Project-11: The Transistor Amplifier

Objective

The objective of this project is to demonstrate how AC voltage gain changes when you use resistors of different values and transistors with different current gain in a transistor amplifier circuit.

General Instructions

When the circuit is set up, you measure Vout for each set of resistors, and find AV, using the ratio Vout/Vin. You also determine a calculated A V using the ratio RC/RE in each case to determine how close the calculated AV is to the measured AV. You repeat this measurement with a second transistor for each set of resistors.

Parts List

You need the following equipment and supplies:

• One 1 kΩ, 0.25-watt resistor
• One 100 Ω, 0.25-watt resistor
• One 15 kΩ, 0.25-watt resistor
• One 2.2 kΩ, 0.25-watt resistor
• One 3.3 kΩ, 0.25-watt resistor
• One 220 Ω, 0.25-watt resistor
• One 68 kΩ, 0.25-watt resistor
• One 8.2 kΩ, 0.25-watt resistor
• One PN2222 transistor
• One 0.1 μF capacitor
• One lab type power supply or 9-volt battery
• One function generator.
• One oscilloscope
• One breadboard
• One 2N3904 transistor

Figure 8.11 shows the pinout diagram for 2N3904 and PN2222 transistors.

Figure 8.11

Step-by-Step Instructions

Set up the circuit shown in Figure 8.12 using the components listed for Circuit # 1 in the following table. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help building the circuit, look at the photos of the completed circuit in the “Expected Results” section. (If you don't have a lab type power supply to provide 10 volts as indicated on the schematic, use a 9-volt battery.)

Figure 8.12

Carefully check your circuit against the diagram.
After you check your circuit, follow these steps, and record your measurements in the blank table following the steps.

1. Connect the oscilloscope probe for channel 2 to a jumper wire connected to Vin, and then connect the ground clip to a jumper wire attached to the ground bus.
2. Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and then connect the ground clip to a jumper wire attached to the ground bus.
3. Set the function generator to generate a 10 kHz sine wave with approximately 0.2 Vpp.
4. Measure and record Vin and Vout.
5. Change the components to those listed in the next row of the table (Circuit # 2 in this case.) You should turn off the power to the circuit before changing components to avoid shorting leads together.
6. Measure and record Vin and Vout.
7. Repeat steps 5 and 6 until you have recorded Vin and Vout in the last row of the table.
8. Determine β for each of the transistors used in this project. Insert the transistors one at a time into the circuit you built in Project 3-1 to take this measurement.
9. For each transistor, record β in the following table.

Expected Results

Figure 8.13 shows the breadboarded Circuit # 1.

Figure 8.13

Figure 8.14 shows the breadboarded Circuit # 3.

Figure 8.14

Figure 8.15 shows a function generator and oscilloscope attached to the circuit.

Figure 8.15

The input signal is represented by the upper sine wave shown in Figure 8.16, and the output signal is represented by the lower sine wave.
Read the number of divisions for the peak-to-peak output sine wave, and multiply it by the corresponding VOLTS/DIV setting to determine Vout.

Figure 8.16

As you measure Vin and Vout for each circuit, you may need to adjust the TIME/DIV control, VOLTS/DIV control, and vertical POSITION controls
on the oscilloscope. The controls shown in Figure 8.17 are adjusted to measure Vin and Vout for Circuit #2.

Figure 8.17

Your values should be close to those shown in the following table.

The measured values of AV are quite close to the calculated values of AV, well within variations that could be caused by the ± 5 percent tolerance specified for the resistor values. Also, the variation in transistor β had no effect on the measured values of AV.