Biasing Method : Voltage Divider Method

• This is most popular method of biasing and stabilization. 
• There are two resistance R1 and R2 is connected across supply voltage V_CC to provide biasing. The emitter resistance R_E provides stabilization.
• There are two resistances R1 and R2 provides voltage divider in this method therefore it is called as voltage divider method. 
• The voltage drop across resistance R₂ forward biases base – emitter junction resulting base current flows. 
• This will result in zero signal collector current flows.

Mathematical analysis

• Let us consider that the current I₁ flows through resistance R₁. 
• As the base current I_B is very small, therefore we consider that the current flows through resistance R₂ is also I₁.

Collector current I_C

     I₁ = V_CC / ( R₁ + R₂ )

Voltage drop across resistance R₂ = R₂I₁

     V₂ = [ V_CC / ( R₁ + R₂ ) ] R₂

Apply KVL to base circuit

V₂ = Voltage drop across Base emitter junction + voltage drop across 

         resistance R_E

     = V_BE + V_E

     = V_BE + I_ER_E

As base current is very small I_E ≈ I_C

V₂ = V_BE + I_C R_E ................ ( 1 )

IC = V2 – VBE / RE..... ( 2 )

 

• The value of collector current does not depend upon current amplification factor β. 
• As the collector current I_C depends upon V_BE but V₂ >> V_BE therefore I_C is practically does not depend upon V_BE. 
• We can say that the collector current I_C is independent parameter therefore it provides good stabilization.

Collector to Emitter voltage

Applying KVL to collector circuit

     V_CC = I_CR_C + V_CE + I_ER_E

     AS I_E ≈ I_C

     V_CC = I_CR_C + V_CE + I_CR_E

             = I_C ( R_C + R_E ) + V_CE

     V_CE = V_CC – I_C ( R_C + R_E )

Stabilization

• The stabilization is provided by emitter resistance R_E. Let us consider the equation ( 1 ) 

        V₂ = V_BE + I_C R_E

• As the collector current increases with increase in the temperature therefore the voltage drop across resistance R_E also increases. 
• As the voltage drop across resistance R2 does not depends upon I_C, the voltage across base to emitter ( V_BE ) decreases. 
• As the base to emitter voltage decrease, the base current also decreases and this will result in collector current I_C restore to its original value.

       Voltage drop across R2 = [ V_CC / ( R1 + R2 ) ] R2

You may also like :

Minimum distance between Transmission line conductor and Ground

Function of Hollow Conductor

What is Isolation Transformer?

Class A Commutation - Current Commutation , Resonance Commutation