreases as the applied bias voltage increase. The small potential barrier will never diminish, so there is a potential drop called CUT IN VOLTAGE.
For the silicon diode, it is 0.7V. For conduction V>0.7V.

Reverse Bias:

Applying negative potential at the p-type and positive potential at the n-type.

For an ideal diode, in reverse bias condition, no current flows through it, and it behaves as an ideal insulator. But this is not true for a practical diode. The current that flows under the reverse bias condition is called REVERSE SATURATION CURRENT.

Fig 6 Reverse bias

DIODE CURRENT UNDER FORWARDING BIAS:

 
I=Current through diode
=Reverse saturation current
V=Applied voltage
=material parameter
=Voltage equivalent to temperature 

 

PROS & CONS:

• It is a non-linear device because its IV characteristic curve is non-linear.

• It can conduct current only in 1 direction, and this quality makes them helpful in rectification applications

• The higher the junction temperature greater will be the saturation current

Why does a diode be a non-linear device?

An electronic device is linear if the current flowing through it is directly proportional to its voltage. As we observed the current and voltage behaviour of a diode, it does not act like a linear device.

 If you graph current versus voltage, you will get an exponential curve.

What are the uses of diodes? (Applications of diodes):

As a beginner, you might not aware of the usefulness of this tiny, low-cost semiconductor device. It is a passive two-terminal device. The characteristic of the diode is to flow current only in one direction. There are numerous applications of this tiny device, each of which utilizes the characteristic property of flowing current in one direction.  I have written a series of articles on each of the following applications. 

1. Diode as a rectifier
1. Half wave rectifier
2. Full-wave rectifier
2. Diode as a switch
3. Diode as a limiter or clipper
4. Diode as a clamper
5. Diode as a voltage multiplier
1. Half-wave doubler 
2. Full-wave doubler
3. Tripler
6. Diode as a peak detector
7. Reverse protection Diode
8. Freewheeling or flyback diode

Analysis of a Simple Diode Circuit Using Multisim