PART 3: Audio Amplifier
INTRODUCTION
In this lab, the student will be working with audio amplifiers. The goals of this lab are to become familiar with, analyze, and construct different audio amplifiers. Some of the different types include Common-collector (CC) amp, CE-CC two stage amp, Class AB push-pull amp, and CE-class AB two stage amp. The student will construct the amps on a breadboard and preform different test on them. This will allow the student to find the gain of each amp so they can decide which will be the best choice for their radio.
3.3 Build and Test the Two-Stage Amplifier
To begin, the student prepared the 8 ohm speaker by soldering two, 3 inch wires to the eyelets on the back of the speaker. After that, an audio wave was applied directly to the speaker. To do this, the speakers wire were inserted into the breadboard and then connected to the function generator and oscilloscope, shown in Figure 3.1 below. Removing one of the speakers wires, a 1 kHz sine wave was created with an amplitude of 2Vpp, set by the GTA. After reconnecting the speaker, the voltage amplitude changes to 584mV, shown in Figure 3.2. The student did notice that this produced a loud higher pitched sound.
FIGURE 3.1: Driving the speaker directly from the generator
FIGURE 3.2: Voltage across speaker at 2 Vpp
Then, the student built the CE amplifier from Figure 3.4, shown in Figure 3.5. It was inserted between the generator and the speaker as indicated in Figure 3.3, where RL is replaced with the 8 ohm speaker. The same settings were used on the function generator as before, and the output sound of the speaker is loud and noisier before. The signal noise is shown below in Figure 3.6.
FIGURE 3.3: CE Amp between the generator and speaker
FIGURE 3.4: CE Amp Circuit
FIGURE 3.5: CE Amp breadboarded Circuit
FIGURE 3.6: CE Amp with speaker signal
Next, the student built the CC amplifier from Figure 3.7, and replaced the CE amp with it. The breadboarded circuit is shown in Figure 3.8. Since a 47ohm resistor is was not available, two 100ohm resistors in parallel were used instead. The same settings were used for the function generator as before. The student noticed a lower pitched, clear sound that was not as loud as before, coming from the speaker.
FIGURE 3.7: CC Amp Circuit
FIGURE 3.8: Breadboarded CC Amp Circuit
After that, the student built the two stage amplifier of Figure 3.10 and inserted it between the generator and the speaker as indicated in Figure 3.9. The load resistor is again replaced with the 8ohm speaker, shown in Figure 3.11. Continuing with the same settings on the function generator, the output signal of the speaker seemed to be louder the CC amp, similar to the CE amp, but with a clearer sound. The speaker was then replaced with a 10ohm resistor to determine the gain from the output and input amplitudes from the oscilloscope, shown in Table 3.1.
FIGURE 3.9: Inserting CE and CC amps
FIGURE 3.10: Two Stage CE-CC Amp Circuit
FIGURE 3.11: Breadboarded Two Stage CE-CC Amp Circuit
To conclude this portion of the lab, the student determined the power dissipation of the two stage amplifier. This was done by disconnecting the input signal, and feeding the voltage supply though a 100ohm resistor as shown in Figure 3.12. The 100 ohm resistor was connected between the DC power source and the input of the CE amp. The total current fed to the amplifier circuit was found by measuring the voltage drop across the resistor and dividing by 100. The PQ is then this current multiplied times Vcc (9 V) which resulted in 18.8 mW.
FIGURE 3.12: Voltage supply fed through 100ohm resistor
3.4 The class AB Push-Pull Amplifier
For this part of the lab, the student built and tested different classes of push-pull amplifier. To begin, the class B push-pull amplifier was breadboarded with the 2N3904/2N3906 transistors as shown in Figure 3.13. The supply voltage was run through a 100ohm resistor with an adjusted power supply to maintain 9V on the circuit side of the resistor. This allowed the student to easily check the bias voltage with no input signal applied and determine the circuit’s quiescent dissipated power, shown in Table 3.2. The input level was then set to 2Vpp, shown in Figure 3.14, where the student noticed clipping, which was expected.
FIGURE 3.13: Class B Push-Pull Amplifier Circuit
FIGURE 3.14: Class B Push-Pull Amplifier input and output signals
Next, the student breadboarded the class AB push-pull amplifier by adding the diodes as shown in Figure 3.15. This allows for the bias voltages with no input signal applied to be checked and the circuit’s quiescent dissipated power to be determined, shown in Table 3.2 above. The result were less than the class B push-pull PQ, which was expected because of the turn-on voltage required by the diodes. The input level was then set to 2Vpp, where the student observed that the signal was no longer clipped. The 10 ohm load resistor was then replaced with the speaker to see if this was a sufficient amplifier for the radio, shown in Figure 3.16.
FIGURE 3.15: Class AB Push-Pull Amplifier Circuit
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FIGURE 3.16: Class AB Push-Pull Amplifier with speaker
To conclude this lab, the student replaced the 2nd stage CC amp with the class AB push-pull amp and inserted it between the generator and the speaker as was indicated in Figure 3.9. Using the oscilloscope, the student observed the waveforms of Figure 3.17 and found the output and input amplitudes. Using these values, the gain was calculated, shown in Table 3.3. A sound test was performed and the student noticed that the sound was louder, but didn't seem as clear as it was with the CE-CC amplifier.
FIGURE 3.17: CE-AB Push-Pull input and output signals
CONCLUSION
In conclusion to this lab, the student was able to successfully work with audio amplifiers. The goals to become familiar with, analyze, and construct different audio amplifiers were accomplished. Some of the different types included the Common-collector (CC) amp, CE-CC two stage amp, Class AB push-pull amp, and CE-class AB two stage amp. The student was able to construct the amps on a breadboard and preform different test on them. This allowed the student to find the gain of each amp so they could decide which was the best choice for their radio. The student feels this lab was very beneficial and will be useful for the remainder of the semester.