Biasing Method : Base Resistor Method

Transistor Biasing

• The transistor input circuit always be in forward biased and output circuit remains in reverse biased during all part the AC signal. This is called as Transistor Biasing.

Base resistor method

• As a high value of resistor is connected in the base circuit, it is called as base resistor method. 
• A high value of resistor R_B is connected between base terminal of transistor and positive end of supply in the case of NPN transistor.
• Similar, a resistor R_B is connected between base and negative end of supply in case of PNP transistor. 
• The zero signal ( without AC signal ) base current is provided by the V_CC supply.  
• As the base is positive with respect to emitter, base – emitter junction is forward biased. 
• The required value of base current is determined by selecting proper value of base resistor R_B.

Let us I_C is the required zero signal collector current

I_C = βI_B

Applying KVL ( Kirchoff’s voltage law ) to closed circuit PQIENP

V_CC = I_BR_B + V_BE

I_BR_B = V_CC – V_BE

RB = VCC – VBE / IB

The value of base resistor R_B can be calculated if we know 

• V_CC and I_B
• V_BE from transistor datasheet

Why it is also called as Fixed bias method?

• As the supply voltage V_CC is fixed and I_B is selected some specific value, the R_B is find out easily therefore this method is also called as Fixed bias method.
• R_B = V_CC / I_B ( If value of V_BE is neglected )

Stability factor

The stability factor

S = ( 1 + β ) / [ 1 – β ( dI_B /dI_C )]

As the base current I_B does not depend on I_C, the dI_B / dI_C = 0

S = 1 + β

•  It indicates that the collector current changes ( 1 + β )  times as many as change in I_CO ( Collector leakage current ) therefore the base resistor bias has poor thermal stability.

Advantages

• Only one base resistor R_B requires
• Calculation is very simple for base resistor
• As no resistor is connected between base emitter circuit, no loading of the source by biasing circuit.

Disadvantages

• Poor stabilization because the collector current increases due to rise in the temperature or transistor may be replaced by another transistor
• Higher possibility of thermal runaway due to high value of stability factor
• If value of β = 90, the stability factor

       S = 1 + β

          = 1 + 90

          = 91

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