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Audio Amplifier

Objectives:
  • The objective of the Audio Amplifier lab is to familarize individuals with a few of the various types of amplifiers which could possibly be used to construct an efficient radio.
  • The various Audio Amplifiers tested in this lab are the:​
    • ​Common-emitter amp
    • Common-collector amp
    • CE-CC two stage amp
    • Class AB push-pull amp
    • CE-class AB two stage amp
    • Op-amp 
    • Op-amp class AB two stage amp
    • LM386 amp
 
The pre-lab:
  • The pre-lab for the Audio Amplifier lab was nearly identical to the actual in-lab testing. The difference however was that pre-lab consisted of getting a theoretical idea of how each amplifier should perform in terms of power and gain, while actual lab consisted of real measurements and hands-on experience rather than a simulation. 

Build and test  two  stage  amp

  • The first part of this lab consisted of soldering wires onto the speaker in order to be able to connect it to circuitry. This small step brought back skills from soldering while Co-oping, even though this was fairly simple.​

  • This step involved:​

    • ​Testing the speaker alone with an injected audio signal.

    • Testing  the output with the CE amp connected.

    • Testing the output with the CC amp connected.

    • Testing  the output and gain of the CE-CC pair. 

The Audio Amplifier lab really gets going by applying an audio wave directly to the speaker and inserting the speaker on a breadboard for testing. In this case of a simple parallel circuit, the BK Precision 4040 Function generator generated a 1 kHz sine wave with an input signal of 50mV for the amplitude (vin); both the speaker and the oscilloscope were used to output the signal. When the speaker was connected, the voltage amplitude changes to 20.8 mV. The signal from the speaker sounded acceptionally louder as the amplitude increased.

Figure 1: Speaker/Osilloscope/Function generator parallel connection

Figure 2: Output signal on the oscilloscope, generated by                       the BK Precision 4040 function generator.

Following, the lab asked that the speaker be connected to the CE amplifier. This configuration was such that the CE amp was connected between the Function Generator (input) and the speaker (output). The output signal sounded the same as before with only the speaker connected, this was a pretty clean signal.

1.22 V / 20.8 mV = 58.65

Figure 3: CE amp inserted between Function Generator and the speaker.

Similarly, the lab asked that the Common Collector amplifier be built and placed between the generator and speaker. The output of this configuration sounded considerably quieter (lower) than prior.

Figure 4: CC amp inserted between Function                               Generaor and the speaker.

The final part of this step was to build the two stage amplifier by inserting both the CE and CC amplifiers between the function generator and speaker. The function generator was the input, CE followed by CC, then the speaker was the output. Through this configuration, the output sounded louder than either of the configurations before. Gain was measured by replacing the speaker with a 10Ω resistor. The signal on the oscilloscope was noisey, even picking up noise from the 3311A function generator. There was a lot of error in this part of the lab, this could have been a result of the noise in the system, as a result, gain was not measured to the expected value; as a lesson to engineering, everything will not go as expected.

Table 1: amplitudes and gain for two stage amplifier

Output amplitude

540 mV

Input amplitude

1.12 V

Gain(V / V)

0.482 (V / V )

Although there was error in measuring gain, the quiescent power dissipation was of a much more expected value, which was 63 mW. This was measured throughout lab by removing the input signal and feeding the supply voltage through a 100Ω resistor and adjusting the supply so that Vcc=9V, then by finding the current fed to the amplifier using the voltage drop across the 100Ω, the quiescent power dissipation could be calculated. 

The class AB Push-Pull Amplifier

  • The next step of this lab consisted of the class B and AB Push-Pull amplifiers.​ Unlike in a class A amplifier, such as the CC amplifier, the class B amplifier has an efficiency advantage. The npn BJT is only on during positive half cycles and the pnp BJT is only on during negative half cycles. Thus, little quiescent power dissipation in the transistor portion of the circuit. A drawback to the class B amp is that it takes roughly 0.7 V to turn on forward biased pn junctions in BJTs which results in output signal distortion. To accommodate for this, the class AB Push-Pull amplifier is built. This amplifier adds in a pair of diodes which ensure that the pair of transistors pn junctions get turned on; a drawback of this circuit is that is does not have very high efficiency. 

This lab was very similar to the things learned in Digial Electronics, Circuit Analysis, and Linear Signals and Systems. The lab involved aspects from each of these classes in terms of breadboarding, interpreting signals, and being comfortable with various components. This lab was filled with great information on CE amplifiers and Bode plots. I look forward to the following labs which build onto this one.

