## 01/02/2020

### 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.

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.

• 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.
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