Lab 3 - Audio Amplifier

The goal of this weeks lab was to become familiar with the analysis and construction of several audio amplifiers such as the CE-CC two stage amp, the class AB push-pull amp, a CE-class AB two stage amp, and the LM386 audio amp. My primary goal was to simulate and test these amplifiers in lab in order to realize the one that will be most suitable for my radio.

I began the lab by soldering wires to my speaker. The soldering experience went rather smoothly and I enjoyed being able to solder something for the first time. I tested my speaker by applying an audio wave directly to the speaker using the generator and scope. I created a 1kHz sine wave with an amplitude of 50mV.

Figure 1: 8 ohm speaker with wires soldered on the eyelets

Building the CE amplifier (shown at the top of the breadboard in Figure 2), I inserted it between the function generator and speaker and conducted a soundcheck using the settings from the speaker test. As expected the sound was minimal and I could barely hear anything coming from the speaker. To improve the amplifier I added a CC stage (shown at the bottom of the breadboard in Figure 2) and inserted it between the CE amp and the speaker.

Figure 2: CE/CC multistage amp hooked up to 8 ohm speaker

Conducting another sound check, I noticed a significant improvement as the speaker was quite audible with this setup. The signal at the output was distorted, but not noisy. I hooked a 10 ohm resistor up to the output in lieu of the speaker and took a measurement of the input and output signal as shown in Figure 3. A picture of the table of voltage amplitudes and gain for the two stage amp is shown in Figure 4.

Figure 3: CE-CC multistage amp input and output waveforms

Figure 4: Voltage amplitudes and gain for CE-CC amp

Determining the power dissipated in this two stage amp is easily done by connecting a 100 ohm resistor and adjusting the supply such that Vcc=9V on the circuit side of the resistor. I measured the voltage drop across the resistor and divided it by 100. The power dissipated is Pq = 89.3mW.

In the following lab meeting, I completed the rest of lab 3 by examining the class AB push-pull amplifier and the LM386 audio amplifier.

Figure 5: CE (top left), Class AB push-pull (bottom left), and LM386 (top right)

Figure 6: Class AB push-pull amp schematic

The pair of diodes added to this class AB push-pull amplifier ensures that there is sufficient voltage to turn on the pair of transistor pn junctions and also works to produce an undistorted output signal. A class B push-pull amplifier has the same configuration as the class AB, but the diodes are removed. The quiescent power dissipation was about 38mW with and without the diodes added in the circuit. The push-pull configuration has an ideal voltage gain of 1 and thus can replace the CC amplifier in the CE/CC two stage audio amplifier design.

I wired up the class AB push-pull amplifier, using it as a second stage to my CE amp and tested its performance with a 10 ohm resistor wired to the output. Resulting input and output waveforms are shown in Figure 7 below and a table of the corresponding voltage amplitudes and gain is given in figure 8.

Figure 7: Input/output waveform of CE/push-pull multistage amplifier

Figure 8: Table of voltage amplitudes and gain for CE/push-pull multistage amplifier

Finally, I tested my LM386 audio amplifier by wiring it between the function generator and speaker as shown in Figure 9. I applied a 10mV signal at 1kHz, and the speaker sounded almost as loud as the CC/CE amplifier that I tested in the previous lab. In the screenshot shown in Figure 10, the input/output waveforms are distorted. It seemed like I was getting interference from other students running tests on their circuits, but I know the gain was around 20V/V.

Figure 9: LM386 audio amplifier schematic

Figure 10: Input/output waveform of LM386 audio amplifier

This lab took two entire lab meetings to complete, but I now have a firm grasp on the design and simulation of most of the amplifier circuits that were presented to me in the prelab. We did not have time to test the TL071 operational amplifier. The most efficient setups I tested were the CE/CC amp and the LM386, so I will likely be using one of these for my radio. Both of these configurations produced adequate gain and a loud audio output at the speaker.