Lab 2 - CE Amplifiers
Figure 1 - DC Portion of Amplifier
In the second lab, we were to learn about common-emitter amplifiers through simulations and testing in the lab. The prelab portion of this experiment was simulated using LTSpice, and the lab was meant to verify our simulations.
For the first part of the lab, we breadboarded the circuit shown in Figure 1. Once this was built and operating, we used the DMM to measure the volatges at the base, collector, and emitter of the transistor. The results are listed below:
Vb = 1.82 V Vbe = 0.72 V
Vc = 5.60 V
Ve = 1.10 V Vce = 4.50 V
As the above results indicate, the transistor is operating in the forward-active region since Vbe is near 0.7V.
The next part of the lab had us add the AC circuit to our existing DC circuit. The end result matched the circuit shown in Figure 2.
Using the oscilloscope, we could compare the input signal from the function generator to the output of the amplifier. An example of this is shown in Figure 4 for the 1k Rload.
Different values of Rload were tested for voltage gain, and the results are listed in Table 1. A graph of the data from Table 1 can be seen in Figure 3.
If this CE amp was driving an 8 ohm speaker, it would not perform very well. This amp provides poor gain for that low of a resistance.
Figure 2 - AC Portion added to Amplifier
Table 1 - Gain vs. Resistance
Figure 3 - Plot of Voltage Gain vs. Resistance
Figure 4 - Oscilloscope for Input vs. Output of Amplifier
Table 2 - Gain vs. Frequency
For the last part of the lab, we made sure the circuit was rebuilt to resemble the circuit in Figure 2. We then varied the frequency on the function generator and observed how the voltage gain varied. Data collected for this part of the experiment is shown in Table 2, while a dB gain vs. frequency graph of the data is shown in Figure 5.
Based on the plot in Figure 5, I estimate the 3dB bandwidth to be about 3 MHz.
This lab helped me to better understand how a CE amplifier behaves when the load resistance and frequency are changed. This could be especially helpful for when I am trying to design an amplifier for an item such as an antenna.
Figure 5 - Plot of Gain (dB) vs. Frequency