1 Answer
BJTs (Bipolar Junction Transistors) are key components in electronics. They work by controlling the flow of current between two terminals using a small current. The main thing to remember about BJTs is that they are current-controlled devices.
Here are some important rules to remember when working with BJTs:
### 1. **BJT has Three Terminals:**
- **Emitter (E)**: The terminal where the current flows into or out of the transistor.
- **Base (B)**: The terminal that controls the current between the emitter and collector. A small current at the base controls the larger current between emitter and collector.
- **Collector (C)**: The terminal where the current flows out of or into the transistor.
### 2. **Two Types of BJTs:**
- **NPN (Negative-Positive-Negative)**: The majority carriers are electrons. In this type, current flows from the collector to the emitter when the base is positive.
- **PNP (Positive-Negative-Positive)**: The majority carriers are holes. Here, current flows from the emitter to the collector when the base is negative.
### 3. **Biasing:**
- To make the transistor work properly, it needs to be biased correctly.
- **Active region**: The transistor is βon,β and current can flow.
- **Cut-off region**: The transistor is βoff,β and no current flows.
- **Saturation region**: The transistor is fully on, and maximum current flows from the collector to the emitter.
### 4. **Base-Emitter Junction:**
- This junction behaves like a **diode**.
- In an NPN transistor, the base-emitter junction must be forward biased (positive voltage on the base relative to the emitter).
- In a PNP transistor, the base-emitter junction must be reverse biased (negative voltage on the base relative to the emitter).
### 5. **Collector-Base Junction:**
- This junction must be **reverse-biased** for normal transistor operation.
### 6. **Current Flow and Gain:**
- The current at the **base** controls the larger current flowing between the **collector** and **emitter**.
- The ratio of the collector current to the base current is called **current gain (Ξ² or hFE)**. For example, if Ξ² = 100, it means that for every 1 mA of base current, the collector current will be 100 mA.
### 7. **Switching Behavior:**
- **On state (Saturation)**: When the transistor is fully on, both junctions (base-emitter and base-collector) are forward-biased. The current flows freely from collector to emitter.
- **Off state (Cut-off)**: When the transistor is off, both junctions are reverse-biased, and no current flows.
### 8. **Working Principle:**
- In an NPN transistor, when a small current flows into the base, it allows a much larger current to flow from the collector to the emitter.
- In a PNP transistor, a small current leaving the base allows a larger current to flow from the emitter to the collector.
### 9. **Voltage and Current Relations (for NPN):**
- **V_BE** (Base-Emitter Voltage): For NPN, this is typically around 0.7V for the transistor to turn on.
- **V_CE** (Collector-Emitter Voltage): In the active region, the transistor will work if **V_CE** is above a certain threshold (usually greater than 0.2V).
- **I_C = Ξ² * I_B**: The collector current is equal to the current gain (Ξ²) multiplied by the base current.
### 10. **Thermal Runaway:**
- BJTs are sensitive to temperature. If the temperature increases, the current through the transistor increases, which can cause even higher temperatures, leading to thermal runaway. Proper heat sinking is necessary in high-power applications.
### Summary:
- **NPN transistor**: Current flows from **collector** to **emitter** when base is positive.
- **PNP transistor**: Current flows from **emitter** to **collector** when base is negative.
- Transistors need to be **biased correctly** to work as switches or amplifiers.
- **Ξ²** (current gain) defines how much current is amplified by the transistor.
- Proper biasing and understanding of current and voltage conditions are crucial for transistor operation.
BJTs are widely used in amplifiers, switches, and other circuits where amplification or switching is required.
Here are some important rules to remember when working with BJTs:
### 1. **BJT has Three Terminals:**
- **Emitter (E)**: The terminal where the current flows into or out of the transistor.
- **Base (B)**: The terminal that controls the current between the emitter and collector. A small current at the base controls the larger current between emitter and collector.
- **Collector (C)**: The terminal where the current flows out of or into the transistor.
### 2. **Two Types of BJTs:**
- **NPN (Negative-Positive-Negative)**: The majority carriers are electrons. In this type, current flows from the collector to the emitter when the base is positive.
- **PNP (Positive-Negative-Positive)**: The majority carriers are holes. Here, current flows from the emitter to the collector when the base is negative.
### 3. **Biasing:**
- To make the transistor work properly, it needs to be biased correctly.
- **Active region**: The transistor is βon,β and current can flow.
- **Cut-off region**: The transistor is βoff,β and no current flows.
- **Saturation region**: The transistor is fully on, and maximum current flows from the collector to the emitter.
### 4. **Base-Emitter Junction:**
- This junction behaves like a **diode**.
- In an NPN transistor, the base-emitter junction must be forward biased (positive voltage on the base relative to the emitter).
- In a PNP transistor, the base-emitter junction must be reverse biased (negative voltage on the base relative to the emitter).
### 5. **Collector-Base Junction:**
- This junction must be **reverse-biased** for normal transistor operation.
### 6. **Current Flow and Gain:**
- The current at the **base** controls the larger current flowing between the **collector** and **emitter**.
- The ratio of the collector current to the base current is called **current gain (Ξ² or hFE)**. For example, if Ξ² = 100, it means that for every 1 mA of base current, the collector current will be 100 mA.
### 7. **Switching Behavior:**
- **On state (Saturation)**: When the transistor is fully on, both junctions (base-emitter and base-collector) are forward-biased. The current flows freely from collector to emitter.
- **Off state (Cut-off)**: When the transistor is off, both junctions are reverse-biased, and no current flows.
### 8. **Working Principle:**
- In an NPN transistor, when a small current flows into the base, it allows a much larger current to flow from the collector to the emitter.
- In a PNP transistor, a small current leaving the base allows a larger current to flow from the emitter to the collector.
### 9. **Voltage and Current Relations (for NPN):**
- **V_BE** (Base-Emitter Voltage): For NPN, this is typically around 0.7V for the transistor to turn on.
- **V_CE** (Collector-Emitter Voltage): In the active region, the transistor will work if **V_CE** is above a certain threshold (usually greater than 0.2V).
- **I_C = Ξ² * I_B**: The collector current is equal to the current gain (Ξ²) multiplied by the base current.
### 10. **Thermal Runaway:**
- BJTs are sensitive to temperature. If the temperature increases, the current through the transistor increases, which can cause even higher temperatures, leading to thermal runaway. Proper heat sinking is necessary in high-power applications.
### Summary:
- **NPN transistor**: Current flows from **collector** to **emitter** when base is positive.
- **PNP transistor**: Current flows from **emitter** to **collector** when base is negative.
- Transistors need to be **biased correctly** to work as switches or amplifiers.
- **Ξ²** (current gain) defines how much current is amplified by the transistor.
- Proper biasing and understanding of current and voltage conditions are crucial for transistor operation.
BJTs are widely used in amplifiers, switches, and other circuits where amplification or switching is required.