1 February 2020

Biasing Method : Base Resistor Method

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 RB is connected between base terminal of transistor and positive end of supply in the case of NPN transistor.
  • Similar, a resistor RB 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 VCC 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 RB.

base-resistor-bias-method


Let us IC is the required zero signal collector current
IC = βIB
Applying KVL ( Kirchoff’s voltage law ) to closed circuit PQIENP
VCC = IBRB + VBE
IBRB = VCC – VBE
RB = VCC – VBE / IB

The value of base resistor RB can be calculated if we know 
  • VCC and IB
  • VBE from transistor datasheet
Why it is also called as Fixed bias method?
  • As the supply voltage VCC is fixed and IB is selected some specific value, the RB is find out easily therefore this method is also called as Fixed bias method.
  • RB = VCC / IB ( If value of VBE is neglected )
Stability factor
The stability factor
S = ( 1 + β ) / [ 1 – β ( dIB /dIC )]
As the base current IB does not depend on IC, the dIB / dIC = 0
    S = 1 + β
  •  It indicates that the collector current changes ( 1 + β )  times as many as change in ICO ( Collector leakage current ) therefore the base resistor bias has poor thermal stability.

Advantages
  • Only one base resistor RB 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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