what is stability factor
Answers
The extent to which the collector current IC is stabilized with varying ICO is measured by a stability factor S. It is defined as the rate of change of collector current IC with respect to the collector base leakage current ICO, keeping both the current IB and the current gain β constant.
S=∂IC∂ICO=dICdICO=ΔICΔICO
The collector current for a CE amplifier is given by
IC=βIB+(1+β)ICO
Differentiating the above equation with respect to IC, we get
I =β(dIB)(dIC)+(1+β)(dICO)(dIC)
Therefore, 1−β(dIB)(dIC)=1+βS
S = ((1+β))((1−β(dIB)(dIC)
From this equation it is clear that this factor S should be as small as possible to have better thermal stability.
Stability factor S’ and S”:
The Stability factor S’ is defined as the rate of change of IC with VBE, keeping ICO and β constant.
S’=∂IC∂VBE=ΔICΔVBE
The stability factor S” is defined as the rate of change of IC with respect to β, keeping ICO and VBE constant.
S’=∂IC∂β=ΔICΔβ
The small value of stability factor indicates good bias stability whereas large value of stability factor indicates poor bias stability. Ideal value of stability factor is zero.
For fixed bias stability factor reduces to
S = 1 + β
Since β is large quantity, this is a very poor bias stable circuit.
For collector to base bias stability factor is
S =((1+β))(1+β(Rc(Rc+RB))
As can be seen, this value of the stability factor is smaller than the value obtained by fixed bias circuit.
For self-bias,
S =(1+β)(1+RBRE)(1+β+RBRE)
As can be seen, the value of S is equal to one if the ratio RB/RE is very small as compared to 1. As this ratio becomes comparable to unity, and beyond towards infinity, the value of the stability factor goes on increasing till S = 1 + β. This improvement is the stability up to a factor equal to 1 is achieved at the cost of power dissipation.