Lecture Notes: BJT Transistors as Electronic Switches and Inverters

Introduction to BJT Transistors

• NPN Transistor
  • Symbol with Base, Collector, Emitter
  • Beta (β): Ratio of collector current (I_C) to base current (I_B)
  • Small base current controls a larger collector current
  • Emitter Current (I_E): Sum of base current and collector current

Transistor as an Electronic Switch

• Example Circuit
  • Components: Chemical battery, switch (S1), resistors (R1, R2), LED, multimeter
  • Circuit Configuration:
    • R1 (current limiting) connected to the base
    • R2 and a green LED connected to the collector
  • Function:
    • Closing the switch allows a small current through R1 to the base
    • Activates larger current from collector to emitter, lighting the LED

Circuit Analysis

• Calculating Currents and Voltages
  • Adjust R1 to 500 kΩ, β to 200
  • Use Ohm’s Law (V = IR) to calculate currents
  • Kirchhoff’s Voltage Law: Sum of voltages in a loop = 0
  • Base Current (I_B): 0.0108 mA
  • Collector Current (I_C): 2.16 mA
  • Voltage across components calculated using potential differences

Voltage Calculations

• Voltage at Multimeter (V_CE):
  • Voltage drop across LED and R2
  • Typical green LED forward voltage: 2 to 2.4 V
  • Calculated V_CE: 1.64 V

Transistor as an Inverter

• Inverter Circuit Setup
  • R1 (10 kΩ) and R2 (1 kΩ) resistors
  • Input and Output voltage relationships
• Function:
  • High input voltage (5 V) leads to low output voltage
  • Low input voltage (0 V) leads to high output voltage (9 V)
  • Saturation: V_CE approaches 0 V driving the output low
• Inverter Behavior:
  • High input = Low output
  • Low input = High output

Conclusion

• BJT transistors can function as switches and inverters in electronic circuits
• Useful in controlling currents and driving digital circuit behavior

Additional Resources

• Mention of more videos and resources available in the description section.