Lab 3 - Audio Amplifiers
In the third lab of ELEC 3030, we tested many common audio amplifiers to determine which would be most suitable for our design project.
As with all labs, results were simulated beforehand in LT Spice to establish an idea of what we should expect in the lab. While not all results were as expected, the gain did follow the trend from the prelab.
In this lab memo, I will discuss the results of each type of amplifier and list our choice of amplifier we will continue to use in subsequent labs.
Figure 1 - Amplifier Circuitry
Before we tested any amplifiers, we tried sending a direct signal to the speaker. What I observed was a large voltage drop across the speaker and a relatively muted sound.
Vin = 508mV
Vin(w/ speaker) = 102mV
The first amplifier we tested was a simple CE ampifier. With this amplifier inserted between the signal and the speaker, I could not hear any sound.
Next, we built the CC amplifier and tested it similar to the CE amplifier. Again, I could not hear any sound.
Figure 2 - CE & CC Amplifiers
When we connected the CE and CC amplifiers together, we finally received a decent sound output. While the amplifier was giving us a nice gain, the output signal was quite noisy. Below, gain and power dissipation are calculated:
Input Amplitude: 370mV
Output Amplitude: 2.1 V
Gain: 5.58 V/V
Power Dissipated: 420mW
Figure 3 - CECC Amplifier Input vs. Output
The next amplifier we built was the Class AB push-pull amplifier. In the first iteration of this design, there were no diodes added to the circuit. The output signal was roughly 10% of the input signal and the power dissipated was 40.5mW. This had a much better power consumption, but provided negative gain.
Next, we added the diodes to the circuit. The power consumption nearly doubled to 106mW compared to without diodes. With this circuit, we recieved just a slight gain, but also a much cleaner signal output. The sound from this amplifier was not bad.
Finally, we added this amplifer to the CE amplifier and observed a much higher sound output, but with some interference.
Input Amplitude: 1.22V
Output Amplitude: 2.86V
Gain: 2.34 V/V
Figure 4 - CE/push-pull Amplifier Input vs. Output
Figure 5 - op-amp/push-pull Amplifier Input vs. Output
Next, we built the op-amp circuit and tested it. I immediately noticed a significant gain increase.
Input Amplitude: 84mV
Output Amplitude: 1.88V
Gain: 22.4 V/V
When the op-amp circuit was combined with the class AB stage, we noticed an even higher signal gain. The sound was very clear and loud.
Input Amplitude: 66mV
Output Amplitude: 3.06V
Gain: 46.4V/V
Table 1 - Gain vs. Power
The last amplifier circuit that we tested was the LM386 amplifier. While this is usually the best amplifier in the class, I had many difficulties getting a clean signal from it.
Input Amplitude: 760mV
Output Amplitude: 7.0V
Gain: 9.2 V/V
Power Dissipated: 495mW
In the end, I ended up deciding on the op-amp / push-pull amplifier to use in my design. This amplifier gave me the best gain and signal clarity of the amplifiers I tested.
