7ème leçon - 2ème partie - Étude du Transistor Bipolaire en Régime Saturé ou Bloqué

Introduction

This tutorial focuses on understanding the operation of bipolar junction transistors (BJTs) in saturation and cutoff modes, similar to how an electrical switch functions. It highlights the key characteristics and calculations necessary for working with BJTs in electronic circuits.

Step 1: Understanding Transistor Operation Modes

• Saturation Mode: The transistor acts as a closed switch, allowing current to flow through the circuit.
• Cutoff Mode: The transistor acts as an open switch, preventing current from flowing.

Key Points

• When in saturation, a small voltage (VCE SAT) remains across the transistor, similar to contact resistance in a real switch.
• In cutoff, a small residual collector current (ICB0) flows, akin to a switch that is not fully open.

Step 2: Important Characteristics of BJTs

• VCE SAT: The minimum voltage across the collector-emitter junction when the transistor is in saturation. It typically ranges from 0.15V to 3V, depending on the transistor type and current.
• ICB0: The minimum collector current when the emitter current is zero, usually in the microampere range. This is influenced by temperature.

Practical Tip

Both VCE SAT and ICB0 can indicate potential defects in the circuit. Always check these values to ensure proper transistor function.

Step 3: Power Dissipation in the Collector

• The maximum power that can be dissipated by the transistor is specified by the manufacturer.
• This information can be represented graphically in the output characteristic curves (quadrant 1).

Practical Advice

Refer to the manufacturer's datasheet for power ratings and ensure your circuit does not exceed these limits to avoid damage.

Step 4: Using Ohm's Law in Circuit Design

To determine the appropriate resistor (R) to use in the collector circuit:

• Use the formula:

  U = R * I
  

  Where:
  • U = voltage in volts
  • R = resistance in ohms
  • I = current in amperes

Steps to Calculate Resistor Value

1. Identify the desired output voltage (U).
2. Determine the expected collector current (I).
3. Rearrange the formula to find R:

   R = U / I
   

Step 5: Circuit Configuration

• Series Configuration: Connect the load (e.g., a lamp) in series with the transistor. This setup ensures that the current through the load is controlled by the transistor's operation.

Visual Reference

Refer to Figure 14 in the video for examples of circuit configurations.

Conclusion

In summary, understanding the operation of BJTs in saturation and cutoff modes is essential for effective circuit design. Familiarize yourself with key characteristics such as VCE SAT and ICB0, and use Ohm's Law to calculate necessary resistor values for your circuits. As you practice, always refer to manufacturer specifications to ensure safe and effective use of transistors in your electronic projects. For further exploration, consider watching related videos or tutorials on transistor circuits.