Correct Answer

Concept & PrincipleElectricalElectronics

Option A

Current

Quick Summary: A Bipolar Junction Transistor (BJT) is defined as a current-operated device because the output collector current ($I_C$) is controlled by the input base current ($I_B$). The relationship between these currents is governed by the common-emitter current gain, denoted as $\beta$ or $h_{fe}$.

💡 Explanation

A Bipolar Junction Transistor (BJT) is defined as a current-operated device because the output collector current ( $I_C$ ) is controlled by the input base current ( $I_B$ ). The relationship between these currents is governed by the common-emitter current gain, denoted as $\beta$ or $h_{fe}$ .

🔢 Key Formulas

$I_C = \beta I_B$ — Collector current as a function of base current and gain

$I_E = I_B + I_C$ — Conservation of current in a BJT

⚙️ Working Principle

In a BJT, the movement of charge carriers across the base-emitter junction establishes the base current. This small input current modulates the wider depletion region at the collector-base junction, allowing a significantly larger flow of electrons (or holes) from emitter to collector. The fundamental operation is summarized by the gain equation $I_C = \beta I_B$ .

📌 Key Points

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The BJT operates based on the minority carrier injection principle.
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BJT is current-controlled, whereas Field Effect Transistors (FETs) are voltage-controlled devices.
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The base region is intentionally kept very thin and lightly doped to facilitate carrier transition.

✅ Advantages

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High current gain
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Fast switching speeds in small-signal applications

❌ Disadvantages / Limitations

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Higher power consumption compared to MOSFETs
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Subject to thermal runaway if not biased correctly

🛠️ Applications / Uses

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Signal amplification in audio circuits
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Digital switching in logic gates

🔄 Comparison Table

Feature	BJT	FET/MOSFET
Control Parameter	Current	Voltage

📄 Additional Information

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The term 'current-operated' refers to the fact that the output collector current is a direct function of the input base current.
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Option B is incorrect because transistors categorized as voltage-operated devices are typically FETs or MOSFETs, where the gate-source voltage ( $V_{GS}$ ) controls the drain-source current ( $I_D$ ).

📊 Diagram / Illustration

✅

A is correct — The Bipolar Junction Transistor is classified as a current-operated device because the output current is fundamentally dependent on the input base current.

Core Concepts Used

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Bipolar Junction Transistor Current Control Mechanism Carrier Diffusion

💡 EXAM TIP

Always remember: BJT = Current controlled ( $I_B$ controls $I_C$ ), FET = Voltage controlled ( $V_{GS}$ controls $I_D$ ).

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