The graph of current indicates that the current gain

Question

Examoogle · Accepted Answer

Varies slightly

Explanation: In a Bipolar Junction Transistor (BJT), the DC current gain, denoted as $\beta_{DC}$ or $h_{FE}$, is defined as the ratio of collector current ($I_C$) to base current ($I_B$). Because the physical processes of recombination and transport efficiency depend on the operating point, the current gain is not a perfectly linear constant but varies slightly across different operating regions.

Working Principle: The gain $\beta$ is primarily determined by the base transport factor and emitter injection efficiency. As the collector-emitter voltage ($V_{CE}$) increases, the effective base width decreases (Early Effect), which slightly increases the collector current for a fixed base current. Consequently, the ratio $\frac{I_C}{I_B}$ exhibits minor sensitivity to changes in the bias point.

Key Formulas: $\beta = \frac{I_C}{I_B}$ — Definition of DC current gain; $\alpha = \frac{I_C}{I_E}$ — Common base current gain

Key Points: The parameter $\beta$ is temperature-sensitive and dependent on the bias current level.; BJT datasheet specifications typically provide a range for $\beta$ (min to max) due to manufacturing tolerances and operating dependency.; The Early Effect contributes to the slight variation in collector current relative to $V_{CE}$.

Advantages: Provides current amplification for signal processing; Essential for transistor biasing and switching stability

Disadvantages: Sensitivity to thermal variations; Non-linear characteristics across wide ranges of operation

Applications: Common Emitter Amplifier design; Switching circuits and logic gates; Current mirrors

Additional Info: While often approximated as constant for simple circuit analysis, the 'small signal' current gain $\beta_{ac} = \frac{\Delta I_C}{\Delta I_B}$ and 'DC' gain $\beta_{dc} = \frac{I_C}{I_B}$ differ slightly.. Option D is an algebraic definition, but Option B accurately describes the physical behavior of the gain parameter on a characteristic curve.

Answer: B is correct — The current gain is not a fixed constant and typically exhibits slight variations based on the transistor's collector-emitter voltage ($V_{CE}$) and the magnitude of the collector current.

Exam Tip: Always remember that in practical circuit design, $\beta$ is treated as a variable range rather than a single fixed number to account for manufacturing spreads and thermal drift.

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