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.
 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_{B}R_{B} + V_{BE}
I_{B}R_{B} = V_{CC} – V_{BE}
R_{B}
= V_{CC} – V_{BE} / I_{B}

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

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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