Audio Amplifier

The purpose of lab 3 was to become familiar with, analyze, and construct the Common-Collector Amp, Common Emitter-Common Collector Amp, Class AB Push-Pull Amp, Common Emitter-class AB Two Stage Amp, Op-Amp Audio Amplifier, Op-Amp-Class AB Two Stage Amp, and the LM386 Audio Amp.

First, we were instructed to prepare our speakers by soldering wires to it. My speaker was already ready to use when I got it so I did not have to do this part. I was a little disappointed that I did not get to do this because I was really looking forward to gaining experience in soldering. Next, we were instructed to apply an audio wave directly to the speaker. The oscilloscope and generator was used to create a 1 kHz sine wave. You can see the results in Figure 1. Vin was found to be 50 mV. The speaker was reconnected and Vin decreased to 8.4 mV as seen in Figure 2. I could not hear a sound until I increased the amplitude.

Figure 1: Amplitude without Speaker

Figure 2: Amplitude with Speaker

Next, we were to build a CE amplifier and insert it between the generator and speaker. Figure 3 is a picture of the CE amplifier. I conducted a sound check and it sounded good. It was very loud but had a bit of noise.

Figure 3: CE Amplifier

For part 4, we were to build the CC amplifier between the generator and speaker shown in Figure 4. When I conducted a sound check, I noticed that it was not as loud as the CE amplifier but it was not a noisy.

Figure 4: CC Amplifier

Next, I conducted a sound check of the Two Stage Amplifier shown in Figure 5. It sounded terrible. It was very noisy so I turned the -20 dB off and it sounded much better. Then, I used the oscilloscope to view the output signal at the speaker. The results can be seen in Figure 6. The results show clipping. The speaker was then replaced with a 10 Ohm resistor and I used the oscilloscope to examine the input and output signal. The results are shown in Table 1.

Figure 5: Two Stage Amplifier

Figure 6: Output Signal at the Speaker of the Two Stage Amplifier

Table 1: Voltage Amplitudes and Gain for the Two Stage Amplifier

Next, I was instructed to determine the power dissipation of my two stage amplifier. I did this by feeding the voltage through a 100 Ohm resistor. I adjusted the supply so that Vcc = 9 V on the circuit side of the resistor. The source voltage was determined to be 9.9 V in order to achieve Vcc of 9 V. I then calculated the current: I = (9.9 V - 9 V)/100 Ohm. The current was determine to be 0.009 A. I then calculated quiescent power dissipated P = 0.009 A * 9 V = 0.081 W. This was expected since gain is low.

For part 3.4b, we were instructed to breadboard the class B push-pull amplifier. The supply voltage was run through the 100 Ohm resistor. The supply voltage was set to 9.5 V in order to maintain 9 V on the circuit side of the resistor. A 10 Ohm resistor was used instead of the speaker and I checked the bias voltage with no input signal applied. The circuit's quiescent dissipated power was determined to be P = 0.045 W. This was determined by I = (9.5 V - 9 V)/100 Ohm = 0.005 A. P = 0.005 A * 9 V. I set the input level to 2 V amplitude and the results are shown in Figure 7.

Figure 7: Class B Push Pull Amplifier with Input of 2 V Amplitude

Next, I breadboarded the class AB push-pull amplifier by simply adding diodes. The bias voltage was determined to be 9.04 V. I = (9.5 V - 9 V)/100 Ohm = 0.005 A. The circuits quiescent dissipated power was determined to be P = 0.005 A * 9 V = 0.045 V. This is equal to the value found for the class B push-pull amplifier. The input level was set to 2 V amplitude and the results are shown in Figure 8. First, I got clipping due to too high of amplitude so I decreased the input amplitude to 800 mV. You can see the results of clipping in Figure 9.

Figure 8: Class AB Push-Pull Amplifier

Figure 9: Class AB Push-Pull Amplifier with too high of Amplitude

The 10 Ohm load resistor was then replaced with the speaker and I conducted a sound check. It sounded good. It sounded much better than the CE-CC amplifier.

The 2nd stage CC amp was replaced with my class AB push-pull amp and it was inserted between the generator and the speaker. I determined gain and the results can be seen in Table 2 and Figure 10. You can see the clipping of the output signal. I conducted a sound check on this amplifier and it sounded great. So far, it was the best one yet but the low gain concerns me.

Table 2: Voltage Amplitude and Gain for the CE/Push-Pull Amplifier

Figure 10: CE/Push-Pull Amplifier

Next, I constructed the Op-Amp Audio Amplifier with a class AB second stage. I tested it with a 1 kHz amplitude input signal of amplitude 20 mV and gain was determined to be 11 V/V. Output voltage was determined to be 352 mV and input voltage was 32 mV. The 10 Ohm load resistor was replaced with the speaker in order to conduct a sound test. This amplifier also sounded great and reacted to frequency but due to the fact that is it very large, makes me nervous to use it for the final project.

Finally, I built the LM386 Amplifier shown in Figure 11. I conducted a sound check and it also sounded great. The output voltage was determined to be 2.32 V, the input voltage was determined to be 120 mV, and the gain was determined to be 19.3 V/V.

Figure 11: LM386 Amplifier

In conclusion, all of the amplifiers that I built produced a good sound. I decided to use the LM386 amplifier as my amplifier because it has good gain and it is easy to use! Table 3 summarizes the data from the amplifiers that I built in this lab.

Table 3: Gain and Quiescent P Various Amplifiers