ELEC 3030 Lab 3: Audio Amplifier

In the third lab activity of the semester, I began to study and understand the wonderful uses of the audio amplifier circuit, the vital part of the radio that amplifies a weak radio signal into a booming output over some speakers. Before studying the audio amp in lab, I first created simulations of the circuit in LTspice. During this lab activity, I focused on achieving a few goals: building the common-collector (CC) amp, the CE-CC two stage amp, the class AB push-pull amp, the CE-class AB two stage amp, the op-amp audio amplifier, the op-amp-class AB two stage amp, and the LM386 audio amp.

First, I constructed the two stage CE-CC amplifier as represented by the diagram in Figure 3, constructing and testing the CE amp first and then then adding the CC amp to it (Figure 4). However, before constructing these circuits, I soldering wires to a speaker and conducted a sound check (1 kHz at Vpp = 240 mV) to make sure that the speaker functioned properly. After verifying that the speaker functioned as it should, I constructed the CE amp and connected it to the speaker (Figure 1). The sound from the speaker was very clean and sharp but also very quiet. Next, I added the CC amp between the CE amp and the speaker (Figure 2). With the CC amp, the output was clean and at a suitable volume level.

Figure 1: Simple Schematic Diagram showing CE Amp

Figure 2: Simple Schematic Diagram showing CE + CC Amp

Then, having constructed these two amps, I constructed the two stage amplifier circuit (Figures 3 and 4) and performed a sound check again, which was crystal-clear but very quiet. Using the oscilloscope to view the output signal at the speaker, I noticed a very strange occurrence. The speaker produced a nice, clear sound; however, as soon as I connected the probes of the oscilloscope to the circuit, the signal became very noisy (Figure 5). Then, I replaced the speaker with a 10 Ohm resistor and measured the input and output signals (Figure 6), using measured values to create Table 1. Next, I calculated the quiescent power of the circuit: Pq = 429.5 mW.

Figure 3: Schematic Diagram of Two Stage Audio Amplifier

Figure 4: Two Stage Amplifier Circuit Constructed on Breadboard

Figure 5: Output Signal to Speaker

Figure 6: Output and Input Signals of Two Stage Amp Circuit showing Amplification

Table 1: Output, Input, and Gain of Two Stage Amp Circuit

For the next amplifier, I breadboarded the class AB Push-Pull Amplifier circuit. To do this, first I constructed the class B push-pull amp with 2N3904/2N3906 transistors (Figure 7). Using a 100 Ohm resistor at the load, I measured the quiescent power as Pq = 42.5 mW. Next, setting the input amplitude to 2 V, I measured the input and output waveforms with the oscilloscope (Figure 8), which very closely resembles my simulation results. As evident in Figure 8, the output (yellow waveform) sits at 0 V whenever the input voltage (blue waveform) is in between the transistors' turn on and reverse voltage threshold values.

Figure 7: Class B Push-Pull Amp Circuit

Figure 8: Input vs Output of Class B Push-Pull Amp

Then, after completing this circuit, I built the full AB Push-Pull amp circuit (Figure 9) which just features an addition of two diodes to the previous circuit. Once more, measuring the circuit's quiescent power, I calculated it to be Pq = 38.9 mW, which is less power consumption than the B Push-Pull amplifier from the previous part. Then, setting the input level of the circuit to 2 V amplitude, I measured the output (yellow waveform) of the amplifier (Figure 10). This output waveform differs significantly from that from Figure 8, as it resembles a very shaky-looking sine-wave.

Figure 9: Schematic of AB Push-Pull Amplifier Circuit

Figure 10: Input and Output of AB Push-Pull Amp at an Input of 2 V Amplitude

Next, after tweaking some parameters in my amplifier circuit, I replaced the 2nd stage of the CC amp from Figure 3 with the AB Push-Pull Amp and reduced the input power level to accommodate for the new amp. Measuring the input and output, I calculated the gain of my new audio amplifier circuit (Table 2). After this, I conducted another sound check of 20 mV at 1 kHz, which produced a clear but quiet sound.

Table 2: Gain of Audio Amplifier Circuit

In Part 2 (Chip Level Design), I had much difficulty in getting the LM386 amplifier to work. To eventually use this chip to create an op-amp (pinout shown in Figure 11), I first constructed the circuit shown in Figure 12 and placed it in between my generator and speaker. Using this setup, I did not successfully receive an output from the speaker. I do not know the reasons for the failure, but I do know that the problem was not due to a dysfunctional chip. Because my other amplifier circuits performed well, I did not worry about this one because I knew beforehand that I did not want to use an op-amp in my radio design. The TA in charge of the lab also told me not to worry too much about the op-amp.

Figure 11: Pinout of the LM386 Chip

Figure 12: Amp circuit using the LM386 Chip

Exploring audio amplifiers with the use of analog electronics has made me appreciate the complexity and engineering genius of even simple devices such as AM radios. As I observed and heard the amplification of a signal through my amp and speaker, I began to understand the important uses of audio amplifiers in not only a radio but also music equipment such as speakers. I especially enjoyed observing the two waveforms of the input and the output on the oscilloscope which showed me the almost-perfect amplification in real time. I am excited about learning and constructing the remaining parts of the AM radio.