Experiment: Using a Voltage Divider for Transistor Biasing

Objective

To understand how a voltage divider provides a stable bias voltage to the base of a transistor, allowing it to operate in its active region for amplification purposes.

Materials

• NPN Transistor (e.g., 2N2222 or BC547)
• Two resistors for the voltage divider (e.g., 10kΩ and 4.7kΩ)
• Collector resistor (e.g., 1kΩ)
• Power supply (e.g., 9V battery)
• Breadboard
• Jumper wires
• Multimeter

Circuit Diagram

Connect the circuit as shown below, with a voltage divider providing the base voltage to bias the transistor.

Theory

In a transistor amplifier circuit, biasing is used to set the operating point of the transistor. A voltage divider can provide a stable base voltage, ensuring the transistor remains in its active region. The base voltage V_B can be calculated as:

V_B = V_CC * (R2 / (R1 + R2))

where V_CC is the supply voltage, and R1 and R2 are the resistors in the divider.

Procedure

1. Choose R1 and R2 to achieve the desired base voltage V_B for the transistor. For example, if V_CC is 9V and you want V_B around 1.5V, use resistors in the range of 10kΩ for R1 and 4.7kΩ for R2.
2. Connect R1 between V_CC and the transistor base.
3. Connect R2 between the transistor base and ground.
4. Attach the collector resistor R_C between the collector and V_CC.
5. Connect the emitter to ground.
6. Use a multimeter to measure the base, collector, and emitter voltages.

Observations

Record the base, collector, and emitter voltages. Verify that the base voltage corresponds to the calculated value from the voltage divider. This should place the transistor in its active region for amplification.

Conclusion

This experiment demonstrates how a voltage divider can be used to provide a stable biasing voltage to a transistor’s base, setting it up for predictable operation in its active region. This is a fundamental technique in analog circuit design, especially for amplifiers.