The class AB Push-Pull Amplifier gets going by breadboarding the class B Push-Pull amplifier with a pair of transistors. The 10Ω resistor was used instead of the the speaker in this case as the load. As before, the quiescent power was measured and determined to be 45 mW. Following that, adjusting the input level to 2 V amplitude revealed a clipping output signal waveform as shown below. 

Figure 5: Class B Push-Pull Amplifier 

Figure 6: Output of the Class B Push-Pull Amplifier 

This next part was to breadboard the AB push-pull amplifier by adding the diodes in the circuit. Once again quiescent dissipated power was checked and measured to be 45 mW. This value was approximately the same compared to the value found for the class B push-pull amplifier. This part of refreshed memories of the diodes lab done in Digital Electronics, where diode bridges and rectifiers were constructed. After adjusting the input level amplitude to 2 V, the output signal waveform was a clean signal although there was overclipping, both this waveform and the prior one cliipped in some way.

1.22 V / 20.8 mV = 58.65

Figure 7: Class AB Push-Pull Amplifier 

Figure 8:  Output of Class AB Push-Pull Amplifier 

The second stage CC amp from the prior step was then replaced with the Class AB Push-Pull amp. The new configuration consisted of the function generator being the input, the speaker was the output, and the CE and AB Push-Pull amplifiers between those with the AB Push-Pull amplifier connected to the speaker. The amplitudes and gain for this new configuration turned out to be close to the expected values, which are presented below. A sound check was constructed and it sounded extremely loud. 

Figure 9: Two stage CE- AB Push-Pull Amplifier

                 with speaker for sound testing 

Figure 10: Two stage CE- AB Push-Pull Amplifier

                 with 10Ω resistor for signal testing

Figure 11: Two stage CE- AB Push-Pull Amplifier output signal.

Table 2: amplitudes and gain for CE-AB Push-Pull two stage amplifier

Output amplitude

446 mV

Input amplitude

900 mV

Gain(V / V)

0.4955 ( V / V )

Op Amp Audio Amplifier

  • The Op Amp Audio Amplifier was next on the list to be build and tested. This part of lab is particularly important because Op Amps are a primary subject in Analog Electronics. Op-amps are integrated circuits that amplifies the difference in voltage at a pair of input terminals. Op-amps have extremely high gain, high input resistance, and low output resistance which allow them to be treated as ideal devices, although no component can be completely ideal. An actual ideal Op-Amp would have infinite gain, the + and - terminals would also have infinite resistance. 

The Op-Amp circuit was build and testing to ensure that it was operational. Then, the class AB Push-Pull amplifier stage was added to the Op-amp circuit. This was then tested with a 1 kHz amplitude input signal of amplitude 20 mV. The gain of this two-stage amplifier came out to be approximately (Av =)90. The 10Ω resistor was then replaced by the speaker which allowed for sound testing; the output of this amplifier revealed to be suitable for use in the final design. The new two-stage amplifier and output are shown below:

Figure 13: Two stage Op-Amp - AB Push-Pull circuit

Figure 14: Output of the two stage Op-Amp - AB Push-Pull configuration

LM386 Amplifier

  • The final part of this lab involved the LM386 Amplifier. The LM386 is an integrated circuit amplifier designed for use in low voltage applications. It has low quiescent power dissipation, and adjustable voltage gain ranging from 20 to 200.

The LM386 Amplifier was breadboarded and testing for a gain of 20. This amplifier was inserted between the generator and the speaker. A sound check was constructed and this circuit proved to be rather noisy and unpleasant. The output signal proved to backup the theory that this output was far from the desired output. The voltage gain was measured to be

3.25 (V / V) and the quiescent power was measured to be 0.378 mW.

Figure 15: The LM386 Amplifier circuit with speaker connected

Figure 16: Output of the LM386 Amplifier circuit

Figure 17: Adusted output of the LM386 Amplifier circuit

This Audio Amplifier lab was very useful in inproving breadboarding skills. This lab assisted in learning much about different forms of Audio Amplifiers and how they each have advantages and disadvantages. This lab helped utilize many skills gained in previous classes and workforce. The boardboarding of a couple of the circuits resembled Digital Electronics, the soldering was reinforced through Co-op experience, and the signals seen helped visual linear signals and systems. This was a great lab to improve many engineering skills.

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