Forward & Reverse bias and Rectifier

Forward bias:
An external voltage applied to a PN junction
 is called bias. If, for example, a battery is 
used to supply bias to a PN junction and is
connected so that its voltage opposes the 
junction field , it will reduce the junction 
barrier and ,therefore, aid current flow 
through the junction. This type of bias is 
known as forward bias , and it causes the 
junction to offer only minimum resistance  to the
flow of current.








  

Reverse bias: 

If the battery mentioned earlier is connected
across the junction so that its voltage 
aids the junction, it will increase the junction 
barrier and thereby offer a high resistance
to the current flow through the junction. This 
type of bias is known as reverse bias.

Voltage–Current Characteristic Of PN Junction:

 For the Forward-and reverse-bias region:

ID=IS(eKVD/TK-1)

Where ,  IS=reveres saturation current.

          k=11,600/h with h=1 for Ge  and h=2 for Si for relatively low levels of diode current and h=1 for Ge and Si for higher levels of diode current (in the rapidly increasing section of the curve)

TK=TC+273
  

Rectifier:

Definition : The circuits which convert an ac voltage into dc voltage is called rectifier.

The classification of rectifier is given bellow :

Rectifier are two types.

1. Half Wave Rectifier

2. Full Wave Rectifier

Half – wave rectifier:

 According to the Fig: the a.c voltage across the secondary winding AB changes polarities after every 

 half-cycle. During the positive half-cycle of input a.c. voltage end A becomes positive w.r.t. end B.

 

         This makes the diode forward biased and 

         hence it conducts current. During the negative

         half-cycle, end A is negative w.r.t. end B. Under 

         this condition, the diode is reverse biased and 

          it conducts no current. Therefore, current flows  

          through the diode during positive half-cycle of

           input a.c. voltage only ; it is blocked during the

 negative half-cycle. In this way , current flows through load R always in the same direction. Hence

 d.c. output is obtained across R.

•

Full-Wave Rectifier: In full-wave rectification, current flows through the load in the same direction for both half-cycles of input a.c. voltage.

 

 












This can be achieved with two diodes working alternately. For the positive half-cycles of input

voltage , one diode supplies current to the load and for the negative half-cycles, the other diode

does so; current being always in the same direction through the load.

Bridge rectifier:
According to the Fig:  A bridge rectifier requires four diode instead of two , but avoids the need for s center-tapped transformer . During the positive half-cycle of the secondary voltage , diodes D2 and D4 are conducting and diodes D1 and D3 are nonconducting . Therefore , current flows through the secondary winding , diode D2 , load resister R and diode D4.  during negative half-cycles of the secondary voltage, diodes D1 and D3 conduct , and the diode D2 and D4 do not conduct . The current therefore flows through the secondary winding , diode D1 , load resister R, and diode D3 . In both cases , the current passes through the load resister in the same direction . Therefore , a fluctuating , unidirectional voltage is developed across the load.