Lab 3 - Audio Amplifiers

Soldering the Speaker

For the first part of the lab, I soldered the two leads onto the speaker. I then connected the speaker directly to the function generator. Without the speaker connected, the amplitude of the signal was 50mV. With the speaker connected, the amplitude from the function generator decreased to 15 mV. I then adjusted the amplitude of the function generator until I heard a sound from the speaker. I began to hear a sound at an amplitude of 65 mV.

Two Stage CE / CC Amp

As discovered in the second lab, the CE amplifier is insufficient as an audio amplifier by itself because the gain is less than one for a load resistance of around 8 Ohms. To retest this conclusion, a 40mv 1kHz signal was fed into the CE amp which was driving the speaker. No sound was audible.

Then, I built the CC amp and tested it separately from the CE amp. The components began getting hot. Because of this, I added a 100 Ohm resistor to the collector of the npn transistor. Then I saw if the CC amp alone could drive the speaker. A 40mV 1kHz signal was fed into the CC amplifier which was driving the speaker. The sound was inaudible.

After this, I connected the CE and the CC using coupling capacitors, creating the two stage CC / CE audio amplifier. With a 1kHz signal input into the two stage amplifier, the sound was pretty distorted. The best result was heard when the input was increased to 90mV. The result was also improved by decreasing the power voltage to around 7.2 V. Figure 1 shows the Oscilloscope while measuring the input and output of the CE / CC amplifier.

Figure 1: CE / CC Two Stage Amplifier

Table one shows the data recorded to find the gain of the CE / CC amplifier.

A 100 Ohm resistor was put in between the power supply and 9 V in order to calculate the current supplied by the power source. From this the quiescent power dissipation can be calculated and was found to be 0.9117 W.

Two Stage CE / Class AB Push-Pull Amplifier

At first a class B amplifier was bread-boarded and the power was supplied through a 100 Ohm resistor in order to calculate the quiescent power dissipation. This was measured to be 2.025 mW. A 2 V amplitude input signal was put into the amplifier. This result is shown in Figure 2. There is noticeable distortion around 0 V.

Figure 2: Class B Amplifier Input and Output

I then bread boarded the class AB push-pull amplifier. The quiescent power dissipation of this amplifier was measured to be 51.984 mW. This is noticeably higher than the Pq of the class B amplifier. It is, however, still much lower than the CC amplifier. The top of the waveform is clipped when the input is at 2 V pk/pk. When the input amplitude is reduced to around 1V pk/pk this is solved.

I then replaced the 10 ohm resistor with the speaker in order to understand how the input to the AB push pull effects the sound. With a 1 V pk/pk input the sound is very close to a sine wave. The volume is medium low. I start to hear distortion at around 1.72 V pk/pk. This is also around when distortion becomes visible on the Oscilloscope. The distortion makes the audio sound have a higher timbre. This is due to the higher frequency components present in the clipping. Higher frequency's are louder, however, this may be due to the speaker instead of the amplifier. There is a small bump in the gain of the amplifier at 468 Hz. This may be a result of the inductance of the speaker reacting with capacitance in the circuit.

Now I made a CE / AB push-pull two stage amplifier. Table 2 shows the gain data that I measured from the two stage amplifier.

I had trouble getting this amplifier to work. After debugging it for around 20 minutes, it started working without any changes that I made. I was able to get the measurements that are shown above, but it stopped working again just as I began to take a picture. When it was unconnected to the oscilloscope and terminated in the speaker it was slightly more reliable, however it would still occasionally have noise that was much louder than the audio signal that I was trying to hear.

LM386 Amplifier

The next amplifier that was tested was the LM386 Audio Amplifier. I made the circuit that is shown in Figure 3.

Figure 3: Circuit of LM386 with Gain of 20

I did a sound check of this amplifier with an input of 10 mV at 1 kHz. The audio sounded distorted. The timbre was higher than that of a sine wave which means that clipping is probably present. This chip functioned better with a high input amplitude. Figure 6 is a picture of the input and output when the input was set to 280 mV pk/pk. There is high frequency noise in both the input and the output.

I measured the quiescient power to be 106.93 mW and the Gain to be 24.14 v/v.

Two Stage Op-Amp / AB Push Pull Amplifier

The two stage Op-amp / AB Push-Pull amplifier was constructed as shown in Figure 4.

Figure 4: Two Stage Op Amp / AB Push-Pull Amplifier Schematic

I measured the quiescent power dissipation to be 57.759 mW. The gain of the two stage amplifier was 46.591 v/v. Figure 8 is the input and the output of the amplifier when it was terminated in the speaker.

Figure 5 is a picture of the input and output of this two stage amp when terminated in the speaker.

Conclusion

Table 3 shows the compiled results of all of the audio amplifier that were experimented with.

In choosing which audio amplifier I wanted to use for my radio, I considered the gain, power dissipation, reliability and quality of the output wave form. The latter two were weighted higher in the selection process. The CE/CC had the lowest gain, highest power dissipation, least reliability, and lowest quality wave form so this configuration was automatically out. The CE/class AB amp has decent gain and the lowest power dissipation. However, I wasn't able to make it consistently work. When it did work, the output quality was good. I did not choose the CE/ class AB amp because of the lack of reliability. The LM386 was very reliable. The power dissipation was about twice that of the CE/ Class AB, but it was still a manageable amount. The gain was also decent. However, the quality of the output waveform was not as good as it could have been. I ultimately ended up choosing the op-amp / class AB amp to be my audio amplifier. The gain was the highest and is easily adjustable by changing the resistance of the feedback resistor in the op amp. The power dissipation was close the that of the CE/ class AB amp. This amp was extremely reliable and worked exactly as predicted. The output wave form was the most accurate output of any of the amplifiers which results in a rich sine wave sound from the speaker.