Question:medium
For a fixed bias circuit the stability factor is
For a fixed bias circuit the stability factor is
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Lower stability factor ⇒ better circuit stability.
Updated On: Jul 2, 2026
- \(\beta + 1\)
- \(\beta - 1\)
- \(\frac{1}{\beta - 1}\)
- \(1 + \beta\)
Show Solution
The Correct Option is D
Solution and Explanation
Step 1: Use the general stability factor formula.
The general expression for the stability factor is $S = \dfrac{1+\beta}{1 - \beta \dfrac{dI_B}{dI_C}}$. This comes from differentiating the KCL equation $I_C = \beta I_B + (1+\beta)I_{CO}$ with respect to $I_{CO}$.
Step 2: Evaluate $dI_B/dI_C$ for fixed bias.
In a fixed-bias circuit, the base current is $I_B = (V_{CC}-V_{BE})/R_B$, which depends only on fixed voltages and $R_B$. It does NOT change when $I_C$ changes. Therefore $dI_B/dI_C = 0$.
Step 3: Substitute to get the stability factor.
$S = \dfrac{1+\beta}{1 - \beta \times 0} = 1+\beta$. A large $\beta$ gives a large $S$, meaning poor thermal stability. \[ \boxed{1+\beta} \]
The general expression for the stability factor is $S = \dfrac{1+\beta}{1 - \beta \dfrac{dI_B}{dI_C}}$. This comes from differentiating the KCL equation $I_C = \beta I_B + (1+\beta)I_{CO}$ with respect to $I_{CO}$.
Step 2: Evaluate $dI_B/dI_C$ for fixed bias.
In a fixed-bias circuit, the base current is $I_B = (V_{CC}-V_{BE})/R_B$, which depends only on fixed voltages and $R_B$. It does NOT change when $I_C$ changes. Therefore $dI_B/dI_C = 0$.
Step 3: Substitute to get the stability factor.
$S = \dfrac{1+\beta}{1 - \beta \times 0} = 1+\beta$. A large $\beta$ gives a large $S$, meaning poor thermal stability. \[ \boxed{1+\beta} \]
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