Bipolar Junction Transistor | TechAmbitionX | Notes By BILRED
🔹 1. What is a Transistor (BJT)?
BJT (Bipolar Junction Transistor) is a three-layer, three-terminal semiconductor device that controls a large current using a small one. Simply put — a small base current controls a much larger collector current.
Terminals:
- Emitter (E): Emits charge carriers.
- Base (B): Thin, lightly doped control layer.
- Collector (C): Collects charge carriers.
🔹 2. Types of BJTs
There are two main types based on how the layers are arranged:
- NPN Transistor → Current flows from Collector → Emitter (majority carriers = electrons)
- PNP Transistor → Current flows from Emitter → Collector (majority carriers = holes)
🔹 3. Basic Principle
When a small current is applied to the base-emitter junction, it allows a much larger current to flow between the collector and emitter.
💡 Think of the base as a “gatekeeper” — a small effort here controls a big flow elsewhere.
🔹 4. Current Relationships
The total current relationship in a transistor is:
I_E = I_B + I_C
Where:
- IE = Emitter current
- IB = Base current
- IC = Collector current
🔹 5. Current Gain Formulas
| Parameter | Symbol | Formula | Meaning |
|---|---|---|---|
| Common Base Current Gain | α (alpha) | α = IC / IE | Typically 0.95–0.99 |
| Common Emitter Current Gain | β (beta) | β = IC / IB | Typically 20–200 |
| Relation between α and β | — | β = α / (1−α) | or α = β / (β+1) |
🔹 6. Modes of Operation
BJTs can operate in different regions depending on how the two junctions are biased:
| Region | Base–Emitter | Base–Collector | Use |
|---|---|---|---|
| Active | Forward | Reverse | Amplifier |
| Saturation | Forward | Forward | Switch ON |
| Cut-off | Reverse | Reverse | Switch OFF |
| Inverse Active | Reverse | Forward | Rarely used |
🔹 7. Common Configurations
Depending on which terminal is common between input and output:
- Common Base (CB): Voltage gain high, current gain low.
- Common Emitter (CE): Most widely used — both voltage and current gain.
- Common Collector (CC): High input impedance, used as buffer.
🔹 8. Key Formulas Recap
I_E = I_B + I_C
α = I_C / I_E
β = I_C / I_B
β = α / (1 - α)
I_C = β * I_B
I_E = (β + 1) * I_B
🔹 9. Applications of BJTs
- Amplifiers (audio, RF, signal processing)
- Switching (digital circuits, logic gates)
- Oscillators and waveform generators
- Current regulation and control circuits
💬 Tip: Remember, BJT is current-controlled while FET is voltage-controlled.
🔹 10. Quick Summary
BJT = Bipolar (uses both electrons & holes). Controlled by base current. Used for amplification & switching. NPN conducts when base is positive; PNP conducts when base is negative.
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