Lab 3: Audio Amplifiers
Lab 3 was designed to introduce some audio amplifier topologies, namely the two-stage CE-CC amplifier, the class B push-pull amplifier, and the class AB push-pull amplifier. To prepare for the lab, wires were soldered to the speaker so that it could be driven by the amplifier.
Figure 1: Speaker with wires connected
Part 2 of lab 3.3 involved testing the speaker's output when driven directly from the function generator. A 1kHz, 2Vpp signal was generated and measured with an oscilloscope. The speaker was then connected and the signal was measured again. These signal measurements are shown in Figure 3. Our assigned input amplitude was 2Vpp and the amplitude with the speaker connected was 55.8mVpp.
Figure 2: Function generator, oscilloscope, and speaker connected on a breadboard
Figure 3: Function generator output before (left) and after (right) connecting the speaker
Part 3 of lab 3.3 involved building a common emitter amplifier to drive the speaker and checking the sound quality. Part 4 was similar, but used a common collector amplifier. Both of these are shown below in Figures 4 and 5 respectively. Both amplifiers sounded very similar and somewhat distorted.
Figure 4: Common emitter amplifier constructed on a breadboard
Figure 5: Common collector amplifier constructed on a breadboard
Part 5 of lab 3.3 involves combining the common emitter and common collector amplifiers into a two-stage amplifier, then measuring the output signal using the speaker as a load, then measuring again using a 10Ω resistor. The constructed amplifier is shown below in Figure 6 and output waveforms are shown in Figure 7.
Figure 6: Two-stage CE-CC amplifier constructed on a breadboard
Figure 7: Two-stage amplifier output with speaker load (left) and 10Ω resistor load (right)
The gain of the amplifier with a 10Ω load is given by Vout/Vin:
0.76/1.84 = 0.413 V/V
Part 6 of lab 3.3 involves calculating the power dissipated by the two-stage amplifier. This was done by supplying the voltage through a 100Ω resistor and measuring the voltage drop. In the case of this amplifier, the voltage was 9.05V at the circuit and 19.2V at the supply, meaning the power dissipated is given by 9.05(19.2-9.05)/100 = 918.6mW
Part 1 of lab 3.4 involves building a class B push-pull amplifier and measuring its quiescent power dissipation. This was done using the same procedure described above for the two-stage amplifier. The assembled class B amplifier is shown below in Figure 8. Its power dissipation is given by 9(19.6-9)/100 = 954mW
Figure 8: Class B push-pull amplifier
Part 2 of lab 3.4 involves viewing the output waveform when driving the class B amplifier with a 2Vpp input signal. The measured input and output signals are shown in Figure 9 below. The signal is attenuated compared to the input signal, and it does not perfectly replicate the input signal. It looks very similar to the LTspice graph in Figure 3.12 of the lab manual.
Figure 9: Output waveform (ch 2) of class B push-pull amplifier
Part 3 of lab 3.4 involves modifying the class B amplifier into a class AB push-pull amplifier by adding two diodes. Its quiescent power consumption was then measured and compared with that of the class B amplifier.. The assembled class AB amplifier is shown below in Figure 10. Its power dissipation is given by 9(19.7-9)/100 = 963mW. This is a higher value than the class B amplifier, but the class AB amplifier produces more gain, as shown later in part 4.
Figure 10: Class AB push-pull amplifier
Part 4 involves viewing the output waveform when driving the class AB amplifier with a 2Vpp input signal and comparing the output signal to the one observed with the class B amplifier. The measured input and output signals are shown in Figure 11 below. Compared to the class B amplifier, the class AB amplifier has a much higher output amplitude and is more faithful to the original input signal.
Figure 11: Output waveform (ch 2) of class B push-pull amplifier
Part 6 of lab 3.4 involves replacing the second stage of the CE-CC amplifier with the class AB push-pull amplifier. The completed circuit for this is shown below in Figure 12
Figure 12: CE amplifier driving a class AB push-pull amplifier
Part 7 involves calculating the gain of the two-stage CE-AB amplifier. The input and output waveforms are shown in Figure 13 below. The gain for this amplifier is 364/26.15 = 13.92 V/V. The output of this amplifier sounds very accurate due to the reduced input signal.
Figure 13: Output waveform (ch 2) of CE-AB two-stage amplifier