EET2222 – Electronic Devices and Circuits

Lab 7 – Multistage Amplifier

BACKGROUND:

The amplified signal out of a CE stage can be used as the input to another CE stage. In this way, we can build a multistage amplifier with a very large voltage gain. However, because a CE stage has an input impedance, there is a loading effect on the preceding stage. Therefore, the loaded voltage gain is less than the unloaded voltage gain. In this lab, you will build a two-stage amplifier with swamped stages to stabilize the overall voltage gain.

PROCEDURE

Calculations:

1. Using the schematic above, calculate the DC voltages at the base, emitter, and collector for each stage. Also calculate I_B, I_E, and I_C for both stages.

2. Calculate the peak-to-peak AC voltage at the base, emitter, and collector of each stage using an h_fe value of 140.

Tests:

3. Connect the two-stage amplifier circuit shown on the first page.

4. Measure and record the DC voltage at the base, emitter and collector of each stage. Within the tolerance of the resistors being used, the measured voltages should agree with your calculated voltages.

5. Measure and record the peak-to-peak AC voltage at the base, emitter, and collector of each stage. These measured AC voltages should agree with your calculated values.

Loading Effects:

6. Open the coupling capacitor between the first and second stage. Look at the AC voltage on the first collector. Reconnect the coupling capacitor and notice the change to the AC signal on the first collector.

7. Open the coupling capacitor between the second stage and the load resistor. Look at the AC voltage on the second collector. Reconnect the coupling capacitor and notice the change to the AC signal on the second collector.

Troubleshooting:

8. Assume that C5 (the first stage’s bypass capacitor) is open. Will this produce a problem (distorted signal) in the first or second stage?

9. Insert the foregoing problem in your circuit (easiest way is to simply remove C5). Measure DC and AC Voltages in both stages to confirm your answer in step 8.

Questions:

1. How well did the calculated and measured values agree in step 4?

2. How well did the calculated and measured values agree in step 5?

3. Why did the AC signal change when the coupling capacitor was reconnected in step 6?

4. Explain what happens when an emitter bypass capacitor opens.

Your report is due at the end of the lab period. Please use the green engineering paper.