Consider a fixed bias circuit using a NPN BJT transistor in CE configuration along with base resistance Rb, collector resistance Rc and supply voltage Vcc. If β is the current gain of the BJT, then the stability factor of the fixed bias circuit is:

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Answer 1

Concept

Stability factor (S): The stability factor is determined by the rate of change of the collector current IC with respect to the ICBO.

\(S= \frac{\partial I_C}{\partial I_{CBO}}\)

Stability factor for a fixed bias circuit:

F1 Madhuri Engineering 23.02.2023 D12

\(I_C = β I_B \space +\space (1+β)I_{CBO}\)

\(S= \frac{\partial {(β I_B \space +\space (1+β)I_{CBO})}}{\partial I_{CBO}}\)

Since IC is independent of ICBO in a fixed bias (no feedback), its derivative is 0.

\(S= 0+(1+β)\times 1\)

S = (1 + β) , where β current gain

S α β

Therefore, the stability factor of the fixed bias circuit is directly proportional to β.