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DIODE :

1)A diode is a one-way valve for electric current.

 

2)Diodes are a basic building block of all electronics and are used to control the direction of current flowing in circuits

 

 

 

Basic operation

 

Ideally it conducts current in only one direction

 

 

and acts like an open in the opposite direction

 

 

Operating Conditions

 

• No Bias

• Forward Bias

• Reverse Bias

No Bias Condition

 

            No external voltage is applied: V_D = 0V and no current is

            flowing I_D = 0A.

 

Forward Bias Condition:

 

The Forward bias voltage required for a :

• Silicon diode V_T = 0.7V (proximately ) 

• Germanium diode V_T = 0.3V (proximately )

 

 

Reverse Bias Condition:

 

 

Characteristics of an ideal diode:

Conduction Region

 

 

Look at the vertical line!
In the conduction region, ideally
            • the voltage across the diode is 0V,
            • the current is infinity ,
            • the forward resistance (R_F) is defined as R_F = V_F/I_F,
            • the diode acts like a short.

 

Characteristics of an ideal diode:

Non-Conduction Region

 

Look at the horizontal line!
In the non-conduction region, ideally
            • all of the voltage is across the diode,
            • the current is 0A,
            • the reverse resistance (R_R) is defined as R_R = V_R/I_R,
            • the diode acts like open.

 

Equivalent Circuits:

Approximate model

Simplified Model

Ideal Model

Example:

Determine the V_f and If for the diode in Figure 1.2 for each of the diode model. Also find the voltage across the limiting resistor in each case. Asume r_d=10 ohm and determined value of forward current.

 

 

Ideal diode	Practical Model	Complete Model
Vf=0V If =10V/1k    =10mA VRlimit=(10m)(1k)          =10V	Vf=0.7V If=(10V – 0.7V)/1k   =9.3/1k   = 9.3mA VRlimit=(9.3m)(1k)          =9.3V	If=(10V – 0.7V)/1k+10   =9.3/1010   = 9.21mA Vf=0.7V+Ifrd     =0.7 +(9.21m)(10)     = 792mV VRlimit=(If)(Rlimit)          =9.21m(1k)          =9.21V